Unfocus textfield if it is disabled (#18097)

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

@@ -344,6 +344,11 @@ class _TextFieldState extends State<TextField> with AutomaticKeepAliveClientMixi
       _controller = new TextEditingController.fromValue(oldWidget.controller.value);
     else if (widget.controller != null && oldWidget.controller == null)
       _controller = null;
+    final bool isEnabled = widget.enabled ?? widget.decoration?.enabled ?? true;
+    final bool wasEnabled = oldWidget.enabled ?? oldWidget.decoration?.enabled ?? true;
+    if (wasEnabled && !isEnabled) {
+      _effectiveFocusNode.unfocus();
+    }
   }
 
   @override

packages/flutter/test/material/text_field_test.dart

@@ -2138,4 +2138,30 @@ void main() {
     expect(exception.toString(), endsWith(':\n  $textField\nThe ancestors of this widget were:\n  [root]'));
   });
 
+  testWidgets('TextField loses focus when disabled', (WidgetTester tester) async {
+    final FocusNode focusNode = new FocusNode();
+
+    await tester.pumpWidget(
+      boilerplate(
+        child: new TextField(
+          focusNode: focusNode,
+          autofocus: true,
+          enabled: true,
+        ),
+      ),
+    );
+    expect(focusNode.hasFocus, isTrue);
+
+    await tester.pumpWidget(
+      boilerplate(
+        child: new TextField(
+          focusNode: focusNode,
+          autofocus: true,
+          enabled: false,
+        ),
+      ),
+    );
+    expect(focusNode.hasFocus, isFalse);
+  });
+
 }