Add the ability to lose focus

Fixes #1308

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

@@ -90,6 +90,7 @@ class InputState extends ScrollableState<Input> {
   }
 
   void _handleTextSubmitted() {
+    Focus.clear(context);
     if (config.onSubmitted != null)
       config.onSubmitted(_value);
   }

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

@@ -136,6 +136,7 @@ class EditableString implements KeyboardClient {
   }
 
   void submit(SubmitAction action) {
+    composing = const TextRange.empty();
     onSubmitted();
   }
 }

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

@@ -25,7 +25,7 @@ class _FocusScope extends InheritedWidget {
     Widget child
   }) : super(key: key, child: child);
 
-  final FocusState focusState;
+  final _FocusState focusState;
   final bool scopeFocused;
 
   // These are mutable because we implicitly change them when they're null in
@@ -144,6 +144,12 @@ class Focus extends StatefulComponent {
     }
   }
 
+  static void clear(BuildContext context) {
+    _FocusScope focusScope = context.ancestorWidgetOfExactType(_FocusScope);
+    if (focusScope != null)
+      focusScope.focusState._clearFocusedWidget();
+  }
+
   /// Focuses a particular focus scope, identified by its GlobalKey. The widget
   /// must be in the widget tree.
   ///
@@ -157,10 +163,10 @@ class Focus extends StatefulComponent {
       focusScope.focusState._setFocusedScope(key);
   }
 
-  FocusState createState() => new FocusState();
+  _FocusState createState() => new _FocusState();
 }
 
-class FocusState extends State<Focus> {
+class _FocusState extends State<Focus> {
   GlobalKey _focusedWidget; // when null, the first component to ask if it's focused will get the focus
   GlobalKey _currentlyRegisteredWidgetRemovalListenerKey;
 
@@ -181,13 +187,20 @@ class FocusState extends State<Focus> {
     }
   }
 
-  void _handleWidgetRemoved(GlobalKey key) {
-    assert(_focusedWidget == key);
+  void _clearFocusedWidget() {
+    if (_focusedWidget != null) {
       _updateWidgetRemovalListener(null);
       setState(() {
         _focusedWidget = null;
       });
     }
+  }
+
+  void _handleWidgetRemoved(GlobalKey key) {
+    assert(key != null);
+    assert(_focusedWidget == key);
+    _clearFocusedWidget();
+  }
 
   void _updateWidgetRemovalListener(GlobalKey key) {
     if (_currentlyRegisteredWidgetRemovalListenerKey != key) {

packages/flutter/test/widget/focus_test.dart

@@ -7,11 +7,19 @@ import 'package:flutter/widgets.dart';
 import 'package:test/test.dart';
 
 class TestFocusable extends StatelessComponent {
-  TestFocusable(this.no, this.yes, GlobalKey key) : super(key: key);
+  TestFocusable({
+    GlobalKey key,
+    this.no,
+    this.yes,
+    this.autofocus: true
+  }) : super(key: key);
+
   final String no;
   final String yes;
+  final bool autofocus;
+
   Widget build(BuildContext context) {
-    bool focused = Focus.at(context, autofocus: true);
+    bool focused = Focus.at(context, autofocus: autofocus);
     return new GestureDetector(
       onTap: () { Focus.moveTo(key); },
       child: new Text(focused ? yes : no)
@@ -29,8 +37,16 @@ void main() {
           child: new Column(
             children: <Widget>[
               // reverse these when you fix https://github.com/flutter/engine/issues/1495
-              new TestFocusable('b', 'B FOCUSED', keyB),
-              new TestFocusable('a', 'A FOCUSED', keyA),
+              new TestFocusable(
+                key: keyB,
+                no: 'b',
+                yes: 'B FOCUSED'
+              ),
+              new TestFocusable(
+                key: keyA,
+                no: 'a',
+                yes: 'A FOCUSED'
+              ),
             ]
           )
         )
@@ -65,4 +81,35 @@ void main() {
       expect(tester.findText('B FOCUSED'), isNull);
     });
   });
+
+  test('Can blur', () {
+    testWidgets((WidgetTester tester) {
+      GlobalKey keyA = new GlobalKey();
+      tester.pumpWidget(
+        new Focus(
+          child: new TestFocusable(
+            key: keyA,
+            no: 'a',
+            yes: 'A FOCUSED',
+            autofocus: false
+          )
+        )
+      );
+
+      expect(tester.findText('a'),         isNotNull);
+      expect(tester.findText('A FOCUSED'), isNull);
+
+      Focus.moveTo(keyA);
+      tester.pump();
+
+      expect(tester.findText('a'),         isNull);
+      expect(tester.findText('A FOCUSED'), isNotNull);
+
+      Focus.clear(keyA.currentContext);
+      tester.pump();
+
+      expect(tester.findText('a'),         isNotNull);
+      expect(tester.findText('A FOCUSED'), isNull);
+    });
+  });
 }