setOffset

setOffset这个方法是Aniamted.Value和Animated.ValueXY都有的。但是更多的用在Animated.ValueXY实例，以及PanResponder和onPanResponderGrant调用中。

使用setOffset方法，可以给我们的动画值设定一个基础值（或者偏差值）。比如，如果你设置了offset为100，而Animated.Value的值为50，那么动画执行的时候使用的值是150.

  componentWillMount() {
    this._animatedValue = new Animated.ValueXY()
    this._value = {x: 0, y: 0}

    this._animatedValue.addListener((value) => this._value = value);
    this._panResponder = PanResponder.create({
      onMoveShouldSetResponderCapture: () => true, //Tell iOS that we are allowing the movement
      onMoveShouldSetPanResponderCapture: () => true, // Same here, tell iOS that we allow dragging
      onPanResponderGrant: (e, gestureState) => {
        this._animatedValue.setOffset({x: this._value.x, y: this._value.y});
        this._animatedValue.setValue({x: 0, y: 0});
      },
      onPanResponderMove: Animated.event([
        null, {dx: this._animatedValue.x, dy: this._animatedValue.y}
      ]), // Creates a function to handle the movement and set offsets
      onPanResponderRelease: () => {
        this._animatedValue.flattenOffset(); // Flatten the offset so it resets the default positioning
      }
    });
  },


// To use

<Animated.View style={[styles.box, {transform: this._animatedValue.getTranslateTransform()}]} {...this._panResponder.panHandlers} />