summaryrefslogtreecommitdiffstats
path: root/servo/components/style/animation.rs
blob: b865120abacadb85730798dc71c6d3e2d9c1bfb9 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

//! CSS transitions and animations.

// NOTE(emilio): This code isn't really executed in Gecko, but we don't want to
// compile it out so that people remember it exists.

use crate::context::{CascadeInputs, SharedStyleContext};
use crate::dom::{OpaqueNode, TDocument, TElement, TNode};
use crate::properties::animated_properties::{AnimationValue, AnimationValueMap};
use crate::properties::longhands::animation_direction::computed_value::single_value::T as AnimationDirection;
use crate::properties::longhands::animation_fill_mode::computed_value::single_value::T as AnimationFillMode;
use crate::properties::longhands::animation_play_state::computed_value::single_value::T as AnimationPlayState;
use crate::properties::AnimationDeclarations;
use crate::properties::{
    ComputedValues, Importance, LonghandId, PropertyDeclarationBlock, PropertyDeclarationId,
    PropertyDeclarationIdSet,
};
use crate::rule_tree::CascadeLevel;
use crate::selector_parser::PseudoElement;
use crate::shared_lock::{Locked, SharedRwLock};
use crate::style_resolver::StyleResolverForElement;
use crate::stylesheets::keyframes_rule::{KeyframesAnimation, KeyframesStep, KeyframesStepValue};
use crate::stylesheets::layer_rule::LayerOrder;
use crate::values::animated::{Animate, Procedure};
use crate::values::computed::{Time, TimingFunction};
use crate::values::generics::easing::BeforeFlag;
use crate::Atom;
use fxhash::FxHashMap;
use parking_lot::RwLock;
use servo_arc::Arc;
use std::fmt;

/// Represents an animation for a given property.
#[derive(Clone, Debug, MallocSizeOf)]
pub struct PropertyAnimation {
    /// The value we are animating from.
    from: AnimationValue,

    /// The value we are animating to.
    to: AnimationValue,

    /// The timing function of this `PropertyAnimation`.
    timing_function: TimingFunction,

    /// The duration of this `PropertyAnimation` in seconds.
    pub duration: f64,
}

impl PropertyAnimation {
    /// Returns the given property longhand id.
    pub fn property_id(&self) -> PropertyDeclarationId {
        debug_assert_eq!(self.from.id(), self.to.id());
        self.from.id()
    }

    fn from_property_declaration(
        property_declaration: &PropertyDeclarationId,
        timing_function: TimingFunction,
        duration: Time,
        old_style: &ComputedValues,
        new_style: &ComputedValues,
    ) -> Option<PropertyAnimation> {
        // FIXME(emilio): Handle the case where old_style and new_style's writing mode differ.
        let property_declaration = property_declaration.to_physical(new_style.writing_mode);
        let from = AnimationValue::from_computed_values(property_declaration, old_style)?;
        let to = AnimationValue::from_computed_values(property_declaration, new_style)?;
        let duration = duration.seconds() as f64;

        if from == to || duration == 0.0 {
            return None;
        }

        Some(PropertyAnimation {
            from,
            to,
            timing_function,
            duration,
        })
    }

    /// The output of the timing function given the progress ration of this animation.
    fn timing_function_output(&self, progress: f64) -> f64 {
        let epsilon = 1. / (200. * self.duration);
        // FIXME: Need to set the before flag correctly.
        // In order to get the before flag, we have to know the current animation phase
        // and whether the iteration is reversed. For now, we skip this calculation
        // by treating as if the flag is unset at all times.
        // https://drafts.csswg.org/css-easing/#step-timing-function-algo
        self.timing_function
            .calculate_output(progress, BeforeFlag::Unset, epsilon)
    }

    /// Update the given animation at a given point of progress.
    fn calculate_value(&self, progress: f64) -> Result<AnimationValue, ()> {
        let procedure = Procedure::Interpolate {
            progress: self.timing_function_output(progress),
        };
        self.from.animate(&self.to, procedure)
    }
}

/// This structure represents the state of an animation.
#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
pub enum AnimationState {
    /// The animation has been created, but is not running yet. This state
    /// is also used when an animation is still in the first delay phase.
    Pending,
    /// This animation is currently running.
    Running,
    /// This animation is paused. The inner field is the percentage of progress
    /// when it was paused, from 0 to 1.
    Paused(f64),
    /// This animation has finished.
    Finished,
    /// This animation has been canceled.
    Canceled,
}

impl AnimationState {
    /// Whether or not this state requires its owning animation to be ticked.
    fn needs_to_be_ticked(&self) -> bool {
        *self == AnimationState::Running || *self == AnimationState::Pending
    }
}

/// This structure represents a keyframes animation current iteration state.
///
/// If the iteration count is infinite, there's no other state, otherwise we
/// have to keep track the current iteration and the max iteration count.
#[derive(Clone, Debug, MallocSizeOf)]
pub enum KeyframesIterationState {
    /// Infinite iterations with the current iteration count.
    Infinite(f64),
    /// Current and max iterations.
    Finite(f64, f64),
}

/// A temporary data structure used when calculating ComputedKeyframes for an
/// animation. This data structure is used to collapse information for steps
/// which may be spread across multiple keyframe declarations into a single
/// instance per `start_percentage`.
struct IntermediateComputedKeyframe {
    declarations: PropertyDeclarationBlock,
    timing_function: Option<TimingFunction>,
    start_percentage: f32,
}

impl IntermediateComputedKeyframe {
    fn new(start_percentage: f32) -> Self {
        IntermediateComputedKeyframe {
            declarations: PropertyDeclarationBlock::new(),
            timing_function: None,
            start_percentage,
        }
    }

