1 files changed, 94 insertions, 0 deletions
diff --git a/layout/generic/ScrollSnap.h b/layout/generic/ScrollSnap.h
new file mode 100644
index 0000000000..a5817543fa
--- /dev/null
+++ b/layout/generic/ScrollSnap.h
@@ -0,0 +1,94 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layout_ScrollSnap_h_
+#define mozilla_layout_ScrollSnap_h_
+
+#include <memory>
+#include "mozilla/ScrollTypes.h"
+#include "mozilla/ScrollSnapTargetId.h"
+#include "mozilla/Maybe.h"
+
+class nsIContent;
+class nsIFrame;
+struct nsPoint;
+struct nsRect;
+struct nsSize;
+
+namespace mozilla {
+
+namespace layers {
+struct ScrollSnapInfo;
+}
+
+struct ScrollSnapUtils {
+  /**
+   * GetSnapPointForDestination determines which point to snap to after
+   * scrolling. |aStartPos| gives the position before scrolling and
+   * |aDestination| gives the position after scrolling, with no snapping.
+   * Behaviour is dependent on the value of |aUnit|.
+   * |aSnapInfo| and |aScrollRange| are characteristics of the scroll frame for
+   * which snapping is being performed.
+   * If a suitable snap point could be found, it is returned. Otherwise, an
+   * empty Maybe is returned.
+   * IMPORTANT NOTE: This function is designed to be called both on and off
+   *                 the main thread. If modifying its implementation, be sure
+   *                 not to touch main-thread-only data structures without
+   *                 appropriate locking.
+   */
+  static Maybe<SnapTarget> GetSnapPointForDestination(
+      const layers::ScrollSnapInfo& aSnapInfo, ScrollUnit aUnit,
+      ScrollSnapFlags aSnapFlags, const nsRect& aScrollRange,
+      const nsPoint& aStartPos, const nsPoint& aDestination);
+
+  /**
+   * Similar to above GetSnapPointForDestination but for re-snapping.
+   *
+   * |aCurrentPosition| are the snap point(s) last time when we scrolled.
+   * |aLastSnapTargetIds| are the snap point(s) last time when we scrolled if
+   * exists.
+   * |aFocusedContent| is the focused content in the document if exists.
+   * Other parameters are same as GetSnapPointForDestination.
+   */
+
+  static mozilla::Maybe<mozilla::SnapTarget> GetSnapPointForResnap(
+      const layers::ScrollSnapInfo& aSnapInfo, const nsRect& aScrollRange,
+      const nsPoint& aCurrentPosition,
+      const UniquePtr<ScrollSnapTargetIds>& aLastSnapTargetIds,
+      const nsIContent* aFocusedContent);
+
+  static ScrollSnapTargetId GetTargetIdFor(const nsIFrame* aFrame);
+
+  // Post a pending re-snap request if the given |aFrame| is one of the snap
+  // points on the last scroll operation.
+  static void PostPendingResnapIfNeededFor(nsIFrame* aFrame);
+
+  // Similar to above PostPendingResnapIfNeededFor but post a pending re-snap
+  // request even if the given |aFrame| is not one of the last snap point.
+  // This is basically used for cases there was no valid snap point on the last
+  // scroll operation but the given |aFrame| might be a valid snap point now,
+  // e.g changing the scroll-snap-align property from `none` to something.
+  static void PostPendingResnapFor(nsIFrame* aFrame);
+
+  // Returns true if the writing-mode of the snap target element needs to be
+  // respected to resolve scroll-snap-align property.
+  // Note that usually the scroll container's writing-mode is used for resolving
+  // the property but there's a special case defined in the CSS scroll snap
+  // spec.
+  static bool NeedsToRespectTargetWritingMode(const nsSize& aSnapAreaSize,
+                                              const nsSize& aSnapportSize);
+
+  // Returns the scroll snap area for the snap target frame |aFrame| inside the
+  // nearest scroll container |aScrolledFrame| and its scrolled rect
+  // |aScrolledRect|.
+  static nsRect GetSnapAreaFor(const nsIFrame* aFrame,
+                               const nsIFrame* aScrolledFrame,
+                               const nsRect& aScrolledRect);
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_layout_ScrollSnap_h_