View跟手拖动

重写View的onTouch方法实现，最后一定要返回true，否则不会起作用

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getActionMasked()) {
            case MotionEvent.ACTION_DOWN:
                mDownX = event.getX();
                mDownY = event.getY();
                break;
            case MotionEvent.ACTION_MOVE:
                float xOff = event.getX() - mDownX;
                float yOff = event.getY() - mDownY;
                if (xOff != 0 && yOff != 0) {
                    int l = (int) (getLeft() + xOff);
                    int t = (int) (getTop() + yOff);
                    int r = (int) (getRight() + xOff);
                    int b = (int) (getBottom() + yOff);
                    this.layout(l, t, r, b);
                }
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                setPressed(false);
                break;
        }
        return true;
    }

自定义View的wrap_content宽度, 自定义的View设置wrap_content，需要手动给一个默认尺寸

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int defaultWidth = 100;
        int defaultHeight = 100;
        int specWidth = MeasureSpec.getSize(widthMeasureSpec);
        int specHeight = MeasureSpec.getSize(heightMeasureSpec);
        if (getLayoutParams().width == ViewGroup.LayoutParams.WRAP_CONTENT &&
                getLayoutParams().height == ViewGroup.LayoutParams.WRAP_CONTENT) {
            setMeasuredDimension(defaultWidth, defaultHeight);
        } else if (getLayoutParams().width == ViewGroup.LayoutParams.WRAP_CONTENT) {
            setMeasuredDimension(defaultWidth, specHeight);
        } else if (getLayoutParams().height == ViewGroup.LayoutParams.WRAP_CONTENT) {
            setMeasuredDimension(specWidth, specHeight);
        }
    }