    /// Walk through all keyframe declarations and combine all declarations with the
    /// same `start_percentage` into individual `IntermediateComputedKeyframe`s.
    fn generate_for_keyframes(
        animation: &KeyframesAnimation,
        context: &SharedStyleContext,
        base_style: &ComputedValues,
    ) -> Vec<Self> {
        let mut intermediate_steps: Vec<Self> = Vec::with_capacity(animation.steps.len());
        let mut current_step = IntermediateComputedKeyframe::new(0.);
        for step in animation.steps.iter() {
            let start_percentage = step.start_percentage.0;
            if start_percentage != current_step.start_percentage {
                let new_step = IntermediateComputedKeyframe::new(start_percentage);
                intermediate_steps.push(std::mem::replace(&mut current_step, new_step));
            }

            current_step.update_from_step(step, context, base_style);
        }
        intermediate_steps.push(current_step);

        // We should always have a first and a last step, even if these are just
        // generated by KeyframesStepValue::ComputedValues.
        debug_assert!(intermediate_steps.first().unwrap().start_percentage == 0.);
        debug_assert!(intermediate_steps.last().unwrap().start_percentage == 1.);

        intermediate_steps
    }

    fn update_from_step(
        &mut self,
        step: &KeyframesStep,
        context: &SharedStyleContext,
        base_style: &ComputedValues,
    ) {
        // Each keyframe declaration may optionally specify a timing function, falling
        // back to the one defined global for the animation.
        let guard = &context.guards.author;
        if let Some(timing_function) = step.get_animation_timing_function(&guard) {
            self.timing_function = Some(timing_function.to_computed_value_without_context());
        }

        let block = match step.value {
            KeyframesStepValue::ComputedValues => return,
            KeyframesStepValue::Declarations { ref block } => block,
        };

        // Filter out !important, non-animatable properties, and the
        // 'display' property (which is only animatable from SMIL).
        let guard = block.read_with(&guard);
        for declaration in guard.normal_declaration_iter() {
            if let PropertyDeclarationId::Longhand(id) = declaration.id() {
                if id == LonghandId::Display {
                    continue;
                }

                if !id.is_animatable() {
                    continue;
                }
            }

            self.declarations.push(
                declaration.to_physical(base_style.writing_mode),
                Importance::Normal,
            );
        }
    }

    fn resolve_style<E>(
        self,
        element: E,
        context: &SharedStyleContext,
        base_style: &Arc<ComputedValues>,
        resolver: &mut StyleResolverForElement<E>,
    ) -> Arc<ComputedValues>
    where
        E: TElement,
    {
        if !self.declarations.any_normal() {
            return base_style.clone();
        }

        let document = element.as_node().owner_doc();
        let locked_block = Arc::new(document.shared_lock().wrap(self.declarations));
        let mut important_rules_changed = false;
        let rule_node = base_style.rules().clone();
        let new_node = context.stylist.rule_tree().update_rule_at_level(
            CascadeLevel::Animations,
            LayerOrder::root(),
            Some(locked_block.borrow_arc()),
            &rule_node,
            &context.guards,
            &mut important_rules_changed,
        );

        if new_node.is_none() {
            return base_style.clone();
        }

        let inputs = CascadeInputs {
            rules: new_node,
            visited_rules: base_style.visited_rules().cloned(),
            flags: base_style.flags.for_cascade_inputs(),
        };
        resolver
            .cascade_style_and_visited_with_default_parents(inputs)
            .0
    }
}

/// A single computed keyframe for a CSS Animation.
#[derive(Clone, MallocSizeOf)]
struct ComputedKeyframe {
    /// The timing function to use for transitions between this step
    /// and the next one.
    timing_function: TimingFunction,

    /// The starting percentage (a number between 0 and 1) which represents
    /// at what point in an animation iteration this step is.
    start_percentage: f32,

    /// The animation values to transition to and from when processing this
    /// keyframe animation step.
    values: Vec<AnimationValue>,
}

impl ComputedKeyframe {
    fn generate_for_keyframes<E>(
        element: E,
        animation: &KeyframesAnimation,
        context: &SharedStyleContext,
        base_style: &Arc<ComputedValues>,
        default_timing_function: TimingFunction,
        resolver: &mut StyleResolverForElement<E>,
    ) -> Vec<Self>
    where
        E: TElement,
    {
        let mut animating_properties = PropertyDeclarationIdSet::default();
        for property in animation.properties_changed.iter() {
            debug_assert!(property.is_animatable());
            animating_properties.insert(property.to_physical(base_style.writing_mode));
        }

        let animation_values_from_style: Vec<AnimationValue> = animating_properties
            .iter()
            .map(|property| {
                AnimationValue::from_computed_values(property, &**base_style)
                    .expect("Unexpected non-animatable property.")
            })
            .collect();

        let intermediate_steps =
            IntermediateComputedKeyframe::generate_for_keyframes(animation, context, base_style);

        let mut computed_steps: Vec<Self> = Vec::with_capacity(intermediate_steps.len());
        for (step_index, step) in intermediate_steps.into_iter().enumerate() {
            let start_percentage = step.start_percentage;
            let properties_changed_in_step = step.declarations.property_ids().clone();
            let step_timing_function = step.timing_function.clone();
            let step_style = step.resolve_style(element, context, base_style, resolver);
            let timing_function =
                step_timing_function.unwrap_or_else(|| default_timing_function.clone());

            let values = {
                // If a value is not set in a property declaration we use the value from
                // the style for the first and last keyframe. For intermediate ones, we
                // use the value from the previous keyframe.
                //
                // TODO(mrobinson): According to the spec, we should use an interpolated
                // value for properties missing from keyframe declarations.
                let default_values = if start_percentage == 0. || start_percentage == 1.0 {
                    &animation_values_from_style
                } else {
                    debug_assert!(step_index != 0);
                    &computed_steps[step_index - 1].values
                };

