准确判断listview上下滚动

boolean isTabShow = true;
float downY = 0;
// 拖动listview时，如果点击到的地方是item里的一些view，可能出现ACTION_DOWN触发不了的问题。
// 利用isActionDown，当为false时就触发了ACTION_MOVE，第一个action需要当成ACTION_DOWN处理
boolean isActionDown = false;
OnTouchListener onTouchListener = new OnTouchListener() {

        @Override
        public boolean onTouch(View v, MotionEvent event) {
            switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                isActionDown = true;
                downY = event.getY();
                Use.trace("onTouchListener", "downY:" + downY);
                break;
            case MotionEvent.ACTION_MOVE:
                if (!isActionDown) {
                    // 当为false时就触发了ACTION_MOVE，第一个action需要当成ACTION_DOWN处理
                    isActionDown = true;
                    downY = event.getY();
                    Use.trace("onTouchListener", "downY:" + downY + " no action down");
                } else {
                    float currentY = event.getY();
                    Use.trace("onTouchListener", "downY:" + downY + " currentY::"
                            + currentY + " currentY - downY:"
                            + (currentY - downY));
                    if (currentY - downY < -20 && isTabShow) {
                        // 向下，隐藏
                        isTabShow = false;
                        Use.trace("onTouchListener", "downY:" + downY + " currentY::"
                                + currentY + " currentY - downY:"
                                + (currentY - downY) + " hide");
                    } else if (currentY - downY > 20 && !isTabShow) {
                        // 向上，显示
                        isTabShow = true;
                        Use.trace("onTouchListener", "downY:" + downY + " currentY::"
                                + currentY + " currentY - downY:"
                                + (currentY - downY) + " show");
                    }

                }

                break;
            case MotionEvent.ACTION_UP:
                isActionDown = false;// isActionDown重置
                break;
            default:
                break;
            }
            return false;
        }
};