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diff --git a/js/gdm/batch.js b/js/gdm/batch.js
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+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+/*
+ * Copyright 2011 Red Hat, Inc
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * In order for transformation animations to look good, they need to be
+ * incremental and have some order to them (e.g., fade out hidden items,
+ * then shrink to close the void left over). Chaining animations in this way can
+ * be error-prone and wordy using just ease() callbacks.
+ *
+ * The classes in this file help with this:
+ *
+ * - Task.  encapsulates schedulable work to be run in a specific scope.
+ *
+ * - ConsecutiveBatch.  runs a series of tasks in order and completes
+ *                      when the last in the series finishes.
+ *
+ * - ConcurrentBatch.  runs a set of tasks at the same time and completes
+ *                     when the last to finish completes.
+ *
+ * - Hold.  prevents a batch from completing the pending task until
+ *          the hold is released.
+ *
+ * The tasks associated with a batch are specified in a list at batch
+ * construction time as either task objects or plain functions.
+ * Batches are task objects, themselves, so they can be nested.
+ *
+ * These classes aren't specific to GDM, but were found to be unintuitive and so
+ * are not used elsewhere. These APIs may ultimately get dropped entirely and
+ * replaced by something else.
+ */
+/* exported ConcurrentBatch, ConsecutiveBatch */
+
+const { GObject } = imports.gi;
+const Signals = imports.misc.signals;
+
+var Task = class extends Signals.EventEmitter {
+    constructor(scope, handler) {
+        super();
+
+        if (scope)
+            this.scope = scope;
+        else
+            this.scope = this;
+
+        this.handler = handler;
+    }
+
+    run() {
+        if (this.handler)
+            return this.handler.call(this.scope);
+
+        return null;
+    }
+};
+
+var Hold = class extends Task {
+    constructor() {
+        super(null, () => this);
+
+        this._acquisitions = 1;
+    }
+
+    acquire() {
+        if (this._acquisitions <= 0)
+            throw new Error("Cannot acquire hold after it's been released");
+        this._acquisitions++;
+    }
+
+    acquireUntilAfter(hold) {
+        if (!hold.isAcquired())
+            return;
+
+        this.acquire();
+        let signalId = hold.connect('release', () => {
+            hold.disconnect(signalId);
+            this.release();
+        });
+    }
+
+    release() {
+        this._acquisitions--;
+
+        if (this._acquisitions == 0)
+            this.emit('release');
+    }
+
+    isAcquired() {
+        return this._acquisitions > 0;
+    }
+};
+
+var Batch = class extends Task {
+    constructor(scope, tasks) {
+        super();
+
+        this.tasks = [];
+
+        for (let i = 0; i < tasks.length; i++) {
+            let task;
+
+            if (tasks[i] instanceof Task)
+                task = tasks[i];
+            else if (typeof tasks[i] == 'function')
+                task = new Task(scope, tasks[i]);
+            else
+                throw new Error('Batch tasks must be functions or Task, Hold or Batch objects');
+
+            this.tasks.push(task);
+        }
+    }
+
+    process() {
+        throw new GObject.NotImplementedError(`process in ${this.constructor.name}`);
+    }
+
+    runTask() {
+        if (!(this._currentTaskIndex in this.tasks))
+            return null;
+
+        return this.tasks[this._currentTaskIndex].run();
+    }
+
+    _finish() {
+        this.hold.release();
+    }
+
+    nextTask() {
+        this._currentTaskIndex++;
+
+        // if the entire batch of tasks is finished, release
+        // the hold and notify anyone waiting on the batch
+        if (this._currentTaskIndex >= this.tasks.length) {
+            this._finish();
+            return;
+        }
+
+        this.process();
+    }
+
+    _start() {
+        // acquire a hold to get released when the entire
+        // batch of tasks is finished
+        this.hold = new Hold();
+        this._currentTaskIndex = 0;
+        this.process();
+    }
+
+    run() {
+        this._start();
+
+        // hold may be destroyed at this point
+        // if we're already done running
+        return this.hold;
+    }
+
+    cancel() {
+        this.tasks = this.tasks.splice(0, this._currentTaskIndex + 1);
+    }
+};
+
+var ConcurrentBatch = class extends Batch {
+    process() {
+        let hold = this.runTask();
+
+        if (hold)
+            this.hold.acquireUntilAfter(hold);
+
+        // Regardless of the state of the just run task,
+        // fire off the next one, so all the tasks can run
+        // concurrently.
+        this.nextTask();
+    }
+};
+
+var ConsecutiveBatch = class extends Batch {
+    process() {
+        let hold = this.runTask();
+
+        if (hold && hold.isAcquired()) {
+            // This task is inhibiting the batch. Wait on it
+            // before processing the next one.
+            let signalId = hold.connect('release', () => {
+                hold.disconnect(signalId);
+                this.nextTask();
+            });
+        } else {
+            // This task finished, process the next one
+            this.nextTask();
+        }
+    }
+};