1 files changed, 86 insertions, 0 deletions
diff --git a/devtools/client/inspector/shared/walker-event-listener.js b/devtools/client/inspector/shared/walker-event-listener.js
new file mode 100644
index 0000000000..322c3be6df
--- /dev/null
+++ b/devtools/client/inspector/shared/walker-event-listener.js
@@ -0,0 +1,86 @@
+/* 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";
+
+/**
+ * WalkerEventListener provides a mechanism to listen the walker event of the inspector
+ * while reflecting the updating of TargetCommand.
+ */
+class WalkerEventListener {
+  /**
+   * @param {Inspector} - inspector
+   * @param {Object} - listenerMap
+   *        The structure of listenerMap should be as follows.
+   *        {
+   *          walkerEventName1: eventHandler1,
+   *          walkerEventName2: eventHandler2,
+   *          ...
+   *        }
+   */
+  constructor(inspector, listenerMap) {
+    this._inspector = inspector;
+    this._listenerMap = listenerMap;
+    this._onTargetAvailable = this._onTargetAvailable.bind(this);
+    this._onTargetDestroyed = this._onTargetDestroyed.bind(this);
+
+    this._init();
+  }
+
+  /**
+   * Clean up function.
+   */
+  destroy() {
+    this._inspector.commands.targetCommand.unwatchTargets({
+      types: [this._inspector.commands.targetCommand.TYPES.FRAME],
+      onAvailable: this._onTargetAvailable,
+      onDestroyed: this._onTargetDestroyed,
+    });
+
+    const targets = this._inspector.commands.targetCommand.getAllTargets([
+      this._inspector.commands.targetCommand.TYPES.FRAME,
+    ]);
+    for (const targetFront of targets) {
+      this._onTargetDestroyed({
+        targetFront,
+      });
+    }
+
+    this._inspector = null;
+    this._listenerMap = null;
+  }
+
+  _init() {
+    this._inspector.commands.targetCommand.watchTargets({
+      types: [this._inspector.commands.targetCommand.TYPES.FRAME],
+      onAvailable: this._onTargetAvailable,
+      onDestroyed: this._onTargetDestroyed,
+    });
+  }
+
+  async _onTargetAvailable({ targetFront }) {
+    const inspectorFront = await targetFront.getFront("inspector");
+    // In case of multiple fast navigations, the front may already be destroyed,
+    // in such scenario bail out and ignore this short lived target.
+    if (inspectorFront.isDestroyed() || !this._listenerMap) {
+      return;
+    }
+    const { walker } = inspectorFront;
+    for (const [name, listener] of Object.entries(this._listenerMap)) {
+      walker.on(name, listener);
+    }
+  }
+
+  _onTargetDestroyed({ targetFront }) {
+    const inspectorFront = targetFront.getCachedFront("inspector");
+    if (inspectorFront) {
+      const { walker } = inspectorFront;
+      for (const [name, listener] of Object.entries(this._listenerMap)) {
+        walker.off(name, listener);
+      }
+    }
+  }
+}
+
+module.exports = WalkerEventListener;