                // For each property that is animating, pull the value from the resolved
                // style for this step if it's in one of the declarations. Otherwise, we
                // use the default value from the set we calculated above.
                animating_properties
                    .iter()
                    .zip(default_values.iter())
                    .map(|(property_declaration, default_value)| {
                        if properties_changed_in_step.contains(property_declaration) {
                            AnimationValue::from_computed_values(property_declaration, &step_style)
                                .unwrap_or_else(|| default_value.clone())
                        } else {
                            default_value.clone()
                        }
                    })
                    .collect()
            };

            computed_steps.push(ComputedKeyframe {
                timing_function,
                start_percentage,
                values,
            });
        }
        computed_steps
    }
}

/// A CSS Animation
#[derive(Clone, MallocSizeOf)]
pub struct Animation {
    /// The name of this animation as defined by the style.
    pub name: Atom,

    /// The properties that change in this animation.
    properties_changed: PropertyDeclarationIdSet,

    /// The computed style for each keyframe of this animation.
    computed_steps: Vec<ComputedKeyframe>,

    /// The time this animation started at, which is the current value of the animation
    /// timeline when this animation was created plus any animation delay.
    pub started_at: f64,

    /// The duration of this animation.
    pub duration: f64,

    /// The delay of the animation.
    pub delay: f64,

    /// The `animation-fill-mode` property of this animation.
    pub fill_mode: AnimationFillMode,

    /// The current iteration state for the animation.
    pub iteration_state: KeyframesIterationState,

    /// Whether this animation is paused.
    pub state: AnimationState,

    /// The declared animation direction of this animation.
    pub direction: AnimationDirection,

    /// The current animation direction. This can only be `normal` or `reverse`.
    pub current_direction: AnimationDirection,

    /// The original cascade style, needed to compute the generated keyframes of
    /// the animation.
    #[ignore_malloc_size_of = "ComputedValues"]
    pub cascade_style: Arc<ComputedValues>,

    /// Whether or not this animation is new and or has already been tracked
    /// by the script thread.
    pub is_new: bool,
}

impl Animation {
    /// Whether or not this animation is cancelled by changes from a new style.
    fn is_cancelled_in_new_style(&self, new_style: &Arc<ComputedValues>) -> bool {
        let new_ui = new_style.get_ui();
        let index = new_ui
            .animation_name_iter()
            .position(|animation_name| Some(&self.name) == animation_name.as_atom());
        let index = match index {
            Some(index) => index,
            None => return true,
        };

        new_ui.animation_duration_mod(index).seconds() == 0.
    }

    /// Given the current time, advances this animation to the next iteration,
    /// updates times, and then toggles the direction if appropriate. Otherwise
    /// does nothing. Returns true if this animation has iterated.
    pub fn iterate_if_necessary(&mut self, time: f64) -> bool {
        if !self.iteration_over(time) {
            return false;
        }

        // Only iterate animations that are currently running.
        if self.state != AnimationState::Running {
            return false;
        }

        if self.on_last_iteration() {
            return false;
        }

        self.iterate();
        true
    }

    fn iterate(&mut self) {
        debug_assert!(!self.on_last_iteration());

        if let KeyframesIterationState::Finite(ref mut current, max) = self.iteration_state {
            *current = (*current + 1.).min(max);
        }

        if let AnimationState::Paused(ref mut progress) = self.state {
            debug_assert!(*progress > 1.);
            *progress -= 1.;
        }

        // Update the next iteration direction if applicable.
        self.started_at += self.duration;
        match self.direction {
            AnimationDirection::Alternate | AnimationDirection::AlternateReverse => {
                self.current_direction = match self.current_direction {
                    AnimationDirection::Normal => AnimationDirection::Reverse,
                    AnimationDirection::Reverse => AnimationDirection::Normal,
                    _ => unreachable!(),
                };
            },
            _ => {},
        }
    }

    /// A number (> 0 and <= 1) which represents the fraction of a full iteration
    /// that the current iteration of the animation lasts. This will be less than 1
    /// if the current iteration is the fractional remainder of a non-integral
    /// iteration count.
    pub fn current_iteration_end_progress(&self) -> f64 {
        match self.iteration_state {
            KeyframesIterationState::Finite(current, max) => (max - current).min(1.),
            KeyframesIterationState::Infinite(_) => 1.,
        }
    }

    /// The duration of the current iteration of this animation which may be less
    /// than the animation duration if it has a non-integral iteration count.
    pub fn current_iteration_duration(&self) -> f64 {
        self.current_iteration_end_progress() * self.duration
    }

    /// Whether or not the current iteration is over. Note that this method assumes that
    /// the animation is still running.
    fn iteration_over(&self, time: f64) -> bool {
        time > (self.started_at + self.current_iteration_duration())
    }

    /// Assuming this animation is running, whether or not it is on the last iteration.
    fn on_last_iteration(&self) -> bool {
        match self.iteration_state {
            KeyframesIterationState::Finite(current, max) => current >= (max - 1.),
            KeyframesIterationState::Infinite(_) => false,
        }
    }

