Various Input and Focus fixes

Require a Key on Input.

Simplify the API for Focus.at() and Focus.moveTo().
Fixes #236.
This will require an e-mail to flutter-dev.

Make Input grab focus onTap not onPointerDown.
Fixes #189.

Complain when you use Focus.at() with two different GlobalKeys that
are both in the tree at the same time.
Fixes #181.

Add dartdocs for Focus.moveTo() and Focus.moveScopeTo().

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

@@ -27,7 +27,9 @@ class Input extends Scrollable {
     key: key,
     initialScrollOffset: 0.0,
     scrollDirection: ScrollDirection.horizontal
-  );
+  ) {
+    assert(key != null);
+  }
 
   final String initialValue;
   final KeyboardType keyboardType;
@@ -78,7 +80,7 @@ class InputState extends ScrollableState<Input> {
   Widget buildContent(BuildContext context) {
     assert(debugCheckHasMaterial(context));
     ThemeData themeData = Theme.of(context);
-    bool focused = Focus.at(context, config);
+    bool focused = Focus.at(context);
 
     if (focused && !_keyboardHandle.attached) {
       _keyboardHandle = keyboard.show(_editableValue.stub, config.keyboardType);
@@ -118,7 +120,17 @@ class InputState extends ScrollableState<Input> {
       scrollOffset: scrollOffsetVector
     ));
 
-    return new Listener(
+    return new GestureDetector(
+      behavior: HitTestBehavior.opaque,
+      onTap: () {
+        if (Focus.at(context)) {
+          assert(_keyboardHandle.attached);
+          _keyboardHandle.showByRequest();
+        } else {
+          Focus.moveTo(config.key);
+          // we'll get told to rebuild and we'll take care of the keyboard then
+        }
+      },
       child: new SizeObserver(
         onSizeChanged: _handleContainerSizeChanged,
         child: new Container(
@@ -136,18 +148,7 @@ class InputState extends ScrollableState<Input> {
             )
           )
         )
-      ),
-      behavior: HitTestBehavior.opaque,
-      onPointerDown: (_) {
-        // TODO(ianh): https://github.com/flutter/engine/issues/1530
-        if (Focus.at(context, config)) {
-          assert(_keyboardHandle.attached);
-          _keyboardHandle.showByRequest();
-        } else {
-          Focus.moveTo(context, config);
-          // we'll get told to rebuild and we'll take care of the keyboard then
-        }
-      }
+      )
     );
   }
 

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

@@ -82,51 +82,71 @@ class Focus extends StatefulComponent {
   final bool autofocus;
   final Widget child;
 
-  static bool at(BuildContext context, Widget widget, { bool autofocus: true }) {
-    assert(widget != null);
-    assert(widget.key is GlobalKey);
+  static GlobalKey debugOnlyFocusedKey;
+
+  static bool at(BuildContext context, { bool autofocus: true }) {
+    assert(context != null);
+    assert(context.widget != null);
+    assert(context.widget.key != null);
+    assert(context.widget.key is GlobalKey);
     _FocusScope focusScope = context.inheritFromWidgetOfType(_FocusScope);
     if (focusScope != null) {
       if (autofocus)
-        focusScope._setFocusedWidgetIfUnset(widget.key);
+        focusScope._setFocusedWidgetIfUnset(context.widget.key);
       return focusScope.scopeFocused &&
              focusScope.focusedScope == null &&
-             focusScope.focusedWidget == widget.key;
+             focusScope.focusedWidget == context.widget.key;
     }
+    assert(() {
+      if (debugOnlyFocusedKey?.currentContext == null)
+        debugOnlyFocusedKey = context.widget.key;
+      if (debugOnlyFocusedKey != context.widget.key) {
+        debugPrint('Tried to focus widgets with two different keys: $debugOnlyFocusedKey and ${context.widget.key}');
+        assert('If you have more than one focusable widget, then you should put them inside a Focus.' == true);
+      }
+      return true;
+    });
     return true;
   }
 
-  static bool _atScope(BuildContext context, Widget widget, { bool autofocus: true }) {
-    assert(widget != null);
+  static bool _atScope(BuildContext context, { bool autofocus: true }) {
+    assert(context != null);
+    assert(context.widget != null);
+    assert(context.widget is Focus);
     _FocusScope focusScope = context.inheritFromWidgetOfType(_FocusScope);
     if (focusScope != null) {
+      assert(context.widget.key != null);
       if (autofocus)
-        focusScope._setFocusedScopeIfUnset(widget.key);
-      assert(widget.key != null);
+        focusScope._setFocusedScopeIfUnset(context.widget.key);
       return focusScope.scopeFocused &&
-             focusScope.focusedScope == widget.key;
+             focusScope.focusedScope == context.widget.key;
     }
     return true;
   }
 
-  // Don't call moveTo() and moveScopeTo() from your build()
-  // functions, it's intended to be called from event listeners, e.g.
-  // in response to a finger tap or tab key.
-
-  static void moveTo(BuildContext context, Widget widget) {
-    assert(widget != null);
-    assert(widget.key is GlobalKey);
-    _FocusScope focusScope = context.inheritFromWidgetOfType(_FocusScope);
+  /// Focuses a particular widget, identified by its GlobalKey.
+  /// The widget must be in the widget tree.
+  /// 
+  /// Don't call moveTo() from your build() functions, it's intended to be
+  /// called from event listeners, e.g. in response to a finger tap or tab key.
+  static void moveTo(GlobalKey key) {
+    assert(key.currentContext != null);
+    _FocusScope focusScope = key.currentContext.ancestorWidgetOfType(_FocusScope);
     if (focusScope != null)
-      focusScope.focusState._setFocusedWidget(widget.key);
-  }
-
-  static void moveScopeTo(BuildContext context, Focus component) {
-    assert(component != null);
-    assert(component.key != null);
-    _FocusScope focusScope = context.inheritFromWidgetOfType(_FocusScope);
+      focusScope.focusState._setFocusedWidget(key);
+  }
+
+  /// Focuses a particular focus scope, identified by its GlobalKey. The widget
+  /// must be in the widget tree.
+  /// 
+  /// Don't call moveScopeTo() from your build() functions, it's intended to be
+  /// called from event listeners, e.g. in response to a finger tap or tab key.
+  static void moveScopeTo(GlobalKey key) {
+    assert(key.currentWidget is Focus);
+    assert(key.currentContext != null);
+    _FocusScope focusScope = key.currentContext.ancestorWidgetOfType(_FocusScope);
     if (focusScope != null)
-      focusScope.focusState._setFocusedScope(component.key);
+      focusScope.focusState._setFocusedScope(key);
   }
 
   FocusState createState() => new FocusState();
@@ -222,7 +242,7 @@ class FocusState extends State<Focus> {
   Widget build(BuildContext context) {
     return new _FocusScope(
       focusState: this,
-      scopeFocused: Focus._atScope(context, config),
+      scopeFocused: Focus._atScope(context),
       focusedScope: _focusedScope == _noFocusedScope ? null : _focusedScope,
       focusedWidget: _focusedWidget,
       child: config.child

packages/flutter/test/widget/focus_test.dart

@@ -11,9 +11,9 @@ class TestFocusable extends StatelessComponent {
   final String no;
   final String yes;
   Widget build(BuildContext context) {
-    bool focused = Focus.at(context, this);
+    bool focused = Focus.at(context);
     return new GestureDetector(
-      onTap: () { Focus.moveTo(context, this); },
+      onTap: () { Focus.moveTo(key); },
       child: new Text(focused ? yes : no)
     );
   }