Android Launcher修改--左右滑動屏幕切換源碼追蹤

在Android的源代碼中，屏幕之間的跳轉是如何實現的呢？在workspace.java中開始。在這個類中，為實現屏幕切換主要重寫了以下幾個方法：onMeasure（）、onLayout（）、onInterceptTouchEvent（）、onTouchEvent（）方法，另外還是用了CustomScroller mScroller來平滑過渡各個頁面之間的切換。

首先，我們看一下onMeasure()方法：

1. protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
2. super.onMeasure(widthMeasureSpec, heightMeasureSpec);
3. final int width = MeasureSpec.getSize(widthMeasureSpec);
4. final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
5. if (widthMode != MeasureSpec.EXACTLY) {
6. throw new IllegalStateException("Workspace can only be used in EXACTLY mode.");
7. }
8. final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
9. if (heightMode != MeasureSpec.EXACTLY) {
10. throw new IllegalStateException("Workspace can only be used in EXACTLY mode.");
11. }
13. // The children are given the same width and height as the workspace
14. final int count = getChildCount();
15. for (int i = 0; i < count; i++) {
16. getChildAt(i).measure(widthMeasureSpec, heightMeasureSpec);
17. }
18. //ADW: measure wallpaper when using old rendering
19. if(!lwpSupport){
20. if (mWallpaperLoaded) {
21. mWallpaperLoaded = false;
22. mWallpaperWidth = mWallpaperDrawable.getIntrinsicWidth();
23. mWallpaperHeight = mWallpaperDrawable.getIntrinsicHeight();
24. }
26. final int wallpaperWidth = mWallpaperWidth;
27. mWallpaperOffset = wallpaperWidth > width ? (count * width - wallpaperWidth) /
28. ((count - 1) * (float) width) : 1.0f;
29. }
30. if (mFirstLayout) {
31. scrollTo(mCurrentScreen * width, 0);
32. mScroller.startScroll(0, 0, mCurrentScreen * width, 0, 0);
33. if(lwpSupport)updateWallpaperOffset(width * (getChildCount() - 1));
34. mFirstLayout = false;
35. }
36. /*int max = 3;
37. int aW = getMeasuredWidth();
38. float w = aW / max;
39. maxPreviewWidth=(int) w;
40. maxPreviewHeight=(int) (getMeasuredHeight()*(w/getMeasuredWidth()));*/
41. }

在這裡，得到屏幕的寬高，然後再枚舉其中所有的子view，設置它們的布局（使他們的高和父控件一樣），這樣每一個子view就是充滿屏幕可以滑動顯示的其中一頁。
下面��onLayout（）方法：

1. protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
2. int childLeft = 0;
3. final int count = getChildCount();
4. for (int i = 0; i < count; i++) {
5. final View child = getChildAt(i);
6. if (child.getVisibility() != View.GONE) {
7. final int childWidth = child.getMeasuredWidth();
8. child.layout(childLeft, 0, childLeft + childWidth, child.getMeasuredHeight());
9. childLeft += childWidth;
10. }
11. }
12. //ADW:updateWallpaperoffset
13. if(lwpSupport){
14. if(mWallpaperScroll)
15. updateWallpaperOffset();
16. else
17. centerWallpaperOffset();
18. }
19. }

