Add initialDelay to AnimationGenerator and add Animation class.

This patch allows for an initial delay before an animation begins and also adds an Animation class which encapsulates a value which is long-lived, can be explicitly set and also animated from its current value to another value.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/994143002

examples/fn/widgets/drawer.dart

@@ -6,39 +6,34 @@ const double _kBaseSettleDurationMS = 246.0;
 const double _kMaxSettleDurationMS = 600.0;
 const Cubic _kAnimationCurve = easeOut;
 
-class DrawerAnimation {
+class DrawerAnimation extends Animation {
 
-  Stream<double> get onPositionChanged => _controller.stream;
+  Stream<double> get onPositionChanged => onValueChanged;
 
-  StreamController _controller;
-  AnimationGenerator _animation;
-  double _position;
-  bool get _isAnimating => _animation != null;
-  bool get _isMostlyClosed => _position <= -_kWidth / 2;
+  bool get _isMostlyClosed => value <= -_kWidth / 2;
 
   DrawerAnimation() {
-    _controller = new StreamController(sync: true);
-    _setPosition(-_kWidth);
+    value = -_kWidth;
   }
 
   void toggle(_) => _isMostlyClosed ? _open() : _close();
 
   void handleMaskTap(_) => _close();
 
-  void handlePointerDown(_) => _cancelAnimation();
+  void handlePointerDown(_) => stop();
 
   void handlePointerMove(sky.PointerEvent event) {
-    assert(_animation == null);
-    _setPosition(_position + event.dx);
+    assert(!isAnimating);
+    value = math.min(0.0, math.max(value + event.dx, -_kWidth));
   }
 
   void handlePointerUp(_) {
-    if (!_isAnimating)
+    if (!isAnimating)
       _settle();
   }
 
   void handlePointerCancel(_) {
-    if (!_isAnimating)
+    if (!isAnimating)
       _settle();
   }
 
@@ -48,35 +43,12 @@ class DrawerAnimation {
 
   void _settle() => _isMostlyClosed ? _close() : _open();
 
-  void _setPosition(double value) {
-    _position = math.min(0.0, math.max(value, -_kWidth));
-    _controller.add(_position);
-  }
-
-  void _cancelAnimation() {
-    if (_animation != null) {
-      _animation.cancel();
-      _animation = null;
-    }
-  }
-
-  void _animate(double duration, double begin, double end, Curve curve) {
-    _cancelAnimation();
-
-    _animation = new AnimationGenerator(duration, begin: begin, end: end,
-        curve: curve);
-
-    _animation.onTick.listen(_setPosition, onDone: () {
-      _animation = null;
-    });
-  }
-
   void _animateToPosition(double targetPosition) {
-    double distance = (targetPosition - _position).abs();
+    double distance = (targetPosition - value).abs();
     if (distance != 0) {
       double targetDuration = distance / _kWidth * _kBaseSettleDurationMS;
       double duration = math.min(targetDuration, _kMaxSettleDurationMS);
-      _animate(duration, _position, targetPosition, _kAnimationCurve);
+      animateTo(targetPosition, duration, curve: _kAnimationCurve);
     }
   }
 
@@ -87,10 +59,10 @@ class DrawerAnimation {
       return;
 
     double targetPosition = direction < 0.0 ? -_kWidth : 0.0;
-    double distance = (targetPosition - _position).abs();
+    double distance = (targetPosition - value).abs();
     double duration = distance / velocityX;
 
-    _animate(duration, _position, targetPosition, linear);
+    animateTo(targetPosition, duration, curve: linear);
   }
 }