Android開發教程： 觸摸屏模擬實現方向鍵

改寫Android的Snake例子，使之運行於我的三星手機上。判斷規則如下：如果x方向移動距離大於y方向，則認為是水平移動，反之則是上下移動。如果水平移動，x移動正距離x-x0>0 則認為向右移動，負距離x-x0<0 則認為向左移動；上下移動的判斷同理。

代碼如下，需要注意的是MotionEvent的ACTION_DOWN, ACTION_UP 是這麼理解的:

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. -

簡而言之，ACTION_DOWN, ACTION_UP 類似於Javascript裡面鍵盤事件OnKeyDown, OnKeyUp 或鼠標事件OnMouseDown, OnMouseUp, 而不是說手指往上劃拉或往下劃拉了一下。

1. /**
2. * Re write onKeyDown() for SAMSUNG
3. */
4. public boolean onTouchEvent(MotionEvent event) {
5. // Set the game status to running
6. if (event.getAction() == MotionEvent.ACTION_DOWN) {
7. if (mMode == READY | mMode == LOSE) {
8. initNewGame();
9. setMode(RUNNING);
10. update();
11. return true;
12. }
14. if (mMode == PAUSE) {
15. setMode(RUNNING);
16. update();
17. return true;
18. }
19. }
21. float x = event.getX();
22. float y = event.getY();
24. switch (event.getAction()) {
25. case MotionEvent.ACTION_DOWN:
26. mX = x;
27. mY = y;
28. update();
29. return true;
30. case MotionEvent.ACTION_UP:
31. float dx = x - mX;
32. float dy = y - mY;
33. if (Math.abs(dx) >= TOUCH_TOLERANCE
34. || Math.abs(dy) >= TOUCH_TOLERANCE) {
36. if (Math.abs(dx) >= Math.abs(dy)) { // move from left -> right
37. // or right -> left
38. if (dx > 0.0f) {
39. turnTo(EAST);
40. } else {
41. turnTo(WEST);
42. }
44. } else { // move from top -> bottom or bottom -> top
45. if (dy > 0.0f) {
46. turnTo(SOUTH);
47. } else {
48. turnTo(NORTH);
49. }
50. }
51. update();
52. return true;
53. }
54. }
56. return super.onTouchEvent(event);
57. }
59. private void turnTo(int direction) {
60. if (direction == WEST & mDirection != EAST) {
61. mNextDirection = WEST;
62. }
64. if (direction == EAST & mDirection != WEST) {
65. mNextDirection = EAST;
66. }
68. if (direction == SOUTH & mDirection != NORTH) {
69. mNextDirection = SOUTH;
70. }
72. if (direction == NORTH & mDirection != SOUTH) {
73. mNextDirection = NORTH;
74. }
75. }