桌面的左右滑動功能主要是在PagedView類中實現的，而WorkSpace是PagedView類的子類，所以會繼承PagedView中的方法。當我們的手指點擊WorkSpace時，首先就會觸發PageView中的onInterceptTouchEvent()方法，會根據相應的條件來判斷是否對Touch事件進行攔截，如果onInterceptTouchEvent()方法返回爲true，則會對Touch事件進行攔截，PageView類的onTouch方法會進行響應從而得到調用。如果返回false，就分兩鍾情況：（1）我們是點擊在它的子控鍵上進行滑動時，比如我們是點擊在桌面的圖標上進行左右滑動的，workspace則會把Touch事件分發給它的子控件。（2）而如果僅僅是點擊到桌面的空白出Touch事件就不會發生響應。
在我們手指第一次觸摸到屏幕時，首先會對onInterceptTouchEvent中的事件進行判斷，如果是按下事件（MotionEvent.ACTION_DOWN）,則會記錄按下時的X座標、Y座標等等數據，同時改變現在Workspace的狀態爲滾動狀態（OUCH_STATE_SCROLLING），這時會返回ture，把事件交給onTouchEvent函數來處理，onTouchEvent中同樣會對事件類型進行判斷，當事件方法爲（otionEvent.ACTION_DOWN）的時候，就可以開始顯示滾動的指示條了（就是Hotseat上顯示第幾屏的屏點）。當我們按着屏幕不放進行滑動的時候，又會在onInterceptTouchEvent進行事件攔截，但是現在的事件類型變爲了MotionEvent.ACTION_MOVE，因爲是移動的操作，所以會在攔截的時候取消桌面長按的事件的響應，同時轉到onTouchEvent中對ACTION_MOVE事件的響應中，判斷我們移動了多少距離，使用scrollBy方法來對桌面進行移動，並刷新屏幕。最後我們放開手後會觸發onTouchEvent中的MotionEvent.ACTION_UP事件，這時會根據滑動的情況來判斷是朝左滑動還是朝右滑動，如果手指只滑動了屏幕寬度的少一半距離，則會彈回原來的頁面，滑動多於屏幕寬度的一半則會進行翻頁。同時要注意無論在什麼情況下觸發了WorkSpace滑動的事件，則系統會不斷調用computeScroll()方法，我們重寫這個方法同時在這個方法中調用刷新界面等操作。
滑動過程中所要注意的主要方法如下，具體見代碼註釋:
//對Touch事件進行攔截 主要用於在攔截各種Touch事件時，設置mTouchState的各種狀態
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
/*
* This method JUST determines whether we want to intercept the motion.
* If we return true, onTouchEvent will be called and we do the actual
* scrolling there.
* 這個方法僅僅決定了我們是否願意去對滑動事件進行攔截，如果返回爲true,則會調用onTouchEvent我們將會在那裏進行事件處理
*/
//對滑動的速率進行跟蹤。
acquireVelocityTrackerAndAddMovement(ev);
// Skip touch handling if there are no pages to swipe
// 如果沒有頁面，則跳過操作。
if (getChildCount() <= 0) return super.onInterceptTouchEvent(ev);
/*
* Shortcut the most recurring case: the user is in the dragging
* state and he is moving his finger. We want to intercept this
* motion.
* shortcut最常見的情況是：用戶處於拖動的狀態下，同時在移動它的手指，這時候我們需要攔截這個動作。
*
*/
final int action = ev.getAction();
//如果是在MOVE的情況下，則進行Touch事件攔截
if ((action == MotionEvent.ACTION_MOVE) &&
(mTouchState == TOUCH_STATE_SCROLLING)) {
return true;
}
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_MOVE: {
/*
* mIsBeingDragged == false, otherwise the shortcut would have caught it. Check
* whether the user has moved far enough from his original down touch.
* 如果mIsBeingDragged==false ,否則快捷方式應該捕獲到該事件，檢查一下用戶從它點擊的地方位移是否足夠
*/
if (mActivePointerId != INVALID_POINTER) {
//根據移動的距離判斷是翻頁還是移動一段位移，同時設置lastMotionX或者mTouchState這些值。同時取消桌面長按事件。
determineScrollingStart(ev);
break;
}
// if mActivePointerId is INVALID_POINTER, then we must have missed an ACTION_DOWN
// event. in that case, treat the first occurence of a move event as a ACTION_DOWN
// i.e. fall through to the next case (don't break)
// (We sometimes miss ACTION_DOWN events in Workspace because it ignores all events
// while it's small- this was causing a crash before we checked for INVALID_POINTER)
// 如果mActivePointerId 是 INVALID_POINTER，這時候我們應該已經錯過了ACTION_DOWN事件。在這種情況下，把
// 第一次發生移動的事件當作ACTION——DOWN事件，直接進入下一個情況下。
// 我們有時候會錯過workspace中的ACTION_DOWN事件，因爲在workspace變小的時候會忽略掉所有的事件。
}
case MotionEvent.ACTION_DOWN: {
final float x = ev.getX();
final float y = ev.getY();
// Remember location of down touch
// 記錄按下的位置
mDownMotionX = x;
mLastMotionX = x;
mLastMotionY = y;
mLastMotionXRemainder = 0;
mTotalMotionX = 0;
//Return the pointer identifier associated with a particular pointer data index is this event.
