Android文档笔记:滚动手势动画

- 通过滚动器来收集由触摸事件产生滚动动画的数据

- Scroller, OverScroller(包含相关方法,可以指出手势已经到底内容的边界)

- 建议使用OverScroller:更好的兼容老设备

- 滚动器跟踪滚动位移,但实际上并不绘制内容,不会自动将位置数据作用于你的视图

- 你自己负责获取位置并按一定的频率使用新的坐标:这样可以使滚动平滑


理解滚动术语

- 滚动是移动视口的一般过程
- 若滚动中,x和y坐标都有变化,那么称为摇动
  • 拖动:
  • 划动:快速划过屏幕。当抬起手指,我们通常期望滚动继续,并且慢慢减速直到停止。覆盖GestureDetector.OnGestureListener的onFling()方法以实现
- 划动手势通常与滚动器连用
- 触摸也会用到,如翻页效果

实现基于触摸的滚动

GestureDetector,
- InteractiveChart示例:
  • 一个可以缩放、平移、滚动的图表。
  • mContentRect:表示内容区域的矩形坐标,整数,Rect类型
  • mCurrentViewport:表示图标当前在屏幕上可见的区域,浮点数据,RectF类型
  • 第一部分代码是onFling()的实现:
// The current viewport. This rectangle represents the currently visible 
// chart domain and range. The viewport is the part of the app that the
// user manipulates via touch gestures.
private RectF mCurrentViewport = 
        new RectF(AXIS_X_MIN, AXIS_Y_MIN, AXIS_X_MAX, AXIS_Y_MAX);

// The current destination rectangle (in pixel coordinates) into which the 
// chart data should be drawn.
private Rect mContentRect;

private OverScroller mScroller;
private RectF mScrollerStartViewport;
...
private final GestureDetector.SimpleOnGestureListener mGestureListener
        = new GestureDetector.SimpleOnGestureListener() {
    @Override
    public boolean onDown(MotionEvent e) {
        // Initiates the decay phase of any active edge effects.
        releaseEdgeEffects();
        mScrollerStartViewport.set(mCurrentViewport);
        // Aborts any active scroll animations and invalidates.
        mScroller.forceFinished(true);
        ViewCompat.postInvalidateOnAnimation(InteractiveLineGraphView.this);
        return true;
    }
    ...
    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, 
            float velocityX, float velocityY) {
        fling((int) -velocityX, (int) -velocityY);
        return true;
    }
};

private void fling(int velocityX, int velocityY) {
    // Initiates the decay phase of any active edge effects.
    releaseEdgeEffects();
    // Flings use math in pixels (as opposed to math based on the viewport).
    Point surfaceSize = computeScrollSurfaceSize();
    mScrollerStartViewport.set(mCurrentViewport);
    int startX = (int) (surfaceSize.x * (mScrollerStartViewport.left - 
            AXIS_X_MIN) / (
            AXIS_X_MAX - AXIS_X_MIN));
    int startY = (int) (surfaceSize.y * (AXIS_Y_MAX - 
            mScrollerStartViewport.bottom) / (
            AXIS_Y_MAX - AXIS_Y_MIN));
    // Before flinging, aborts the current animation.
    mScroller.forceFinished(true);
    // Begins the animation
    mScroller.fling(
            // Current scroll position
            startX,
            startY,
            velocityX,
            velocityY,
            /*
             * Minimum and maximum scroll positions. The minimum scroll 
             * position is generally zero and the maximum scroll position 
             * is generally the content size less the screen size. So if the 
             * content width is 1000 pixels and the screen width is 200  
             * pixels, the maximum scroll offset should be 800 pixels.
             */
            0, surfaceSize.x - mContentRect.width(),
            0, surfaceSize.y - mContentRect.height(),
            // The edges of the content. This comes into play when using
            // the EdgeEffect class to draw "glow" overlays.
            mContentRect.width() / 2,
            mContentRect.height() / 2);
    // Invalidates to trigger computeScroll()
    ViewCompat.postInvalidateOnAnimation(this);
}

  • 当onFling()中调用postInvalidateOnAnimation(),即触发了computeScroll()来刷新x和y的值。
  • 多数视图将滚动器对象的x/y位置直接传给scrollTo()
  • 下面的computeScroll实现则采取了不同的方法:调用computeScrollOffset(),获取当前位置的x和y值,然后判断是否到达边界,是否展示相应的效果。
// Edge effect / overscroll tracking objects.
private EdgeEffectCompat mEdgeEffectTop;
private EdgeEffectCompat mEdgeEffectBottom;
private EdgeEffectCompat mEdgeEffectLeft;
private EdgeEffectCompat mEdgeEffectRight;

private boolean mEdgeEffectTopActive;
private boolean mEdgeEffectBottomActive;
private boolean mEdgeEffectLeftActive;
private boolean mEdgeEffectRightActive;

