[webui] Trigger TextInputAction.next/newline on tab/enter keypresses in single line text input elements.

lib/web_ui/lib/src/engine/text_editing.dart

@@ -42,6 +42,14 @@ void _setStaticStyleAttributes(html.HtmlElement domElement) {
   }
 }
 
+enum _InputAction {
+  /// Procced to next enput element.
+  next,
+
+  /// Newline has been entered in current input element.
+  newline,
+}
+
 /// The current text and selection state of a text field.
 class EditingState {
   EditingState({this.text, this.baseOffset = 0, this.extentOffset = 0});
@@ -162,6 +170,7 @@ class InputConfiguration {
 }
 
 typedef _OnChangeCallback = void Function(EditingState editingState);
+typedef _OnActionCallback = void Function(_InputAction inputAction);
 
 enum ElementType {
   /// The backing element is an `<input>`.
@@ -225,6 +234,7 @@ class TextEditingElement {
   html.HtmlElement domElement;
   EditingState _lastEditingState;
   _OnChangeCallback _onChange;
+  _OnActionCallback _onAction;
 
   final List<StreamSubscription<html.Event>> _subscriptions =
       <StreamSubscription<html.Event>>[];
@@ -268,12 +278,14 @@ class TextEditingElement {
   void enable(
     InputConfiguration inputConfig, {
     @required _OnChangeCallback onChange,
+    @required _OnActionCallback onAction,
   }) {
     assert(!_enabled);
 
     _initDomElement(inputConfig);
     _enabled = true;
     _onChange = onChange;
+    _onAction = onAction;
 
     // Chrome on Android will hide the onscreen keyboard when you tap outside
     // the text box. Instead, we want the framework to tell us to hide the
@@ -304,7 +316,8 @@ class TextEditingElement {
     // Subscribe to text and selection changes.
     _subscriptions
       ..add(html.document.onSelectionChange.listen(_handleChange))
-      ..add(domElement.onInput.listen(_handleChange));
+      ..add(domElement.onInput.listen(_handleChange))
+      ..add(domElement.onKeyDown.listen(_handleKeyDown));
   }
 
   /// Disables the element so it's no longer used for text editing.
@@ -440,6 +453,27 @@ class TextEditingElement {
     _onChange(_lastEditingState);
   }
 
+  // Map KeyboardEvent to InputAction.
+  void _handleKeyDown(html.KeyboardEvent event) {
+    // Tab and enter key can only be mapped to actions for single line inputs.
+    if (_elementType == ElementType.input) {
+      // Trigger newline action if enter key was pressed.
+      if (event.which == 13) {
+        _onAction(_InputAction.newline);
+      }
+      // Trigger next action if tab key was pressed.
+      if (event.which == 9) {
+        _onAction(_InputAction.next);
+        // The default action of the browser is to focus the next element.
+        // As the browser is not able to focus flutter widgets, it will
+        // focus some part of its UI (e.g. the location bar).
+        // We prevent this action and leave it to the corresponding widget
+        // to change the focus.
+        event.preventDefault();
+      }
+    }
+  }
+
   @visibleForTesting
   EditingState calculateEditingState() {
     assert(domElement != null);
@@ -700,6 +734,7 @@ class HybridTextEditing {
     editingElement.enable(
       InputConfiguration.fromFlutter(_configuration),
       onChange: _syncEditingStateToFlutter,
+      onAction: _sendInputActionToFlutter,
     );
   }
 
@@ -790,6 +825,30 @@ class HybridTextEditing {
   bool get doesKeyboardShiftInput =>
     browserEngine == BrowserEngine.webkit &&
     operatingSystem == OperatingSystem.iOs;
+  _InputActionToFlutter(_InputAction inputAction) {
+    switch (inputAction) {
+      case _InputAction.next:
+        return 'TextInputAction.next';
+        break;
+      case _InputAction.newline:
+        return 'TextInputAction.newline';
+        break;
+    }
+    return 'TextInputAction.unspecified';
+  }
+
+  void _sendInputActionToFlutter(_InputAction inputAction) {
+    ui.window.onPlatformMessage(
+      'flutter/textinput',
+      const JSONMethodCodec().encodeMethodCall(
+        MethodCall('TextInputClient.performAction', <dynamic>[
+          _clientId,
+          _InputActionToFlutter(inputAction),
+        ]),
+      ),
+      _emptyCallback,
+    );
+  }
 
   /// These style attributes are dynamic throughout the life time of an input
   /// element.