ScrollTricks是一个开源控件，实现了两个简单功能：

1、Quick Return:向上滑动时，View也向上滑动并且消失，当向下滑动时，View马上出现。例如Google Now的搜索功能。

2、Sticky：类似的同步滚动，特定的View最多滑动到顶部并保持固定不动。例如大众点评或美团的“立即购买”功能。

    ScrollTricks的两个效果原理是两个相同的View同在一个FrameLayout布局，这里是android:id="@+id/placeholder"，android:id="@+id/sticky"两个View。监控ScrollView的滑动，根据android:id="@+id/placeholder" View的位置控制android:id="@+id/sticky"View的位置。主要是对ScrollView滚动的Y值得监听。

看一下sticky的实现：

public class StickyFragment extends Fragment implements ObservableScrollView.Callbacks {    private TextView mStickyView;    private View mPlaceholderView;    private ObservableScrollView mObservableScrollView;    public StickyFragment() {    }    @Override    public View onCreateView(LayoutInflater inflater, ViewGroup container,            Bundle savedInstanceState) {        ViewGroup rootView = (ViewGroup) inflater                .inflate(R.layout.fragment_content, container, false);        mObservableScrollView = (ObservableScrollView) rootView.findViewById(R.id.scroll_view);        mObservableScrollView.setCallbacks(this);        mStickyView = (TextView) rootView.findViewById(R.id.sticky);        mStickyView.setText(R.string.sticky_item);        mPlaceholderView = rootView.findViewById(R.id.placeholder);        mObservableScrollView.getViewTreeObserver().addOnGlobalLayoutListener(                new ViewTreeObserver.OnGlobalLayoutListener() {                    @Override                    public void onGlobalLayout() {                        onScrollChanged(mObservableScrollView.getScrollY());                    }                });        return rootView;    }    @Override    public void onScrollChanged(int scrollY) {    	    	Log.d("onScroll", "Y:"+scrollY+"|"+mPlaceholderView.getTop());        mStickyView.setTranslationY(Math.max(mPlaceholderView.getTop(), scrollY));    }    @Override    public void onDownMotionEvent() {    }    @Override    public void onUpOrCancelMotionEvent() {    }}

ObservableScrollView的实现：

public class ObservableScrollView extends ScrollView {    private Callbacks mCallbacks;    public ObservableScrollView(Context context, AttributeSet attrs) {        super(context, attrs);    }    @Override    protected void onScrollChanged(int l, int t, int oldl, int oldt) {        super.onScrollChanged(l, t, oldl, oldt);        if (mCallbacks != null) {            mCallbacks.onScrollChanged(t);        }    }    @Override    public boolean onTouchEvent(MotionEvent ev) {        if (mCallbacks != null) {            switch (ev.getActionMasked()) {                case MotionEvent.ACTION_DOWN:                    mCallbacks.onDownMotionEvent();                    break;                case MotionEvent.ACTION_UP:                case MotionEvent.ACTION_CANCEL:                    mCallbacks.onUpOrCancelMotionEvent();                    break;            }        }        return super.onTouchEvent(ev);    }    @Override    public int computeVerticalScrollRange() {        return super.computeVerticalScrollRange();    }    public void setCallbacks(Callbacks listener) {        mCallbacks = listener;    }    public static interface Callbacks {        public void onScrollChanged(int scrollY);        public void onDownMotionEvent();        public void onUpOrCancelMotionEvent();    }}

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