Switch thumb decorations don't load (#4209)

We weren't listening to the decorations to see when they loaded.

Fixes #4185

packages/flutter/lib/src/material/switch.dart

@@ -195,19 +195,43 @@ class _RenderSwitch extends RenderToggleable {
   set activeThumbDecoration(Decoration value) {
     if (value == _activeThumbDecoration)
       return;
+    _removeActiveThumbListenerIfNeeded();
     _activeThumbDecoration = value;
+    _addActiveThumbListenerIfNeeded();
     markNeedsPaint();
   }
 
+  void _addActiveThumbListenerIfNeeded() {
+    if (attached && _activeThumbDecoration != null && _activeThumbDecoration.needsListeners)
+      _activeThumbDecoration.addChangeListener(markNeedsPaint);
+  }
+
+  void _removeActiveThumbListenerIfNeeded() {
+    if (attached && _activeThumbDecoration != null && _activeThumbDecoration.needsListeners)
+      _activeThumbDecoration.removeChangeListener(markNeedsPaint);
+  }
+
   Decoration get inactiveThumbDecoration => _inactiveThumbDecoration;
   Decoration _inactiveThumbDecoration;
   set inactiveThumbDecoration(Decoration value) {
     if (value == _inactiveThumbDecoration)
       return;
+    _removeInactiveThumbListenerIfNeeded();
     _inactiveThumbDecoration = value;
+    _addInactiveThumbListenerIfNeeded();
     markNeedsPaint();
   }
 
+  void _addInactiveThumbListenerIfNeeded() {
+    if (attached && _inactiveThumbDecoration != null && _inactiveThumbDecoration.needsListeners)
+      _inactiveThumbDecoration.addChangeListener(markNeedsPaint);
+  }
+
+  void _removeInactiveThumbListenerIfNeeded() {
+    if (attached && _inactiveThumbDecoration != null && _inactiveThumbDecoration.needsListeners)
+      _inactiveThumbDecoration.removeChangeListener(markNeedsPaint);
+  }
+
   Color get activeTrackColor => _activeTrackColor;
   Color _activeTrackColor;
   set activeTrackColor(Color value) {
@@ -228,6 +252,20 @@ class _RenderSwitch extends RenderToggleable {
     markNeedsPaint();
   }
 
+  @override
+  void attach(PipelineOwner owner) {
+    super.attach(owner);
+    _addInactiveThumbListenerIfNeeded();
+    _addActiveThumbListenerIfNeeded();
+  }
+
+  @override
+  void detach() {
+    _removeActiveThumbListenerIfNeeded();
+    _removeInactiveThumbListenerIfNeeded();
+    super.detach();
+  }
+
   double get _trackInnerLength => size.width - 2.0 * kRadialReactionRadius;
 
   HorizontalDragGestureRecognizer _drag;

packages/flutter/test/material/switch_test.dart

+// Copyright 2016 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:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+class TestBoxPainter extends BoxPainter {
+  @override
+  void paint(Canvas canvas, Rect rect) { }
+}
+
+class TestDecoration extends Decoration {
+  final List<VoidCallback> listeners = <VoidCallback>[];
+
+  @override
+  bool get needsListeners => true;
+
+  @override
+  void addChangeListener(VoidCallback listener) { listeners.add(listener); }
+
+  @override
+  void removeChangeListener(VoidCallback listener) { listeners.remove(listener); }
+
+  @override
+  BoxPainter createBoxPainter() => new TestBoxPainter();
+}
+
+void main() {
+  testWidgets('Switch can toggle on tap', (WidgetTester tester) async {
+    Key switchKey = new UniqueKey();
+    bool value = false;
+
+    await tester.pumpWidget(
+      new StatefulBuilder(
+        builder: (BuildContext context, StateSetter setState) {
+          return new Material(
+            child: new Center(
+              child: new Switch(
+                key: switchKey,
+                value: value,
+                onChanged: (bool newValue) {
+                  setState(() {
+                    value = newValue;
+                  });
+                }
+              )
+            )
+          );
+        }
+      )
+    );
+
+    expect(value, isFalse);
+    await tester.tap(find.byKey(switchKey));
+    expect(value, isTrue);
+  });
+
+  testWidgets('Switch listens to decorations', (WidgetTester tester) async {
+    TestDecoration activeDecoration = new TestDecoration();
+    TestDecoration inactiveDecoration = new TestDecoration();
+
+    Widget build(TestDecoration activeDecoration, TestDecoration inactiveDecoration) {
+      return new Material(
+        child: new Center(
+          child: new Switch(
+            value: false,
+            activeThumbDecoration: activeDecoration,
+            inactiveThumbDecoration: inactiveDecoration
+          )
+        )
+      );
+    }
+
+    await tester.pumpWidget(build(activeDecoration, inactiveDecoration));
+
+    expect(activeDecoration.listeners.length, 1);
+    expect(inactiveDecoration.listeners.length, 1);
+
+    await tester.pumpWidget(build(activeDecoration, null));
+
+    expect(activeDecoration.listeners.length, 1);
+    expect(inactiveDecoration.listeners.length, 0);
+
+    await tester.pumpWidget(new Container(key: new UniqueKey()));
+
+    expect(activeDecoration.listeners.length, 0);
+    expect(inactiveDecoration.listeners.length, 0);
+  });
+}