Android VelocityTracker的介绍

VelocityTracker的介绍：

简单来说就是：X/Y方向的速度相关的帮助类

/**
 * Helper for tracking the velocity of touch events, for implementing flinging and other such gestures. 
 * Use obtain to retrieve a new instance of the class when you are going to begin tracking. 
 * Put the motion events you receive into it with addMovement(MotionEvent). 
 * When you want to determine the velocity call computeCurrentVelocity(int) and then call getXVelocity(int) and getYVelocity(int) to retrieve the velocity for each pointer id.
 */

使用步骤：

• 声明

    private VelocityTracker mVelocityTracker;
  

• 初始化

    private void init() {
        mVelocityTracker = VelocityTracker.obtain();
    }
  

• 在onTouchEvent的开头添加滑动事件

    public boolean onTouchEvent(MotionEvent event) {
        mVelocityTracker.addMovement(event);
        ...
    }
  

• 在onTouchEvent的MotionEvent.ACTION_UP中使用

    case MotionEvent.ACTION_UP: {
        int scrollX = getScrollX();
        int scrollToChildIndex = scrollX / mChildWidth;
        mVelocityTracker.computeCurrentVelocity(1000);
        float xVelocity = mVelocityTracker.getXVelocity();
        if (Math.abs(xVelocity) >= 50) {
            mChildIndex = xVelocity > 0 ? mChildIndex - 1 : mChildIndex + 1;
        } else {
            mChildIndex = (scrollX + mChildWidth / 2) / mChildWidth;
        }
        mChildIndex = Math.max(0, Math.min(mChildIndex, mChildrenSize - 1));
        int dx = mChildIndex * mChildWidth - scrollX;
        smoothScrollBy(dx, 0);
        mVelocityTracker.clear();
        break;
    }
  

• 在onDetachedFromWindow中回收

    @Override
    protected void onDetachedFromWindow() {
        mVelocityTracker.recycle();
        super.onDetachedFromWindow();
    }