自定义可拖拽的LinearLayout

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参照谷歌官方Demo写的一个可拖拽的LinearLayout

参考博客

http://developer.android.com/intl/zh-cn/training/gestures/scale.html

http://stackoverflow.com/questions/9398057/android-move-a-view-on-touch-move-action-move

http://javatechig.com/android/how-to-drag-a-view-in-android

控件相对于屏幕的位置

http://blog.csdn.net/sjf0115/article/details/7306284

多点触控

http://hukai.me/android-training-course-in-chinese/input/gestures/scale.html

一.只是简单的实现拖拽可以用如下代码

public boolean onTouchEvent(MotionEvent ev) {
    // Let the ScaleGestureDetector inspect all events.
    // mScaleDetector.onTouchEvent(ev);

    final int action = MotionEventCompat.getActionMasked(ev);

    switch (action) {
        case MotionEvent.ACTION_DOWN: {             mLastTouchX = ev.getX();
             mLastTouchy= ev.getY();

            break;
        }

        case MotionEvent.ACTION_MOVE: {
            final float x = ev.getX();
            final float y = ev.getY();

            // Calculate the distance moved
            final float dx = x - mLastTouchX;
            final float dy = y - mLastTouchY;
            layout((int) (getLeft() + dx), (int) (getTop() + dy), (int) (getRight() + dx), (int) (getBottom() + dy));
            invalidate();

            break;
        }
        case MotionEvent.ACTION_UP: {
            break;
        }
        case MotionEvent.ACTION_CANCEL: {
            break;
        }
        case MotionEvent.ACTION_POINTER_UP: {
            break;
        }
    }
    return true;
}

二.bug--当该自定义容器中有可点击的子控件（例如 Button）事件会被该控件消费，该自定义容器就不能处理事件，所以要在onInterceptTouchEvent中拦截事件

做如下修改



@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
    int action = event.getAction() & MotionEvent.ACTION_MASK;
    //为了使手指按在Button等可点击的控件上任可以滑动，需要拦截滑动实践
    //并且为了使坐标准确，在此处记录按下的点
    switch (action) {
        case MotionEvent.ACTION_MOVE:
            return true;
        case MotionEvent.ACTION_DOWN:
             mLastTouchX = ev.getX();
             mLastTouchy= ev.getY();

            return false;
    }
    return false;
}

public boolean onTouchEvent(MotionEvent ev) {
    // Let the ScaleGestureDetector inspect all events.
    // mScaleDetector.onTouchEvent(ev);

    final int action = MotionEventCompat.getActionMasked(ev);

    switch (action) {
        case MotionEvent.ACTION_DOWN: {
            break;
        }

        case MotionEvent.ACTION_MOVE: {
            final float x = ev.getX();
            final float y = ev.getY();

            // Calculate the distance moved
            final float dx = x - mLastTouchX;
            final float dy = y - mLastTouchY;
            layout((int) (getLeft() + dx), (int) (getTop() + dy), (int) (getRight() + dx), (int) (getBottom() + dy));
            invalidate();

            break;
        }
        case MotionEvent.ACTION_UP: {
            break;
        }
        case MotionEvent.ACTION_CANCEL: {
            break;
        }
        case MotionEvent.ACTION_POINTER_UP: {
            break;
        }
    }
    return true;
}

三.bug---由于安卓可以处理多点触控，所以如果在拖拽的时候放上第二个手指，然后抬起第一个手指，此时app会把第二个点当做默认的点，并且把图片移到该点的位置。

所以需要在ACTION_POINTER_UP中重新选择操作点，所以程序改为

public class GoogleDragDemo extends LinearLayout {

    private float mLastTouchX;
    private float mLastTouchY;

    public GoogleDragDemo(Context context) {
        super(context);
    }

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

    // The ‘active pointer’ is the one currently moving our object.
    private int INVALID_POINTER_ID = -1000;
    private int mActivePointerId = INVALID_POINTER_ID;

    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        int action = event.getAction() & MotionEvent.ACTION_MASK;
        //为了使手指按在Button等可点击的控件上任可以滑动，需要拦截滑动实践
        //并且为了使坐标准确，在此处记录按下的点
        switch (action) {
            case MotionEvent.ACTION_MOVE:
                return true;
            case MotionEvent.ACTION_DOWN:
                final int pointerIndex = MotionEventCompat.getActionIndex(event);mActivePointerId=MotionEventCompat.getPointerId(event,pointerIndex);

                final float x = MotionEventCompat.getX(event, pointerIndex);
                final float y = MotionEventCompat.getY(event, pointerIndex);

                // Remember where we started (for dragging)
                mLastTouchX = x;
                mLastTouchY = y;
                // Save the ID of this pointer (for dragging)
                mActivePointerId = MotionEventCompat.getPointerId(event, 0);
                mActivePointerId = MotionEventCompat.getPointerId(event, 0);
                return false;
        }
        return false;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        // Let the ScaleGestureDetector inspect all events.
        // mScaleDetector.onTouchEvent(ev);

        final int action = MotionEventCompat.getActionMasked(ev);

        switch (action) {
            case MotionEvent.ACTION_DOWN: {
                break;
            }

            case MotionEvent.ACTION_MOVE: {
                // Find the index of the active pointer and fetch its position
                final int pointerIndex =
                        MotionEventCompat.findPointerIndex(ev, mActivePointerId);

                final float x = MotionEventCompat.getX(ev, pointerIndex);
                final float y = MotionEventCompat.getY(ev, pointerIndex);

                // Calculate the distance moved
                final float dx = x - mLastTouchX;
                final float dy = y - mLastTouchY;
                layout((int) (getLeft() + dx), (int) (getTop() + dy), (int) (getRight() + dx), (int) (getBottom() + dy));
                invalidate();

                // Remember this touch position for the next move event
                //使用视图坐标系（相对做坐标系），不用刷新初始点
                // mLastTouchX = x;
                //mLastTouchY = y;

                break;
            }

            case MotionEvent.ACTION_UP: {
                mActivePointerId = INVALID_POINTER_ID;
                break;
            }

            case MotionEvent.ACTION_CANCEL: {
                mActivePointerId = INVALID_POINTER_ID;
                break;
            }

            case MotionEvent.ACTION_POINTER_UP: {

                final int pointerIndex = MotionEventCompat.getActionIndex(ev);
                final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex);

                if (pointerId == mActivePointerId) {
                    // This was our active pointer going up. Choose a new
                    // active pointer and adjust accordingly.
                    final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                    mLastTouchX = MotionEventCompat.getX(ev, newPointerIndex);
                    mLastTouchY = MotionEventCompat.getY(ev, newPointerIndex);
                    mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
                }
                break;
            }
        }
        return true;
    }
}

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