    /// Whether or not this animation has finished at the provided time. This does
    /// not take into account canceling i.e. when an animation or transition is
    /// canceled due to changes in the style.
    pub fn has_ended(&self, time: f64) -> bool {
        if !self.on_last_iteration() {
            return false;
        }

        let progress = match self.state {
            AnimationState::Finished => return true,
            AnimationState::Paused(progress) => progress,
            AnimationState::Running => (time - self.started_at) / self.duration,
            AnimationState::Pending | AnimationState::Canceled => return false,
        };

        progress >= self.current_iteration_end_progress()
    }

    /// Updates the appropiate state from other animation.
    ///
    /// This happens when an animation is re-submitted to layout, presumably
    /// because of an state change.
    ///
    /// There are some bits of state we can't just replace, over all taking in
    /// account times, so here's that logic.
    pub fn update_from_other(&mut self, other: &Self, now: f64) {
        use self::AnimationState::*;

        debug!(
            "KeyframesAnimationState::update_from_other({:?}, {:?})",
            self, other
        );

        // NB: We shall not touch the started_at field, since we don't want to
        // restart the animation.
        let old_started_at = self.started_at;
        let old_duration = self.duration;
        let old_direction = self.current_direction;
        let old_state = self.state.clone();
        let old_iteration_state = self.iteration_state.clone();

        *self = other.clone();

        self.started_at = old_started_at;
        self.current_direction = old_direction;

        // Don't update the iteration count, just the iteration limit.
        // TODO: see how changing the limit affects rendering in other browsers.
        // We might need to keep the iteration count even when it's infinite.
        match (&mut self.iteration_state, old_iteration_state) {
            (
                &mut KeyframesIterationState::Finite(ref mut iters, _),
                KeyframesIterationState::Finite(old_iters, _),
            ) => *iters = old_iters,
            _ => {},
        }

        // Don't pause or restart animations that should remain finished.
        // We call mem::replace because `has_ended(...)` looks at `Animation::state`.
        let new_state = std::mem::replace(&mut self.state, Running);
        if old_state == Finished && self.has_ended(now) {
            self.state = Finished;
        } else {
            self.state = new_state;
        }

        // If we're unpausing the animation, fake the start time so we seem to
        // restore it.
        //
        // If the animation keeps paused, keep the old value.
        //
        // If we're pausing the animation, compute the progress value.
        match (&mut self.state, &old_state) {
            (&mut Pending, &Paused(progress)) => {
                self.started_at = now - (self.duration * progress);
            },
            (&mut Paused(ref mut new), &Paused(old)) => *new = old,
            (&mut Paused(ref mut progress), &Running) => {
                *progress = (now - old_started_at) / old_duration
            },
            _ => {},
        }

        // Try to detect when we should skip straight to the running phase to
        // avoid sending multiple animationstart events.
        if self.state == Pending && self.started_at <= now && old_state != Pending {
            self.state = Running;
        }
    }

    /// Fill in an `AnimationValueMap` with values calculated from this animation at
    /// the given time value.
    fn get_property_declaration_at_time(&self, now: f64, map: &mut AnimationValueMap) {
        debug_assert!(!self.computed_steps.is_empty());

        let total_progress = match self.state {
            AnimationState::Running | AnimationState::Pending | AnimationState::Finished => {
                (now - self.started_at) / self.duration
            },
            AnimationState::Paused(progress) => progress,
            AnimationState::Canceled => return,
        };

        if total_progress < 0. &&
            self.fill_mode != AnimationFillMode::Backwards &&
            self.fill_mode != AnimationFillMode::Both
        {
            return;
        }
        if self.has_ended(now) &&
            self.fill_mode != AnimationFillMode::Forwards &&
            self.fill_mode != AnimationFillMode::Both
        {
            return;
        }
        let total_progress = total_progress
            .min(self.current_iteration_end_progress())
            .max(0.0);

        // Get the indices of the previous (from) keyframe and the next (to) keyframe.
        let next_keyframe_index;
        let prev_keyframe_index;
        let num_steps = self.computed_steps.len();
        match self.current_direction {
            AnimationDirection::Normal => {
                next_keyframe_index = self
                    .computed_steps
                    .iter()
                    .position(|step| total_progress as f32 <= step.start_percentage);
                prev_keyframe_index = next_keyframe_index
                    .and_then(|pos| if pos != 0 { Some(pos - 1) } else { None })
                    .unwrap_or(0);
            },
            AnimationDirection::Reverse => {
                next_keyframe_index = self
                    .computed_steps
                    .iter()
                    .rev()
                    .position(|step| total_progress as f32 <= 1. - step.start_percentage)
                    .map(|pos| num_steps - pos - 1);
                prev_keyframe_index = next_keyframe_index
                    .and_then(|pos| {
                        if pos != num_steps - 1 {
                            Some(pos + 1)
                        } else {
                            None
                        }
                    })
                    .unwrap_or(num_steps - 1)
            },
            _ => unreachable!(),
        }

        debug!(
            "Animation::get_property_declaration_at_time: keyframe from {:?} to {:?}",
            prev_keyframe_index, next_keyframe_index
        );

        let prev_keyframe = &self.computed_steps[prev_keyframe_index];
        let next_keyframe = match next_keyframe_index {
            Some(index) => &self.computed_steps[index],
            None => return,
        };

