scrollTo(index) {
    if (this.state.isScrolling || this.state.total < 2) return
    let state = this.state
    let diff = (this.props.loop ? 1 : 0) + index + this.state.index
    let x = 0
    let y = 0
    if(state.dir == 'x') x = diff * state.width
    if(state.dir == 'y') y = diff * state.height

    if (Platform.OS === 'android') {
      this.refs.scrollView && this.refs.scrollView.setPage(diff)
    } else {
      this.refs.scrollView && this.refs.scrollView.scrollTo({
        y: y,
        x: x
      })
    }

    // update scroll state
    this.setState({
      isScrolling: true,
      autoplayEnd: false,
    })

    // trigger onScrollEnd manually in android
    if (Platform.OS === 'android') {
      this.setTimeout(() => {
        this.onScrollEnd({
          nativeEvent: {
            position: diff,
          }
        });
      }, 50);
    }

  }
Scroll by index
@param  {number} index offset index