@Override
public void computeScroll() {
    super.computeScroll();

    boolean needsInvalidate = false;

    // The scroller isn't finished, meaning a fling or programmatic pan 
    // operation is currently active.
    if (mScroller.computeScrollOffset()) {
        Point surfaceSize = computeScrollSurfaceSize();
        int currX = mScroller.getCurrX();
        int currY = mScroller.getCurrY();

        boolean canScrollX = (mCurrentViewport.left > AXIS_X_MIN
                || mCurrentViewport.right < AXIS_X_MAX);
        boolean canScrollY = (mCurrentViewport.top > AXIS_Y_MIN
                || mCurrentViewport.bottom < AXIS_Y_MAX);

        /*          
         * If you are zoomed in and currX or currY is
         * outside of bounds and you're not already
         * showing overscroll, then render the overscroll
         * glow edge effect.
         */
        if (canScrollX
                && currX < 0
                && mEdgeEffectLeft.isFinished()
                && !mEdgeEffectLeftActive) {
            mEdgeEffectLeft.onAbsorb((int) 
                    OverScrollerCompat.getCurrVelocity(mScroller));
            mEdgeEffectLeftActive = true;
            needsInvalidate = true;
        } else if (canScrollX
                && currX > (surfaceSize.x - mContentRect.width())
                && mEdgeEffectRight.isFinished()
                && !mEdgeEffectRightActive) {
            mEdgeEffectRight.onAbsorb((int) 
                    OverScrollerCompat.getCurrVelocity(mScroller));
            mEdgeEffectRightActive = true;
            needsInvalidate = true;
        }

        if (canScrollY
                && currY < 0
                && mEdgeEffectTop.isFinished()
                && !mEdgeEffectTopActive) {
            mEdgeEffectTop.onAbsorb((int) 
                    OverScrollerCompat.getCurrVelocity(mScroller));
            mEdgeEffectTopActive = true;
            needsInvalidate = true;
        } else if (canScrollY
                && currY > (surfaceSize.y - mContentRect.height())
                && mEdgeEffectBottom.isFinished()
                && !mEdgeEffectBottomActive) {
            mEdgeEffectBottom.onAbsorb((int) 
                    OverScrollerCompat.getCurrVelocity(mScroller));
            mEdgeEffectBottomActive = true;
            needsInvalidate = true;
        }
        ...
    }
  • 这里是执行缩放的代码:
// Custom object that is functionally similar to Scroller
Zoomer mZoomer;
private PointF mZoomFocalPoint = new PointF();
...

// If a zoom is in progress (either programmatically or via double
// touch), performs the zoom.
if (mZoomer.computeZoom()) {
    float newWidth = (1f - mZoomer.getCurrZoom()) * 
            mScrollerStartViewport.width();
    float newHeight = (1f - mZoomer.getCurrZoom()) * 
            mScrollerStartViewport.height();
    float pointWithinViewportX = (mZoomFocalPoint.x - 
            mScrollerStartViewport.left)
            / mScrollerStartViewport.width();
    float pointWithinViewportY = (mZoomFocalPoint.y - 
            mScrollerStartViewport.top)
            / mScrollerStartViewport.height();
    mCurrentViewport.set(
            mZoomFocalPoint.x - newWidth * pointWithinViewportX,
            mZoomFocalPoint.y - newHeight * pointWithinViewportY,
            mZoomFocalPoint.x + newWidth * (1 - pointWithinViewportX),
            mZoomFocalPoint.y + newHeight * (1 - pointWithinViewportY));
    constrainViewport();
    needsInvalidate = true;
}
if (needsInvalidate) {
    ViewCompat.postInvalidateOnAnimation(this);
}

  • 下面是computeScrollSurfaceSize()方法,它调用上面的代码。它计算当前可滚动平面的大小:
private Point computeScrollSurfaceSize() {
    return new Point(
            (int) (mContentRect.width() * (AXIS_X_MAX - AXIS_X_MIN)
                    / mCurrentViewport.width()),
            (int) (mContentRect.height() * (AXIS_Y_MAX - AXIS_Y_MIN)
                    / mCurrentViewport.height()));
}




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