Factor out GlobalKeyWatcher

This also moves the logic that tracks what Widget is being watched into
GlobalKeyWatcher, and much simplifies Mimic based on this.

packages/flutter/lib/src/widgets/global_key_watcher.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:sky/src/widgets/framework.dart';
+
+abstract class GlobalKeyWatcher extends StatefulComponent {
+  GlobalKeyWatcher({
+    Key key,
+    this.watchedKey
+  });
+
+  GlobalKey watchedKey;
+
+  void syncConstructorArguments(GlobalKeyWatcher source) {
+    if (source != source.watchedKey) {
+      _removeListeners();
+      watchedKey = source.watchedKey;
+      if (mounted)
+        _setWatchedWidget(GlobalKey.getWidget(watchedKey));
+      _addListeners();
+    }
+  }
+
+  Widget get watchedWidget => _watchedWidget;
+  Widget _watchedWidget;
+  void _setWatchedWidget(Widget value) {
+    assert(mounted || value == null);
+    if (watchedWidget != value) {
+      if (watchedWidget != null)
+        stopWatching();
+      assert(debugValidateWatchedWidget(value));
+      setState(() {
+        _watchedWidget = value;
+      });
+      if (watchedWidget != null)
+        startWatching();
+    }
+  }
+
+  bool debugValidateWatchedWidget(Widget candidate) => true;
+
+  void didMount() {
+    super.didMount();
+    _setWatchedWidget(GlobalKey.getWidget(watchedKey));
+    _addListeners();
+  }
+
+  void didUnmount() {
+    super.didUnmount();
+    _removeListeners();
+    _setWatchedWidget(null);
+  }
+
+  void _addListeners() {
+    GlobalKey.registerSyncListener(watchedKey, didSyncWatchedKey);
+    GlobalKey.registerRemoveListener(watchedKey, didRemoveWatchedKey);
+  }
+
+  void _removeListeners() {
+    GlobalKey.unregisterSyncListener(watchedKey, didSyncWatchedKey);
+    GlobalKey.unregisterRemoveListener(watchedKey, didRemoveWatchedKey);
+  }
+
+  void didSyncWatchedKey(GlobalKey key, Widget widget) {
+    assert(key == watchedKey);
+    _setWatchedWidget(widget);
+  }
+
+  void didRemoveWatchedKey(GlobalKey key) {
+    assert(key == watchedKey);
+    _setWatchedWidget(null);
+  }
+
+  void stopWatching() { }
+  void startWatching() { }
+
+}

packages/flutter/lib/src/widgets/mimic.dart

@@ -4,51 +4,7 @@
 
 import 'package:sky/src/widgets/basic.dart';
 import 'package:sky/src/widgets/framework.dart';
-
-abstract class GlobalKeyWatcher extends StatefulComponent {
-  GlobalKeyWatcher({
-    Key key,
-    this.watchedKey
-  });
-
-  GlobalKey watchedKey;
-
-  void syncConstructorArguments(GlobalKeyWatcher source) {
-    if (source != source.watchedKey) {
-      _removeListeners();
-      watchedKey = source.watchedKey;
-      _addListeners();
-    }
-  }
-
-  void didMount() {
-    super.didMount();
-    _addListeners();
-  }
-
-  void didUnmount() {
-    super.didUnmount();
-    _removeListeners();
-  }
-
-  void didSyncWatchedKey(GlobalKey key, Widget widget) {
-    assert(key == watchedKey);
-  }
-
-  void didRemoveWatchedKey(GlobalKey key) {
-    assert(key == watchedKey);
-  }
-
-  void _addListeners() {
-    GlobalKey.registerSyncListener(watchedKey, didSyncWatchedKey);
-    GlobalKey.registerRemoveListener(watchedKey, didRemoveWatchedKey);
-  }
-
-  void _removeListeners() {
-    GlobalKey.unregisterSyncListener(watchedKey, didSyncWatchedKey);
-    GlobalKey.unregisterRemoveListener(watchedKey, didRemoveWatchedKey);
-  }
-}
+import 'package:sky/src/widgets/global_key_watcher.dart';
 
 typedef MimicReadyCallback();
 
@@ -66,48 +22,24 @@ class Mimic extends GlobalKeyWatcher {
     super.syncConstructorArguments(source);
   }
 
-  Mimicable _mimicable;
-
-  void didMount() {
-    super.didMount();
-    if (_mimicable == null)
-      _setMimicable(GlobalKey.getWidget(watchedKey));
+  bool debugValidateWatchedWidget(Widget candidate) {
+    return candidate is Mimicable;
   }
 
-  void didUnmount() {
-    super.didUnmount();
-    _stopMimic();
-  }
+  Mimicable get _mimicable => watchedWidget;
 
   void didSyncWatchedKey(GlobalKey key, Widget widget) {
-    super.didSyncWatchedKey(key, widget);
-    _setMimicable(widget);
-  }
-
-  void didRemoveWatchedKey(GlobalKey key) {
-    super.didRemoveWatchedKey(key);
-    _setMimicable(null);
+    super.didSyncWatchedKey(key, widget); // calls startWatching()
+    if (onMimicReady != null && _mimicable._didBuildPlaceholder)
+      onMimicReady();
   }
 
-  void _stopMimic() {
-    if (_mimicable != null) {
-      _mimicable.stopMimic();
-      _mimicable = null;
-    }
+  void startWatching() {
+    _mimicable.startMimic();
   }
 
-  void _setMimicable(widget) {
-    if (_mimicable != widget) {
-      _stopMimic();
-      if (widget != null) {
-        widget.startMimic();
-      }
-    }
-    setState(() {
-      _mimicable = widget;
-    });
-    if (onMimicReady != null && _mimicable != null && _mimicable._didBuildPlaceholder)
-      onMimicReady();
+  void stopWatching() {
+    _mimicable.stopMimic();
   }
 
   Widget build() {

packages/flutter/lib/widgets.dart

@@ -28,6 +28,7 @@ export 'package:sky/src/widgets/floating_action_button.dart';
 export 'package:sky/src/widgets/focus.dart';
 export 'package:sky/src/widgets/framework.dart';
 export 'package:sky/src/widgets/gesture_detector.dart';
+export 'package:sky/src/widgets/global_key_watcher.dart';
 export 'package:sky/src/widgets/homogeneous_viewport.dart';
 export 'package:sky/src/widgets/icon.dart';
 export 'package:sky/src/widgets/icon_button.dart';