1 files changed, 190 insertions, 0 deletions
diff --git a/devtools/client/shared/undo.js b/devtools/client/shared/undo.js
new file mode 100644
index 0000000000..759d3a4aed
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+++ b/devtools/client/shared/undo.js
@@ -0,0 +1,190 @@
+/* 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/. */
+
+"use strict";
+
+/**
+ * A simple undo stack manager.
+ *
+ * Actions are added along with the necessary code to
+ * reverse the action.
+ *
+ * @param integer maxUndo Maximum number of undo steps.
+ *   defaults to 50.
+ */
+function UndoStack(maxUndo) {
+  this.maxUndo = maxUndo || 50;
+  this._stack = [];
+}
+
+exports.UndoStack = UndoStack;
+
+UndoStack.prototype = {
+  // Current index into the undo stack.  Is positioned after the last
+  // currently-applied change.
+  _index: 0,
+
+  // The current batch depth (see startBatch() for details)
+  _batchDepth: 0,
+
+  destroy() {
+    this.uninstallController();
+    delete this._stack;
+  },
+
+  /**
+   * Start a collection of related changes.  Changes will be batched
+   * together into one undo/redo item until endBatch() is called.
+   *
+   * Batches can be nested, in which case the outer batch will contain
+   * all items from the inner batches.  This allows larger user
+   * actions made up of a collection of smaller actions to be
+   * undone as a single action.
+   */
+  startBatch() {
+    if (this._batchDepth++ === 0) {
+      this._batch = [];
+    }
+  },
+
+  /**
+   * End a batch of related changes, performing its action and adding
+   * it to the undo stack.
+   */
+  endBatch() {
+    if (--this._batchDepth > 0) {
+      return;
+    }
+
+    // Cut off the end of the undo stack at the current index,
+    // and the beginning to prevent a stack larger than maxUndo.
+    const start = Math.max(this._index + 1 - this.maxUndo, 0);
+    this._stack = this._stack.slice(start, this._index);
+
+    const batch = this._batch;
+    delete this._batch;
+    const entry = {
+      do() {
+        for (const item of batch) {
+          item.do();
+        }
+      },
+      undo() {
+        for (let i = batch.length - 1; i >= 0; i--) {
+          batch[i].undo();
+        }
+      },
+    };
+    this._stack.push(entry);
+    this._index = this._stack.length;
+    entry.do();
+  },
+
+  /**
+   * Perform an action, adding it to the undo stack.
+   *
+   * @param function toDo Called to perform the action.
+   * @param function undo Called to reverse the action.
+   */
+  do(toDo, undo) {
+    this.startBatch();
+    this._batch.push({ do: toDo, undo });
+    this.endBatch();
+  },
+
+  /*
+   * Returns true if undo() will do anything.
+   */
+  canUndo() {
+    return this._index > 0;
+  },
+
+  /**
+   * Undo the top of the undo stack.
+   *
+   * @return true if an action was undone.
+   */
+  undo() {
+    if (!this.canUndo()) {
+      return false;
+    }
+    this._stack[--this._index].undo();
+    return true;
+  },
+
+  /**
+   * Returns true if redo() will do anything.
+   */
+  canRedo() {
+    return this._stack.length > this._index;
+  },
+
+  /**
+   * Redo the most recently undone action.
+   *
+   * @return true if an action was redone.
+   */
+  redo() {
+    if (!this.canRedo()) {
+      return false;
+    }
+    this._stack[this._index++].do();
+    return true;
+  },
+
+  /**
+   * ViewController implementation for undo/redo.
+   */
+
+  /**
+   * Install this object as a command controller.
+   */
+  installController(controllerWindow) {
+    const controllers = controllerWindow.controllers;
+    // Only available when running in a Firefox panel.
+    if (!controllers || !controllers.appendController) {
+      return;
+    }
+
+    this._controllerWindow = controllerWindow;
+    controllers.appendController(this);
+  },
+
+  /**
+   * Uninstall this object from the command controller.
+   */
+  uninstallController() {
+    if (!this._controllerWindow) {
+      return;
+    }
+    this._controllerWindow.controllers.removeController(this);
+  },
+
+  supportsCommand(command) {
+    return command == "cmd_undo" || command == "cmd_redo";
+  },
+
+  isCommandEnabled(command) {
+    switch (command) {
+      case "cmd_undo":
+        return this.canUndo();
+      case "cmd_redo":
+        return this.canRedo();
+    }
+    return false;
+  },
+
+  doCommand(command) {
+    switch (command) {
+      case "cmd_undo":
+        return this.undo();
+      case "cmd_redo":
+        return this.redo();
+      default:
+        return null;
+    }
+  },
+
+  onEvent(event) {},
+};