事实上，有非常多方法能够实现一个Layout的抽屉拉伸效果，最常常的方法就是自己定义一个ViewGroup，然后控制点击事件。控制移动之类的，这样的方法的代码量多，并且实现起来复杂，后期维护添加其它效果也非常麻烦，直到今天看到了 ViewDragHelper这个类，就是专门为实现View的移动而生的。我就试着开发了一个抽屉拉伸的效果，效果图例如以下：

全部移动的控制在ViewDragHelper.Callback里面来实现。移动就用dragHelper.smoothSlideViewTo来实现，并且Callback集成了很多的方法。方便后期的维护或者添加其它功能。

首先看下最核心的DragLayout的代码

public class DragLayout extends LinearLayout {    private ViewDragHelper dragHelper;    private View mDragView, contentView;    private int dragRange;    public DragLayout(Context context) {        super(context);        init();    }    public DragLayout(Context context, AttributeSet attrs) {        super(context, attrs);        init();    }    @TargetApi(Build.VERSION_CODES.LOLLIPOP)    public DragLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {        super(context, attrs, defStyleAttr, defStyleRes);        init();    }    public DragLayout(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);        init();    }    private void init() {        dragHelper = ViewDragHelper.create(this, callback);    }    @Override    protected void onFinishInflate() {        super.onFinishInflate();        mDragView = findViewById(R.id.dragView);        contentView = findViewById(R.id.contentView);    }    private ViewDragHelper.Callback callback = new ViewDragHelper.Callback() {        @Override        public boolean tryCaptureView(View child, int pointerId) {            return child == mDragView;        }        @Override        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {            contentView.layout(0, top + mDragView.getHeight(), getWidth(), top + mDragView.getHeight() + dragRange);        }        @Override        public int clampViewPositionVertical(View child, int top, int dy) {            int topBound = getHeight() - dragRange - mDragView.getHeight();            int bottomBound = getHeight() - mDragView.getHeight();            final int newHeight = Math.min(Math.max(topBound, top), bottomBound);            return newHeight;        }        @Override        public int getViewVerticalDragRange(View child) {            return dragRange;        }        @Override        public void onViewReleased(View releasedChild, float xvel, float yvel) {            super.onViewReleased(releasedChild, xvel, yvel);            if (yvel > 0) {                smoothToBottom();            }else if (yvel < 0) {                smoothToTop();            }        }    };    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        super.onMeasure(widthMeasureSpec, heightMeasureSpec);        dragRange = contentView.getMeasuredHeight();    }    @Override    protected void onLayout(boolean changed, int l, int t, int r, int b) {        super.onLayout(changed, l, t, r, b);        mDragView.layout(0, getHeight() - mDragView.getHeight(), getWidth(), getHeight());        contentView.layout(0, getHeight(), getWidth(), getHeight() + dragRange);    }    @Override    public boolean onInterceptHoverEvent(MotionEvent event) {        final int action = MotionEventCompat.getActionMasked(event);        if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {            dragHelper.cancel();            return false;        }        return dragHelper.shouldInterceptTouchEvent(event);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        dragHelper.processTouchEvent(event);        return true;    }    private void smoothToTop() {        if (dragHelper.smoothSlideViewTo(mDragView, getPaddingLeft(), getHeight() - dragRange - mDragView.getHeight())) {            ViewCompat.postInvalidateOnAnimation(this);        }    }    private void smoothToBottom() {        if (dragHelper.smoothSlideViewTo(mDragView, getPaddingLeft(), getHeight() - mDragView.getHeight())) {            ViewCompat.postInvalidateOnAnimation(this);        }    }    @Override    public void computeScroll() {        if (dragHelper.continueSettling(true)) {            ViewCompat.postInvalidateOnAnimation(this);        }    }}

在这里面，初始化了ViewDragHelper

dragHelper = ViewDragHelper.create(this, callback);

设置垂直方向的移动距离，这里设置为listview的高度：

@Override

        public int clampViewPositionVertical(View child, int top, int dy) {

            int topBound = getHeight() - dragRange - mDragView.getHeight();

            int bottomBound = getHeight() - mDragView.getHeight();

            final int newHeight = Math.min(Math.max(topBound, top), bottomBound);

            return newHeight;

        }

监听位置的移动，移动listview。让他始终挨在DrawView的以下：

@Overridepublic void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {    contentView.layout(0, top + mDragView.getHeight(), getWidth(), top + mDragView.getHeight() + dragRange);}

在OnLayout里面又一次布局。隐藏listView：

@Override    protected void onLayout(boolean changed, int l, int t, int r, int b) {        super.onLayout(changed, l, t, r, b);        mDragView.layout(0, getHeight() - mDragView.getHeight(), getWidth(), getHeight());        contentView.layout(0, getHeight(), getWidth(), getHeight() + dragRange);    }

接下来是XML的布局：

非常easy。自己定义的View里面放置两个View

最后的MainActivity：

public class MainActivity extends AppCompatActivity {    String[] listItems = {"item 1", "item 2 ", "list", "android", "item 3", "foobar", "bar", };    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        ListView listView = (ListView) findViewById(R.id.contentView);        listView.setAdapter(new ArrayAdapter(this,  android.R.layout.simple_list_item_1, listItems));    }}

最后，这个Demo仅仅是实现一个非常easy的功能，只是大概能够看到ViewDragHelper的强大。强烈建议去了解下，这个是两个API的地址

 

最后，附上