自定义ViewGroup

实现ScrollView的效果，且滚动带有“黏性”，滑动距离大于控件高度（屏幕高度）的1/3时，会自动滚动到上一个（下一个）控件

public class DiyViewGroup extends ViewGroup {
    private int mScreenHeight;
    private int mLastY;
    private Scroller mScroller;
    private int mStart;
    private int mEnd;

    public DiyViewGroup(Context context) {
        super(context);
        init(context);
    }

    public DiyViewGroup(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context);
    }

    public DiyViewGroup(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init(context);
    }

    // 初始化mScreenHeight和mScroller
    private void init(Context context) {
        WindowManager wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        DisplayMetrics outMetrics = new DisplayMetrics();
        wm.getDefaultDisplay().getMetrics(outMetrics);
        mScreenHeight = outMetrics.heightPixels;
        mScroller = new Scroller(context);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int childCount = getChildCount();
        // 对子View进行布局
        for (int i = 0; i < childCount; i++) {
            View child = getChildAt(i);
            if(child.getVisibility() != GONE) {
                child.layout(l, mScreenHeight * i, r, mScreenHeight * (i + 1));
            }
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int count = getChildCount();
        // 通知子View对自身进行测量
        for (int i = 0; i <count ; i++) {
            View childView = getChildAt(i);
            // 可能子View的宽高是match_parent，所以要将父控件的宽高传进去
            measureChild(childView, widthMeasureSpec, heightMeasureSpec);
        }
        // 设置ViewGroup自己的宽高
        int width = MeasureSpec.getSize(widthMeasureSpec);
        int height = mScreenHeight * count;
        setMeasuredDimension(width, height);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int y = (int) event.getY();
        switch(event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                Log.i("qqq", "onTouchEvent: down");
                mLastY = y;
                // 记录触摸起点
                // getScrollY()从最原始位置向上滚动的距离
                mStart = getScrollY();
                break;
            case MotionEvent.ACTION_MOVE:
                Log.i("qqq", "onTouchEvent: move");
                if(!mScroller.isFinished()) {
                    // 停止动画，立即移动到终点位置
                    mScroller.abortAnimation();
                }
                int dy = mLastY - y;
                if(getScrollY() < 0) {
                    dy = 0;
                }
                if(getScrollY() + mScreenHeight > getHeight()) {
                    dy = 0;
                }
                scrollBy(0, dy);
                mLastY = y;
                break;
            case MotionEvent.ACTION_UP:
                Log.i("qqq", "onTouchEvent: up");
                // 记录触摸终点
                mEnd = getScrollY();
                int dScrollY = mEnd - mStart;
                if(dScrollY > 0) {
                    if(dScrollY < mScreenHeight / 3) {
                        // 第三个参数第四个参数分别是向左移动的距离和向上移动的距离
                        mScroller.startScroll(0, getScrollY(), 0, -dScrollY);
                    } else {
                        mScroller.startScroll(0, getScrollY(), 0, mScreenHeight - dScrollY);
                    }
                } else {
                    if(-dScrollY < mScreenHeight / 3) {
                        mScroller.startScroll(0, getScrollY(), 0, -dScrollY);
                    } else {
                        mScroller.startScroll(0, getScrollY(), 0, -(mScreenHeight - (-dScrollY)));
                    }
                }
                break;
        }
        postInvalidate();
        return true;
    }

    @Override
    public void computeScroll() {
        super.computeScroll();
        if(mScroller.computeScrollOffset()) {
            scrollTo(0, mScroller.getCurrY());
            postInvalidate();
        }
    }
}