Android - 触摸屏模拟实现方向键

改写Android的Snake例子，使之运行于我的三星手机上。判断规则如下：如果x方向移动距离大于y方向，则认为是水平移动，反之则是上下移动。如果水平移动，x移动正距离x-x0>0 则认为向右移动，负距离x-x0<0 则认为向左移动；上下移动的判断同理。

代码如下，需要注意的是MotionEvent的ACTION_DOWN, ACTION_UP 是这么理解的:
ACTION_DOWN - A pressed gesture has started, the motion contains the initial starting location.

ACTION_UP - A pressed gesture has finished, the motion contains the final release location as well as any intermediate points since the last down or move event.

ACTION_MOVE - A change has happened during a press gesture (between {@link #ACTION_DOWN} and {@link #ACTION_UP}). The motion contains the most recent point, as well as any intermediate points since the last down or move event. -

简而言之，ACTION_DOWN, ACTION_UP 类似于Javascript里面键盘事件OnKeyDown, OnKeyUp 或鼠标事件OnMouseDown, OnMouseUp, 而不是说手指往上划拉或往下划拉了一下。

	/**
	 * Re write onKeyDown() for SAMSUNG
	 */
	public boolean onTouchEvent(MotionEvent event) {
		// Set the game status to running
		if (event.getAction() == MotionEvent.ACTION_DOWN) {
			if (mMode == READY | mMode == LOSE) {
				initNewGame();
				setMode(RUNNING);
				update();
				return true;
			}

			if (mMode == PAUSE) {
				setMode(RUNNING);
				update();
				return true;
			}
		}

		float x = event.getX();
		float y = event.getY();

		switch (event.getAction()) {
		case MotionEvent.ACTION_DOWN:
			mX = x;
			mY = y;
			update();
			return true;
		case MotionEvent.ACTION_UP:
			float dx = x - mX;
			float dy = y - mY;
			if (Math.abs(dx) >= TOUCH_TOLERANCE
					|| Math.abs(dy) >= TOUCH_TOLERANCE) {

				if (Math.abs(dx) >= Math.abs(dy)) { // move from left -> right
														// or right -> left
					if (dx > 0.0f) {
						turnTo(EAST);
					} else {
						turnTo(WEST);
					}
	
				} else { // move from top -> bottom or bottom -> top
					if (dy > 0.0f) {
						turnTo(SOUTH);
					} else {
						turnTo(NORTH);
					}
				}
				update();
				return true;
			}
		}

		return super.onTouchEvent(event);
	}

	private void turnTo(int direction) {
		if (direction == WEST & mDirection != EAST) {
			mNextDirection = WEST;
		}

		if (direction == EAST & mDirection != WEST) {
			mNextDirection = EAST;
		}

		if (direction == SOUTH & mDirection != NORTH) {
			mNextDirection = SOUTH;
		}

		if (direction == NORTH & mDirection != SOUTH) {
			mNextDirection = NORTH;
		}
	}