//The identifier tells you the actual pointer number associated with the data,
//accounting for individual pointers going up and down since the start of the current gesture.
//返回和這個事件關聯的觸點數據id，計算單獨點的id會上下浮動，因爲手勢的起始位置揮發聲改變。
mActivePointerId = ev.getPointerId(0);
mAllowLongPress = true;
/*
* If being flinged and user touches the screen, initiate drag;
* otherwise don't. mScroller.isFinished should be false when
* being flinged.
* 如果被拖動同時用戶觸摸到了屏幕，就開始初始化拖動，否則便不會。
* 當拖動完成後mScroller.isFinished就應該設置爲false.
*
*/
final int xDist = Math.abs(mScroller.getFinalX() - mScroller.getCurrX());
final boolean finishedScrolling = (mScroller.isFinished() || xDist < mTouchSlop);
if (finishedScrolling) {
//標記爲TOUCH_STATE_REST狀態
mTouchState = TOUCH_STATE_REST;
//取消滾動動畫
mScroller.abortAnimation();
} else {
//狀態爲TOUCH_STATE_SCROLLING
mTouchState = TOUCH_STATE_SCROLLING;
}
// check if this can be the beginning of a tap on the side of the pages
// to scroll the current page
// 檢測此事件是不是開始於點擊頁面的邊緣來對當前頁面進行滾動。
if (mTouchState != TOUCH_STATE_PREV_PAGE && mTouchState != TOUCH_STATE_NEXT_PAGE) {
if (getChildCount() > 0) {
//根據觸點的點位來判斷是否點擊到上一頁，從而更新相應的狀態
if (hitsPreviousPage(x, y)) {
mTouchState = TOUCH_STATE_PREV_PAGE;
} else if (hitsNextPage(x, y)) {
mTouchState = TOUCH_STATE_NEXT_PAGE;
}
}
}
break;
}
case MotionEvent.ACTION_UP:
case MotionEvent.ACTION_CANCEL:
//觸點不被相應時，所做的動作
mTouchState = TOUCH_STATE_REST;
mAllowLongPress = false;
mActivePointerId = INVALID_POINTER;
//釋放速率跟蹤
releaseVelocityTracker();
break;
case MotionEvent.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
releaseVelocityTracker();
break;
}
/*
* The only time we want to intercept motion events is if we are in the
* drag mode.
* 我們唯一會去對移動事件進行攔截的情況時我們在拖動模式下
*/
if(DEBUG) Log.d(TAG, "onInterceptTouchEvent "+(mTouchState != TOUCH_STATE_REST));
//只要是mTouchState的狀態不爲TOUCH_STATE_REST,那麼就進行事件攔截
return mTouchState != TOUCH_STATE_REST;
}
onTouchEvent方法，詳細見代碼註釋：
@Override
public boolean onTouchEvent(MotionEvent ev) {
// Skip touch handling if there are no pages to swipe
// 如果沒有子頁面，就直接跳過
if (getChildCount() <= 0) return super.onTouchEvent(ev);
acquireVelocityTrackerAndAddMovement(ev);
final int action = ev.getAction();
switch (action & MotionEvent.ACTION_MASK) {
case MotionEvent.ACTION_DOWN:
/*
* If being flinged and user touches, stop the fling. isFinished
* will be false if being flinged.
* 如果在滑動的過程中下用戶又點擊桌面，則取消滑動，從而響應當前的點擊。
* 在滑動的isFinished將返回false.
*/
if (!mScroller.isFinished()) {
mScroller.abortAnimation();
}
// Remember where the motion event started
mDownMotionX = mLastMotionX = ev.getX();
mLastMotionXRemainder = 0;
mTotalMotionX = 0;
mActivePointerId = ev.getPointerId(0);
//主要用來顯示滾動條，表明要開始滾動了，這裏可以進行調整，滾動條時逐漸顯示還是立刻顯示。
if (mTouchState == TOUCH_STATE_SCROLLING) {
pageBeginMoving();
}
break;
case MotionEvent.ACTION_MOVE:
if (mTouchState == TOUCH_STATE_SCROLLING) {
// Scroll to follow the motion event
final int pointerIndex = ev.findPointerIndex(mActivePointerId);
final float x = ev.getX(pointerIndex);
final float deltaX = mLastMotionX + mLastMotionXRemainder - x;
//總共移動的距離
mTotalMotionX += Math.abs(deltaX);
// Only scroll and update mLastMotionX if we have moved some discrete amount. We
// keep the remainder because we are actually testing if we've moved from the last
// scrolled position (which is discrete).
