Android(安卓)- 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 runningif (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 -> leftif (dx > 0.0f) {turnTo(EAST);} else {turnTo(WEST);}} else { // move from top -> bottom or bottom -> topif (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;}}

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