onLayout方法中，橫向畫出每一個子view，view的高與屏幕高一致，寬度為getChildCount()-1個屏幕寬度的view。

再看一下onInterceptTouchEvent（）方法：

1. public boolean onInterceptTouchEvent(MotionEvent ev) {
2. if(mStatus==SENSE_OPEN){
3. if(ev.getAction()==MotionEvent.ACTION_DOWN){
4. findClickedPreview(ev.getX(),ev.getY());
5. }
6. return true;
7. }
9. //Wysie: If multitouch event is detected
10. if (multiTouchController.onTouchEvent(ev)) {
11. return false;
12. }
14. if (mLocked || mLauncher.isAllAppsVisible()) {
15. return true;
16. }
18. /*
19. * This method JUST determines whether we want to intercept the motion.
20. * If we return true, onTouchEvent will be called and we do the actual
21. * scrolling there.
22. */
24. /*
25. * Shortcut the most recurring case: the user is in the dragging
26. * state and he is moving his finger. We want to intercept this
27. * motion.
28. */
29. final int action = ev.getAction();
30. if ((action == MotionEvent.ACTION_MOVE) && (mTouchState != TOUCH_STATE_REST)) {
31. return true;
32. }
34. final float x = ev.getX();
35. final float y = ev.getY();
37. switch (action) {
38. case MotionEvent.ACTION_MOVE:
39. /*
40. * mIsBeingDragged == false, otherwise the shortcut would have caught it. Check
41. * whether the user has moved far enough from his original down touch.
42. */
44. /*
45. * Locally do absolute value. mLastMotionX is set to the y value
46. * of the down event.
47. */
48. final int xDiff = (int) Math.abs(x - mLastMotionX);
49. final int yDiff = (int) Math.abs(y - mLastMotionY);
51. final int touchSlop = mTouchSlop;
52. boolean xMoved = xDiff > touchSlop;
53. boolean yMoved = yDiff > touchSlop;
54. if (xMoved || yMoved) {
55. // If xDiff > yDiff means the finger path pitch is smaller than 45deg so we assume the user want to scroll X axis
56. if (xDiff > yDiff) {
57. // Scroll if the user moved far enough along the X axis
58. mTouchState = TOUCH_STATE_SCROLLING;
59. enableChildrenCache();
61. }
62. // If yDiff > xDiff means the finger path pitch is bigger than 45deg so we assume the user want to either scroll Y or Y-axis gesture
63. else if (getOpenFolder()==null)
64. {
65. // As x scrolling is left untouched (more or less untouched;)), every gesture should start by dragging in Y axis. In fact I only consider useful, swipe up and down.
66. // Guess if the first Pointer where the user click belongs to where a scrollable widget is.
67. mTouchedScrollableWidget = isWidgetAtLocationScrollable((int)mLastMotionX,(int)mLastMotionY);
68. if (!mTouchedScrollableWidget)
69. {
70. // Only y axis movement. So may be a Swipe down or up gesture
71. if ((y - mLastMotionY) > 0){
72. if(Math.abs(y-mLastMotionY)>(touchSlop*2))mTouchState = TOUCH_SWIPE_DOWN_GESTURE;
73. }else{
74. if(Math.abs(y-mLastMotionY)>(touchSlop*2))mTouchState = TOUCH_SWIPE_UP_GESTURE;
75. }
76. }
77. }
78. // Either way, cancel any pending longpress
79. if (mAllowLongPress) {
80. mAllowLongPress = false;
81. // Try canceling the long press. It could also have been scheduled
82. // by a distant descendant, so use the mAllowLongPress flag to block
83. // everything
84. final View currentScreen = getChildAt(mCurrentScreen);
85. currentScreen.cancelLongPress();
86. }
87. }
88. break;
90. case MotionEvent.ACTION_DOWN:
91. // Remember location of down touch
92. mLastMotionX = x;
93. mLastMotionY = y;
94. mAllowLongPress = true;
96. /*
97. * If being flinged and user touches the screen, initiate drag;
98. * otherwise don't. mScroller.isFinished should be false when
99. * being flinged.
100. */
101. mTouchState = mScroller.isFinished() ? TOUCH_STATE_REST : TOUCH_STATE_SCROLLING;
102. break;
104. case MotionEvent.ACTION_CANCEL:
105. case MotionEvent.ACTION_UP:
107. if (mTouchState != TOUCH_STATE_SCROLLING && mTouchState != TOUCH_SWIPE_DOWN_GESTURE && mTouchState != TOUCH_SWIPE_UP_GESTURE) {
108. final CellLayout currentScreen = (CellLayout) getChildAt(mCurrentScreen);
109. if (!currentScreen.lastDownOnOccupiedCell()) {
110. getLocationOnScreen(mTempCell);
111. // Send a tap to the wallpaper if the last down was on empty space
112. if(lwpSupport)
113. mWallpaperManager.sendWallpaperCommand(getWindowToken(),
114. "android.wallpaper.tap",
115. mTempCell[0] + (int) ev.getX(),
116. mTempCell[1] + (int) ev.getY(), 0, null);
117. }
118. }
119. // Release the drag
120. clearChildrenCache();
121. mTouchState = TOUCH_STATE_REST;
122. mAllowLongPress = false;
123. break;
124. }
126. /*
127. * The only time we want to intercept motion events is if we are in the
128. * drag mode.
129. */
130. return mTouchState != TOUCH_STATE_REST;
131. }