        // If we only need to take into account one keyframe, then exit early
        // in order to avoid doing more work.
        let mut add_declarations_to_map = |keyframe: &ComputedKeyframe| {
            for value in keyframe.values.iter() {
                map.insert(value.id().to_owned(), value.clone());
            }
        };
        if total_progress <= 0.0 {
            add_declarations_to_map(&prev_keyframe);
            return;
        }
        if total_progress >= 1.0 {
            add_declarations_to_map(&next_keyframe);
            return;
        }

        let percentage_between_keyframes =
            (next_keyframe.start_percentage - prev_keyframe.start_percentage).abs() as f64;
        let duration_between_keyframes = percentage_between_keyframes * self.duration;
        let direction_aware_prev_keyframe_start_percentage = match self.current_direction {
            AnimationDirection::Normal => prev_keyframe.start_percentage as f64,
            AnimationDirection::Reverse => 1. - prev_keyframe.start_percentage as f64,
            _ => unreachable!(),
        };
        let progress_between_keyframes = (total_progress -
            direction_aware_prev_keyframe_start_percentage) /
            percentage_between_keyframes;

        for (from, to) in prev_keyframe.values.iter().zip(next_keyframe.values.iter()) {
            let animation = PropertyAnimation {
                from: from.clone(),
                to: to.clone(),
                timing_function: prev_keyframe.timing_function.clone(),
                duration: duration_between_keyframes as f64,
            };

            if let Ok(value) = animation.calculate_value(progress_between_keyframes) {
                map.insert(value.id().to_owned(), value);
            }
        }
    }
}

impl fmt::Debug for Animation {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Animation")
            .field("name", &self.name)
            .field("started_at", &self.started_at)
            .field("duration", &self.duration)
            .field("delay", &self.delay)
            .field("iteration_state", &self.iteration_state)
            .field("state", &self.state)
            .field("direction", &self.direction)
            .field("current_direction", &self.current_direction)
            .field("cascade_style", &())
            .finish()
    }
}

/// A CSS Transition
#[derive(Clone, Debug, MallocSizeOf)]
pub struct Transition {
    /// The start time of this transition, which is the current value of the animation
    /// timeline when this transition was created plus any animation delay.
    pub start_time: f64,

    /// The delay used for this transition.
    pub delay: f64,

    /// The internal style `PropertyAnimation` for this transition.
    pub property_animation: PropertyAnimation,

    /// The state of this transition.
    pub state: AnimationState,

    /// Whether or not this transition is new and or has already been tracked
    /// by the script thread.
    pub is_new: bool,

    /// If this `Transition` has been replaced by a new one this field is
    /// used to help produce better reversed transitions.
    pub reversing_adjusted_start_value: AnimationValue,

    /// If this `Transition` has been replaced by a new one this field is
    /// used to help produce better reversed transitions.
    pub reversing_shortening_factor: f64,
}

impl Transition {
    fn update_for_possibly_reversed_transition(
        &mut self,
        replaced_transition: &Transition,
        delay: f64,
        now: f64,
    ) {
        // If we reach here, we need to calculate a reversed transition according to
        // https://drafts.csswg.org/css-transitions/#starting
        //
        //  "...if the reversing-adjusted start value of the running transition
        //  is the same as the value of the property in the after-change style (see
        //  the section on reversing of transitions for why these case exists),
        //  implementations must cancel the running transition and start
        //  a new transition..."
        if replaced_transition.reversing_adjusted_start_value != self.property_animation.to {
            return;
        }

        // "* reversing-adjusted start value is the end value of the running transition"
        let replaced_animation = &replaced_transition.property_animation;
        self.reversing_adjusted_start_value = replaced_animation.to.clone();

        // "* reversing shortening factor is the absolute value, clamped to the
        //    range [0, 1], of the sum of:
        //    1. the output of the timing function of the old transition at the
        //      time of the style change event, times the reversing shortening
        //      factor of the old transition
        //    2.  1 minus the reversing shortening factor of the old transition."
        let transition_progress = ((now - replaced_transition.start_time) /
            (replaced_transition.property_animation.duration))
            .min(1.0)
            .max(0.0);
        let timing_function_output = replaced_animation.timing_function_output(transition_progress);
        let old_reversing_shortening_factor = replaced_transition.reversing_shortening_factor;
        self.reversing_shortening_factor = ((timing_function_output *
            old_reversing_shortening_factor) +
            (1.0 - old_reversing_shortening_factor))
            .abs()
            .min(1.0)
            .max(0.0);

        // "* start time is the time of the style change event plus:
        //    1. if the matching transition delay is nonnegative, the matching
        //       transition delay, or.
        //    2. if the matching transition delay is negative, the product of the new
        //       transition’s reversing shortening factor and the matching transition delay,"
        self.start_time = if delay >= 0. {
            now + delay
        } else {
            now + (self.reversing_shortening_factor * delay)
        };

        // "* end time is the start time plus the product of the matching transition
        //    duration and the new transition’s reversing shortening factor,"
        self.property_animation.duration *= self.reversing_shortening_factor;

        // "* start value is the current value of the property in the running transition,
        //  * end value is the value of the property in the after-change style,"
        let procedure = Procedure::Interpolate {
            progress: timing_function_output,
        };
        match replaced_animation
            .from
            .animate(&replaced_animation.to, procedure)
        {
            Ok(new_start) => self.property_animation.from = new_start,
            Err(..) => {},
        }
    }

    /// Whether or not this animation has ended at the provided time. This does
    /// not take into account canceling i.e. when an animation or transition is
    /// canceled due to changes in the style.
    pub fn has_ended(&self, time: f64) -> bool {
        time >= self.start_time + (self.property_animation.duration)
    }

    /// Update the given animation at a given point of progress.
    pub fn calculate_value(&self, time: f64) -> Option<AnimationValue> {
        let progress = (time - self.start_time) / (self.property_animation.duration);
        if progress < 0.0 {
            return None;
        }

        self.property_animation
            .calculate_value(progress.min(1.0))
            .ok()
    }
}

/// Holds the animation state for a particular element.
#[derive(Debug, Default, MallocSizeOf)]
pub struct ElementAnimationSet {
    /// The animations for this element.
    pub animations: Vec<Animation>,

