Android开发：使用ViewDragHelper实现抽屉拉伸效果

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);
        }
    }

}

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：

@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);
}

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：

<cn.xm.weidongjian.verticaldrawerlayout.DragLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <View
        android:id="@+id/dragView"
        android:layout_width="match_parent"
        android:layout_height="80dp"
        android:background="@android:color/holo_blue_light"/>

    <ListView
        android:scrollbars="none"
        android:id="@+id/contentView"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:background="@android:color/holo_green_light"/>

</cn.xm.weidongjian.verticaldrawerlayout.DragLayout>

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

ViewDraghelper 

Callback

最后，附上源代码链接