原理：

addHeaderView裡做的事：

1.測量出header的寬高，呼叫了measureView方法
2.設定LayoutParams，寬：MATCH_PARENT，高：10
3.設定topMargin的值為負的header的高度,即將header隱藏在螢幕最上方


onInterceptTouchEvent：

如果滑動距離為零，讓onInterceptTouchEvent處理。螢幕沒有滑動效果。

onTouchEvent：
如果滑動距離大於零，讓onTouchEvent處理。在滑動過程中不斷的計算header的topMargin，讓螢幕有滑動效果。

程式碼：

/**
 * android
 
回彈效果 by haidaye
 */
public class HaidayeView extends LinearLayout {
    private int mLastMotionY;
/**
     * 頭部view
     */
private View mHeaderView;
    private LayoutInflater mInflater;
    private ScrollView mScrollView;
/**
     * 頭部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 
 
在此新增,保證是第一個新增到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);
// 設定topMargin的值為負的header View高度,即將其隱藏在最上方
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:
                // 首先攔截down事件,記錄y座標
mLastMotionY = y;
                break;
            case MotionEvent.ACTION_MOVE:
                // deltaY > 0 是向下運動,< 0是向上運動
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()方法中沒有攔截(即onInterceptTouchEvent()方法中 return
     * false)則由PullToRefreshView 的子View來處理;否則由下面的方法來處理(即由HaidayeView自己來處理)
     */
@Override
    public boolean onTouchEvent(MotionEvent event) {
        int y = (int) event.getRawY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                // onInterceptTouchEvent已經記錄
// 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://設定多點觸控
this.mLastMotionY = y;
                break;
}
        return super.onTouchEvent(event);
}

    /**
     * 是否應該到了父View,即PullToRefreshView滑動
*
     * @param deltaY , deltaY > 0 是向下運動,< 0是向上運動
* @return
*/
private boolean isRefreshViewScroll(int deltaY) {
        // 對於ScrollView
if (mScrollView != null) {
            // 子scroll view滑動到最頂端
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 準備重新整理,手指移動過程,還沒有釋放
*
     * @param deltaY ,手指滑動的距離
*/
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;
}

    /**
     * 設定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;
}
}

致敬海大爺