    /// The transitions for this element.
    pub transitions: Vec<Transition>,

    /// Whether or not this ElementAnimationSet has had animations or transitions
    /// which have been added, removed, or had their state changed.
    pub dirty: bool,
}

impl ElementAnimationSet {
    /// Cancel all animations in this `ElementAnimationSet`. This is typically called
    /// when the element has been removed from the DOM.
    pub fn cancel_all_animations(&mut self) {
        self.dirty = !self.animations.is_empty();
        for animation in self.animations.iter_mut() {
            animation.state = AnimationState::Canceled;
        }
        self.cancel_active_transitions();
    }

    fn cancel_active_transitions(&mut self) {
        for transition in self.transitions.iter_mut() {
            if transition.state != AnimationState::Finished {
                self.dirty = true;
                transition.state = AnimationState::Canceled;
            }
        }
    }

    /// Apply all active animations.
    pub fn apply_active_animations(
        &self,
        context: &SharedStyleContext,
        style: &mut Arc<ComputedValues>,
    ) {
        let now = context.current_time_for_animations;
        let mutable_style = Arc::make_mut(style);
        if let Some(map) = self.get_value_map_for_active_animations(now) {
            for value in map.values() {
                value.set_in_style_for_servo(mutable_style);
            }
        }

        if let Some(map) = self.get_value_map_for_active_transitions(now) {
            for value in map.values() {
                value.set_in_style_for_servo(mutable_style);
            }
        }
    }

    /// Clear all canceled animations and transitions from this `ElementAnimationSet`.
    pub fn clear_canceled_animations(&mut self) {
        self.animations
            .retain(|animation| animation.state != AnimationState::Canceled);
        self.transitions
            .retain(|animation| animation.state != AnimationState::Canceled);
    }

    /// Whether this `ElementAnimationSet` is empty, which means it doesn't
    /// hold any animations in any state.
    pub fn is_empty(&self) -> bool {
        self.animations.is_empty() && self.transitions.is_empty()
    }

    /// Whether or not this state needs animation ticks for its transitions
    /// or animations.
    pub fn needs_animation_ticks(&self) -> bool {
        self.animations
            .iter()
            .any(|animation| animation.state.needs_to_be_ticked()) ||
            self.transitions
                .iter()
                .any(|transition| transition.state.needs_to_be_ticked())
    }

    /// The number of running animations and transitions for this `ElementAnimationSet`.
    pub fn running_animation_and_transition_count(&self) -> usize {
        self.animations
            .iter()
            .filter(|animation| animation.state.needs_to_be_ticked())
            .count() +
            self.transitions
                .iter()
                .filter(|transition| transition.state.needs_to_be_ticked())
                .count()
    }

    /// If this `ElementAnimationSet` has any any active animations.
    pub fn has_active_animation(&self) -> bool {
        self.animations
            .iter()
            .any(|animation| animation.state != AnimationState::Canceled)
    }

    /// If this `ElementAnimationSet` has any any active transitions.
    pub fn has_active_transition(&self) -> bool {
        self.transitions
            .iter()
            .any(|transition| transition.state != AnimationState::Canceled)
    }

    /// Update our animations given a new style, canceling or starting new animations
    /// when appropriate.
    pub fn update_animations_for_new_style<E>(
        &mut self,
        element: E,
        context: &SharedStyleContext,
        new_style: &Arc<ComputedValues>,
        resolver: &mut StyleResolverForElement<E>,
    ) where
        E: TElement,
    {
        for animation in self.animations.iter_mut() {
            if animation.is_cancelled_in_new_style(new_style) {
                animation.state = AnimationState::Canceled;
            }
        }

        maybe_start_animations(element, &context, &new_style, self, resolver);
    }

    /// Update our transitions given a new style, canceling or starting new animations
    /// when appropriate.
    pub fn update_transitions_for_new_style(
        &mut self,
        might_need_transitions_update: bool,
        context: &SharedStyleContext,
        old_style: Option<&Arc<ComputedValues>>,
        after_change_style: &Arc<ComputedValues>,
    ) {
        // If this is the first style, we don't trigger any transitions and we assume
        // there were no previously triggered transitions.
        let mut before_change_style = match old_style {
            Some(old_style) => Arc::clone(old_style),
            None => return,
        };

        // If the style of this element is display:none, then cancel all active transitions.
        if after_change_style.get_box().clone_display().is_none() {
            self.cancel_active_transitions();
            return;
        }

        if !might_need_transitions_update {
            return;
        }

        // We convert old values into `before-change-style` here.
        if self.has_active_transition() || self.has_active_animation() {
            self.apply_active_animations(context, &mut before_change_style);
        }

        let transitioning_properties = start_transitions_if_applicable(
            context,
            &before_change_style,
            after_change_style,
            self,
        );

        // Cancel any non-finished transitions that have properties which no longer transition.
        for transition in self.transitions.iter_mut() {
            if transition.state == AnimationState::Finished {
                continue;
            }
            if transitioning_properties.contains(transition.property_animation.property_id()) {
                continue;
            }
            transition.state = AnimationState::Canceled;
            self.dirty = true;
        }
    }

    fn start_transition_if_applicable(
        &mut self,
        context: &SharedStyleContext,
        property_declaration_id: &PropertyDeclarationId,
        index: usize,
        old_style: &ComputedValues,
        new_style: &Arc<ComputedValues>,
    ) {
        let style = new_style.get_ui();
        let timing_function = style.transition_timing_function_mod(index);
        let duration = style.transition_duration_mod(index);
        let delay = style.transition_delay_mod(index).seconds() as f64;
        let now = context.current_time_for_animations;

        // Only start a new transition if the style actually changes between
        // the old style and the new style.
        let property_animation = match PropertyAnimation::from_property_declaration(
            property_declaration_id,
            timing_function,
            duration,
            old_style,
            new_style,
        ) {
            Some(property_animation) => property_animation,
            None => return,
        };

