// ViewDragHelper test

VG.java

public class VG extends ViewGroup {
    private static final String TAG = "VG";
    private final ViewDragHelper mDragHelper;
    private Rect[] mBounds = new Rect[4];

    public VG(Context context) {
        super(context);
        int[] colors = {Color.RED, Color.GREEN, Color.BLUE, Color.YELLOW};
        for (int i = 0; i < 4; i++) {
            TextView tv = new TextView(getContext());
            tv.setTextSize(32);
            tv.setGravity(Gravity.CENTER);
            tv.setText("#" + i);
            tv.setBackgroundColor(colors[i]);
            tv.setClickable(true);
            addView(tv);
        }
        mDragHelper = ViewDragHelper.create(this, mCallback);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        int w2 = w / 2;
        int h2 = h / 2;
        for (int i = 0; i < 4; i++) {
            Rect r = new Rect(0, 0, w2, h2);
            r.offset(w2 * (i % 2), h2 * (i / 2));
            mBounds[i] = r;
            getChildAt(i).layout(r.left, r.top, r.right, r.bottom);
        }
    }

    ViewDragHelper.Callback mCallback = new ViewDragHelper.Callback() {
        @Override
        public boolean tryCaptureView(View view, int i) {
            return true;
        }

        @Override
        public void onViewPositionChanged(View changedView, int left, int top, int dx, int dy) {
            invalidate();
//            requestLayout();
        }

        @Override
        public int getViewHorizontalDragRange(View child) {
            return getWidth();
        }

        @Override
        public int getViewVerticalDragRange(View child) {
            return getHeight();
        }

        @Override
        public void onViewCaptured(View capturedChild, int activePointerId) {
            bringChildToFront(capturedChild);
        }

        @Override
        public void onViewReleased(View releasedChild, float xvel, float yvel) {
            Point pos = computeFinalPosition(releasedChild, xvel, yvel);
            mDragHelper.settleCapturedViewAt(pos.x, pos.y);
            Log.d(TAG, "onViewReleased settleCapturedViewAt " + pos.x + " " + pos.y);
            invalidate();
        }

        private Point computeFinalPosition(View child, float xvel, float yvel) {
            Rect r = new Rect();
            child.getHitRect(r);

            int cx = r.centerX();
            int cy = r.centerY();
            if (xvel != 0 || yvel != 0) {
                Scroller s = new Scroller(getContext());
                int w2 = r.width() / 2;
                int h2 = r.height() / 2;
                s.fling(cx, cy, (int) xvel, (int) yvel, w2, getWidth() - w2, h2, getHeight() - h2);
                cx = s.getFinalX();
                cy = s.getFinalY();
            }

            for (int i = 0; i < 4; i++) {
                Rect b = mBounds[i];
                    if (b.contains(cx, cy)) {
                        return new Point(b.left, b.top);
                    }
            }
            return new Point();
        }

        @Override
        public int clampViewPositionHorizontal(View child, int left, int dx) {
            return Math.max(Math.min(left, getWidth() - child.getWidth()), 0);
        }

        @Override
        public int clampViewPositionVertical(View child, int top, int dy) {
            return Math.max(Math.min(top, getHeight() - child.getHeight()), 0);
        }
    };

    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        int action = event.getAction();
        if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
            mDragHelper.cancel();
            return false;
        }
        return mDragHelper.shouldInterceptTouchEvent(event);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mDragHelper.processTouchEvent(event);
        return true;
    }

    @Override
    public void computeScroll() {
        boolean rc = mDragHelper.continueSettling(true);
        if (rc) {
            invalidate();
        }
    }
}