Add CupertinoActivityIndicator (#7380)

Fixes #7372

packages/flutter/lib/cupertino.dart

@@ -7,6 +7,7 @@
 /// To use, import `package:flutter/cupertino.dart`.
 library cupertino;
 
+export 'src/cupertino/activity_indicator.dart';
 export 'src/cupertino/slider.dart';
 export 'src/cupertino/switch.dart';
 export 'src/cupertino/thumb_painter.dart';

packages/flutter/lib/src/cupertino/activity_indicator.dart

+// Copyright 2017 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 'dart:math' as math;
+
+import 'package:flutter/widgets.dart';
+
+/// An iOS-style activity indicator.
+///
+/// See also:
+///
+///  * <https://developer.apple.com/ios/human-interface-guidelines/ui-controls/progress-indicators/#activity-indicators>
+class CupertinoActivityIndicator extends StatefulWidget {
+  /// Creates an iOS-style activity indicator.
+  CupertinoActivityIndicator({
+    Key key,
+    this.animating: true,
+  }) : super(key: key) {
+    assert(animating != null);
+  }
+
+  /// Whether the activity indicator is running its animation.
+  ///
+  /// Defaults to true.
+  final bool animating;
+
+  @override
+  _CupertinoActivityIndicatorState createState() => new _CupertinoActivityIndicatorState();
+}
+
+const double _kIndicatorWidth = 20.0;
+const double _kIndicatorHeight = 20.0;
+
+class _CupertinoActivityIndicatorState extends State<CupertinoActivityIndicator> with SingleTickerProviderStateMixin {
+  AnimationController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = new AnimationController(
+      duration: const Duration(seconds: 1),
+      vsync: this,
+    );
+
+    if (config.animating)
+      _controller.repeat();
+  }
+
+  @override
+  void didUpdateConfig(CupertinoActivityIndicator oldConfig) {
+    if (config.animating != oldConfig.animating) {
+      if (config.animating)
+        _controller.repeat();
+      else
+        _controller.stop();
+    }
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new SizedBox(
+      width: _kIndicatorWidth,
+      height: _kIndicatorHeight,
+      child: new CustomPaint(
+        painter: new _CupertinoActivityIndicatorPainter(
+          position: _controller,
+        ),
+      ),
+    );
+  }
+}
+
+const double _kTwoPI = math.PI * 2.0;
+const int _kTickCount = 12;
+const int _kHalfTickCount = _kTickCount ~/ 2;
+const Color _kTickColor = const Color(0xFFE0E0E0);
+const Color _kActiveTickColor = const Color(0xFF9D9D9D);
+final RRect _kTickFundamentalRRect = new RRect.fromLTRBXY(-10.0, 1.0, -5.0, -1.0, 1.0, 1.0);
+
+class _CupertinoActivityIndicatorPainter extends CustomPainter {
+  _CupertinoActivityIndicatorPainter({
+    Animation<double> position,
+  }) : position = position, super(repaint: position);
+
+  final Animation<double> position;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    final Paint paint = new Paint();
+
+    canvas.save();
+    canvas.translate(size.width / 2.0, size.height / 2.0);
+
+    final int activeTick = (_kTickCount * position.value).floor();
+
+    for (int i = 0; i < _kTickCount; ++ i) {
+      final double t = (((i + activeTick) % _kTickCount) / _kHalfTickCount).clamp(0.0, 1.0);
+      paint.color = Color.lerp(_kActiveTickColor, _kTickColor, t);
+      canvas.drawRRect(_kTickFundamentalRRect, paint);
+      canvas.rotate(-_kTwoPI / _kTickCount);
+    }
+
+    canvas.restore();
+  }
+
+  @override
+  bool shouldRepaint(_CupertinoActivityIndicatorPainter oldPainter) {
+    return oldPainter.position != position;
+  }
+}

packages/flutter/test/cupertino/activity_indicator.dart

+// Copyright 2017 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/cupertino.dart';
+import 'package:flutter/scheduler.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Activity indicator animate property works', (WidgetTester tester) async {
+    await tester.pumpWidget(new Center(child: new CupertinoActivityIndicator()));
+    expect(SchedulerBinding.instance.transientCallbackCount, equals(1));
+
+    await tester.pumpWidget(new Center(child: new CupertinoActivityIndicator(animating: false)));
+    expect(SchedulerBinding.instance.transientCallbackCount, equals(0));
+
+    await tester.pumpWidget(new Container());
+
+    await tester.pumpWidget(new Center(child: new CupertinoActivityIndicator(animating: false)));
+    expect(SchedulerBinding.instance.transientCallbackCount, equals(0));
+
+    await tester.pumpWidget(new Center(child: new CupertinoActivityIndicator()));
+    expect(SchedulerBinding.instance.transientCallbackCount, equals(1));
+  });
+}