        // Per [1], don't trigger a new transition if the end state for that
        // transition is the same as that of a transition that's running or
        // completed. We don't take into account any canceled animations.
        // [1]: https://drafts.csswg.org/css-transitions/#starting
        if self
            .transitions
            .iter()
            .filter(|transition| transition.state != AnimationState::Canceled)
            .any(|transition| transition.property_animation.to == property_animation.to)
        {
            return;
        }

        // We are going to start a new transition, but we might have to update
        // it if we are replacing a reversed transition.
        let reversing_adjusted_start_value = property_animation.from.clone();
        let mut new_transition = Transition {
            start_time: now + delay,
            delay,
            property_animation,
            state: AnimationState::Pending,
            is_new: true,
            reversing_adjusted_start_value,
            reversing_shortening_factor: 1.0,
        };

        if let Some(old_transition) = self
            .transitions
            .iter_mut()
            .filter(|transition| transition.state == AnimationState::Running)
            .find(|transition| {
                transition.property_animation.property_id() == *property_declaration_id
            })
        {
            // We always cancel any running transitions for the same property.
            old_transition.state = AnimationState::Canceled;
            new_transition.update_for_possibly_reversed_transition(old_transition, delay, now);
        }

        self.transitions.push(new_transition);
        self.dirty = true;
    }

    /// Generate a `AnimationValueMap` for this `ElementAnimationSet`'s
    /// active transitions at the given time value.
    pub fn get_value_map_for_active_transitions(&self, now: f64) -> Option<AnimationValueMap> {
        if !self.has_active_transition() {
            return None;
        }

        let mut map =
            AnimationValueMap::with_capacity_and_hasher(self.transitions.len(), Default::default());
        for transition in &self.transitions {
            if transition.state == AnimationState::Canceled {
                continue;
            }
            let value = match transition.calculate_value(now) {
                Some(value) => value,
                None => continue,
            };
            map.insert(value.id().to_owned(), value);
        }

        Some(map)
    }

    /// Generate a `AnimationValueMap` for this `ElementAnimationSet`'s
    /// active animations at the given time value.
    pub fn get_value_map_for_active_animations(&self, now: f64) -> Option<AnimationValueMap> {
        if !self.has_active_animation() {
            return None;
        }

        let mut map = Default::default();
        for animation in &self.animations {
            animation.get_property_declaration_at_time(now, &mut map);
        }

        Some(map)
    }
}

#[derive(Clone, Debug, Eq, Hash, MallocSizeOf, PartialEq)]
/// A key that is used to identify nodes in the `DocumentAnimationSet`.
pub struct AnimationSetKey {
    /// The node for this `AnimationSetKey`.
    pub node: OpaqueNode,
    /// The pseudo element for this `AnimationSetKey`. If `None` this key will
    /// refer to the main content for its node.
    pub pseudo_element: Option<PseudoElement>,
}

impl AnimationSetKey {
    /// Create a new key given a node and optional pseudo element.
    pub fn new(node: OpaqueNode, pseudo_element: Option<PseudoElement>) -> Self {
        AnimationSetKey {
            node,
            pseudo_element,
        }
    }

    /// Create a new key for the main content of this node.
    pub fn new_for_non_pseudo(node: OpaqueNode) -> Self {
        AnimationSetKey {
            node,
            pseudo_element: None,
        }
    }

    /// Create a new key for given node and pseudo element.
    pub fn new_for_pseudo(node: OpaqueNode, pseudo_element: PseudoElement) -> Self {
        AnimationSetKey {
            node,
            pseudo_element: Some(pseudo_element),
        }
    }
}

#[derive(Clone, Debug, Default, MallocSizeOf)]
/// A set of animations for a document.
pub struct DocumentAnimationSet {
    /// The `ElementAnimationSet`s that this set contains.
    #[ignore_malloc_size_of = "Arc is hard"]
    pub sets: Arc<RwLock<FxHashMap<AnimationSetKey, ElementAnimationSet>>>,
}

impl DocumentAnimationSet {
    /// Return whether or not the provided node has active CSS animations.
    pub fn has_active_animations(&self, key: &AnimationSetKey) -> bool {
        self.sets
            .read()
            .get(key)
            .map_or(false, |set| set.has_active_animation())
    }

    /// Return whether or not the provided node has active CSS transitions.
    pub fn has_active_transitions(&self, key: &AnimationSetKey) -> bool {
        self.sets
            .read()
            .get(key)
            .map_or(false, |set| set.has_active_transition())
    }

    /// Return a locked PropertyDeclarationBlock with animation values for the given
    /// key and time.
    pub fn get_animation_declarations(
        &self,
        key: &AnimationSetKey,
        time: f64,
        shared_lock: &SharedRwLock,
    ) -> Option<Arc<Locked<PropertyDeclarationBlock>>> {
        self.sets
            .read()
            .get(key)
            .and_then(|set| set.get_value_map_for_active_animations(time))
            .map(|map| {
                let block = PropertyDeclarationBlock::from_animation_value_map(&map);
                Arc::new(shared_lock.wrap(block))
            })
    }

    /// Return a locked PropertyDeclarationBlock with transition values for the given
    /// key and time.
    pub fn get_transition_declarations(
        &self,
        key: &AnimationSetKey,
        time: f64,
        shared_lock: &SharedRwLock,
    ) -> Option<Arc<Locked<PropertyDeclarationBlock>>> {
        self.sets
            .read()
            .get(key)
            .and_then(|set| set.get_value_map_for_active_transitions(time))
            .map(|map| {
                let block = PropertyDeclarationBlock::from_animation_value_map(&map);
                Arc::new(shared_lock.wrap(block))
            })
    }

