Android滑动回弹效果

原理：

addHeaderView里做的事：

1.测量出header的宽高，调用了measureView方法
2.设置LayoutParams，宽：MATCH_PARENT，高：10
3.设置topMargin的值为负的header的高度,即将header隐藏在屏幕最上方


onInterceptTouchEvent：

如果滑动距离为零，让onInterceptTouchEvent处理。屏幕没有滑动效果。

onTouchEvent：
如果滑动距离大于零，让onTouchEvent处理。在滑动过程中不断的计算header的topMargin，让屏幕有滑动效果。

代码：

/**  * android回弹效果 by haidaye  */ public class HaidayeView extends LinearLayout {
    private int mLastMotionY;
    /**  * 头部view  */  private View mHeaderView;
    private LayoutInflater mInflater;
    private ScrollView mScrollView;
    /**  * 头部view的高  */  private int mHeaderViewHeight;

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

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

    private void init() {
        // Load all of the animations we need in code rather than through XML
        mInflater = LayoutInflater.from(getContext());
        // header view 在此添加,保证是第一个添加到linearlayout的最上端
        addHeaderView();
    }

    private void addHeaderView() {
        // header view
        mHeaderView = mInflater.inflate(R.layout.header, this, false);
        measureView(mHeaderView);
        mHeaderViewHeight = mHeaderView.getMeasuredHeight();
        LayoutParams params = new LayoutParams(LayoutParams.MATCH_PARENT,
                mHeaderViewHeight);
        // 设置topMargin的值为负的header View高度,即将其隐藏在最上方
        params.topMargin = -(mHeaderViewHeight);
        addView(mHeaderView, params);

    }

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        initContentAdapterView();
    }

    private void initContentAdapterView() {
        mScrollView = (ScrollView) getChildAt(1);
    }

    private void measureView(View child) {
        ViewGroup.LayoutParams p = child.getLayoutParams();
        if (p == null) {
            p = new ViewGroup.LayoutParams(ViewGroup.LayoutParams.FILL_PARENT,
                    ViewGroup.LayoutParams.WRAP_CONTENT);
        }

        int childWidthSpec = ViewGroup.getChildMeasureSpec(0, 0 + 0, p.width);
        int lpHeight = p.height;
        int childHeightSpec;
        if (lpHeight > 0) {
            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight,
                    MeasureSpec.EXACTLY);
        } else {
            childHeightSpec = MeasureSpec.makeMeasureSpec(0,
                    MeasureSpec.UNSPECIFIED);
        }
        child.measure(childWidthSpec, childHeightSpec);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent e) {
        int y = (int) e.getRawY();
        switch (e.getAction()) {
            case MotionEvent.ACTION_DOWN:
                // 首先拦截down事件,记录y坐标
                mLastMotionY = y;
                break;
            case MotionEvent.ACTION_MOVE:
                // deltaY > 0 是向下运动,< 0是向上运动
                int deltaY = y - mLastMotionY;
                if (isRefreshViewScroll(deltaY)) {
                    return true;
                }
                mLastMotionY = y;
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                break;
        }
        return false;
    }

    /*
     * 如果在onInterceptTouchEvent()方法中没有拦截(即onInterceptTouchEvent()方法中 return
     * false)则由PullToRefreshView 的子View来处理;否则由下面的方法来处理(即由HaidayeView自己来处理)
     */
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int y = (int) event.getRawY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                // onInterceptTouchEvent已经记录
                // mLastMotionY = y;
                break;
            case MotionEvent.ACTION_MOVE:
                int deltaY = y - mLastMotionY;
                headerPrepareToRefresh(deltaY);
                mLastMotionY = y;
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                int topMargin = getHeaderTopMargin();
                setHeaderTopMargin(-mHeaderViewHeight);
                break;
            case MotionEvent.ACTION_POINTER_DOWN://设置多点触控
                this.mLastMotionY = y;
                break;
        }
        return super.onTouchEvent(event);
    }

    /**  * 是否应该到了父View,即PullToRefreshView滑动  *  * @param deltaY , deltaY > 0 是向下运动,< 0是向上运动  * @return  */  private boolean isRefreshViewScroll(int deltaY) {
        // 对于ScrollView
        if (mScrollView != null) {
            // 子scroll view滑动到最顶端
            View child = mScrollView.getChildAt(0);
            if (deltaY > 0 && mScrollView.getScrollY() == 0) {
                return true;
            } else if (deltaY < 0
                    && child.getMeasuredHeight() <= getHeight()
                    + mScrollView.getScrollY()) {
                return true;
            }
        }
        return false;
    }

    /**  * header 准备刷新,手指移动过程,还没有释放  *  * @param deltaY ,手指滑动的距离  */  private void headerPrepareToRefresh(int deltaY) {
        int newTopMargin = changingHeaderViewTopMargin(deltaY);
    }

    private int changingHeaderViewTopMargin(int deltaY) {
        LayoutParams params = (LayoutParams) mHeaderView.getLayoutParams();
        float newTopMargin = params.topMargin + deltaY * 0.3f;
        params.topMargin = (int) newTopMargin;
        mHeaderView.setLayoutParams(params);
        invalidate();
        return params.topMargin;
    }

    /**  * 设置header view 的topMargin的值  */  private void setHeaderTopMargin(int topMargin) {
        LayoutParams params = (LayoutParams) mHeaderView.getLayoutParams();
        params.topMargin = topMargin;
        mHeaderView.setLayoutParams(params);
        invalidate();
    }

    private int getHeaderTopMargin() {
        LayoutParams params = (LayoutParams) mHeaderView.getLayoutParams();
        return params.topMargin;
    }
}

致敬海大爷