    /// Get all the animation declarations for the given key, returning an empty
    /// `AnimationDeclarations` if there are no animations.
    pub fn get_all_declarations(
        &self,
        key: &AnimationSetKey,
        time: f64,
        shared_lock: &SharedRwLock,
    ) -> AnimationDeclarations {
        let sets = self.sets.read();
        let set = match sets.get(key) {
            Some(set) => set,
            None => return Default::default(),
        };

        let animations = set.get_value_map_for_active_animations(time).map(|map| {
            let block = PropertyDeclarationBlock::from_animation_value_map(&map);
            Arc::new(shared_lock.wrap(block))
        });
        let transitions = set.get_value_map_for_active_transitions(time).map(|map| {
            let block = PropertyDeclarationBlock::from_animation_value_map(&map);
            Arc::new(shared_lock.wrap(block))
        });
        AnimationDeclarations {
            animations,
            transitions,
        }
    }

    /// Cancel all animations for set at the given key.
    pub fn cancel_all_animations_for_key(&self, key: &AnimationSetKey) {
        if let Some(set) = self.sets.write().get_mut(key) {
            set.cancel_all_animations();
        }
    }
}

/// Kick off any new transitions for this node and return all of the properties that are
/// transitioning. This is at the end of calculating style for a single node.
pub fn start_transitions_if_applicable(
    context: &SharedStyleContext,
    old_style: &ComputedValues,
    new_style: &Arc<ComputedValues>,
    animation_state: &mut ElementAnimationSet,
) -> PropertyDeclarationIdSet {
    let mut properties_that_transition = PropertyDeclarationIdSet::default();
    for transition in new_style.transition_properties() {
        let physical_property = PropertyDeclarationId::Longhand(
            transition.longhand_id.to_physical(new_style.writing_mode),
        );
        if properties_that_transition.contains(physical_property) {
            continue;
        }

        properties_that_transition.insert(physical_property);
        animation_state.start_transition_if_applicable(
            context,
            &physical_property,
            transition.index,
            old_style,
            new_style,
        );
    }

    properties_that_transition
}

/// Triggers animations for a given node looking at the animation property
/// values.
pub fn maybe_start_animations<E>(
    element: E,
    context: &SharedStyleContext,
    new_style: &Arc<ComputedValues>,
    animation_state: &mut ElementAnimationSet,
    resolver: &mut StyleResolverForElement<E>,
) where
    E: TElement,
{
    let style = new_style.get_ui();
    for (i, name) in style.animation_name_iter().enumerate() {
        let name = match name.as_atom() {
            Some(atom) => atom,
            None => continue,
        };

        debug!("maybe_start_animations: name={}", name);
        let duration = style.animation_duration_mod(i).seconds() as f64;
        if duration == 0. {
            continue;
        }

        let keyframe_animation = match context.stylist.get_animation(name, element) {
            Some(animation) => animation,
            None => continue,
        };

        debug!("maybe_start_animations: animation {} found", name);

        // If this animation doesn't have any keyframe, we can just continue
        // without submitting it to the compositor, since both the first and
        // the second keyframes would be synthetised from the computed
        // values.
        if keyframe_animation.steps.is_empty() {
            continue;
        }

        // NB: This delay may be negative, meaning that the animation may be created
        // in a state where we have advanced one or more iterations or even that the
        // animation begins in a finished state.
        let delay = style.animation_delay_mod(i).seconds();

        let iteration_count = style.animation_iteration_count_mod(i);
        let iteration_state = if iteration_count.0.is_infinite() {
            KeyframesIterationState::Infinite(0.0)
        } else {
            KeyframesIterationState::Finite(0.0, iteration_count.0 as f64)
        };

        let animation_direction = style.animation_direction_mod(i);

        let initial_direction = match animation_direction {
            AnimationDirection::Normal | AnimationDirection::Alternate => {
                AnimationDirection::Normal
            },
            AnimationDirection::Reverse | AnimationDirection::AlternateReverse => {
                AnimationDirection::Reverse
            },
        };

        let now = context.current_time_for_animations;
        let started_at = now + delay as f64;
        let mut starting_progress = (now - started_at) / duration;
        let state = match style.animation_play_state_mod(i) {
            AnimationPlayState::Paused => AnimationState::Paused(starting_progress),
            AnimationPlayState::Running => AnimationState::Pending,
        };

        let computed_steps = ComputedKeyframe::generate_for_keyframes(
            element,
            &keyframe_animation,
            context,
            new_style,
            style.animation_timing_function_mod(i),
            resolver,
        );

        let mut new_animation = Animation {
            name: name.clone(),
            properties_changed: keyframe_animation.properties_changed.clone(),
            computed_steps,
            started_at,
            duration,
            fill_mode: style.animation_fill_mode_mod(i),
            delay: delay as f64,
            iteration_state,
            state,
            direction: animation_direction,
            current_direction: initial_direction,
            cascade_style: new_style.clone(),
            is_new: true,
        };

        // If we started with a negative delay, make sure we iterate the animation if
        // the delay moves us past the first iteration.
        while starting_progress > 1. && !new_animation.on_last_iteration() {
            new_animation.iterate();
            starting_progress -= 1.;
        }

        animation_state.dirty = true;

        // If the animation was already present in the list for the node, just update its state.
        for existing_animation in animation_state.animations.iter_mut() {
            if existing_animation.state == AnimationState::Canceled {
                continue;
            }

            if new_animation.name == existing_animation.name {
                existing_animation
                    .update_from_other(&new_animation, context.current_time_for_animations);
                return;
            }
        }

        animation_state.animations.push(new_animation);
    }
}