// 如果我們移動了一小段距離，我們則移動和更新mLastMotionX 。我們保存Remainder變量是因爲會檢測我們 //是否是從最後的滾動點位移動的。
if (Math.abs(deltaX) >= 1.0f) {
mTouchX += deltaX;
mSmoothingTime = System.nanoTime() / NANOTIME_DIV;
if (!mDeferScrollUpdate) {
scrollBy((int) deltaX, 0);
if (DEBUG) Log.d(TAG, "onTouchEvent().Scrolling: " + deltaX);
} else {
invalidate();
}
mLastMotionX = x;
mLastMotionXRemainder = deltaX - (int) deltaX;
} else {
//Trigger the scrollbars to draw. When invoked this method starts an animation to fade the
//scrollbars out after a default delay. If a subclass provides animated scrolling,
//the start delay should equal the duration of the scrolling animation.
//觸發scrollbar進行繪製。 使用這個方法來啓動一個動畫來使scrollbars經過一段時間淡出。如果子類提供了滾動的動畫，則
//延遲的時間等於動畫滾動的時間。
awakenScrollBars();
}
} else {
determineScrollingStart(ev);
}
break;
case MotionEvent.ACTION_UP:
if (mTouchState == TOUCH_STATE_SCROLLING) {
final int activePointerId = mActivePointerId;
final int pointerIndex = ev.findPointerIndex(activePointerId);
final float x = ev.getX(pointerIndex);
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int velocityX = (int) velocityTracker.getXVelocity(activePointerId);
final int deltaX = (int) (x - mDownMotionX);
final int pageWidth = getScaledMeasuredWidth(getPageAt(mCurrentPage));
// 屏幕的寬度*0.4f
boolean isSignificantMove = Math.abs(deltaX) > pageWidth *
SIGNIFICANT_MOVE_THRESHOLD;
final int snapVelocity = mSnapVelocity;
mTotalMotionX += Math.abs(mLastMotionX + mLastMotionXRemainder - x);
boolean isFling = mTotalMotionX > MIN_LENGTH_FOR_FLING &&
Math.abs(velocityX) > snapVelocity;
// In the case that the page is moved far to one direction and then is flung
// in the opposite direction, we use a threshold to determine whether we should
// just return to the starting page, or if we should skip one further.
// 這鐘情況是頁面朝一個方向移動了一段距離，然後又彈回去了。我們使用一個閥值來判斷是進行翻頁還是返回到初始頁面
boolean returnToOriginalPage = false;
if (Math.abs(deltaX) > pageWidth * RETURN_TO_ORIGINAL_PAGE_THRESHOLD &&
Math.signum(velocityX) != Math.signum(deltaX) && isFling) {
returnToOriginalPage = true;
}
int finalPage;
// We give flings precedence over large moves, which is why we short-circuit our
// test for a large move if a fling has been registered. That is, a large
// move to the left and fling to the right will register as a fling to the right.
//朝右移動
if (((isSignificantMove && deltaX > 0 && !isFling) ||
(isFling && velocityX > 0)) && mCurrentPage > 0) {
finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage - 1;
snapToPageWithVelocity(finalPage, velocityX);
//朝左移動
} else if (((isSignificantMove && deltaX < 0 && !isFling) ||
(isFling && velocityX < 0)) &&
mCurrentPage < getChildCount() - 1) {
finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage + 1;
snapToPageWithVelocity(finalPage, velocityX);
//尋找離屏幕中心最近的頁面移動
} else {
snapToDestination();
}
}
//直接移動到前一頁
else if (mTouchState == TOUCH_STATE_PREV_PAGE) {
// at this point we have not moved beyond the touch slop
// (otherwise mTouchState would be TOUCH_STATE_SCROLLING), so
// we can just page
int nextPage = Math.max(0, mCurrentPage - 1);
if (nextPage != mCurrentPage) {
snapToPage(nextPage);
} else {
snapToDestination();
}
}
//直接移動到下一頁
else if (mTouchState == TOUCH_STATE_NEXT_PAGE) {
// at this point we have not moved beyond the touch slop
// (otherwise mTouchState would be TOUCH_STATE_SCROLLING), so
// we can just page
int nextPage = Math.min(getChildCount() - 1, mCurrentPage + 1);
if (nextPage != mCurrentPage) {
snapToPage(nextPage);
} else {
snapToDestination();
}
} else {
onUnhandledTap(ev);
}
mTouchState = TOUCH_STATE_REST;
mActivePointerId = INVALID_POINTER;
releaseVelocityTracker();
break;
//對事件不響應
case MotionEvent.ACTION_CANCEL:
if (mTouchState == TOUCH_STATE_SCROLLING) {
snapToDestination();
}
mTouchState = TOUCH_STATE_REST;
mActivePointerId = INVALID_POINTER;
releaseVelocityTracker();
break;
case MotionEvent.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
}
return true;
}最後有個小知識點要搞清楚，不少網友都問到過我。就是scrollTo和scrollBy的區別。我們查看View類的源代碼如下所示，mScrollX記錄的是當前View針對屏幕座標在水平方向上的偏移量，而mScrollY則是記錄的時當前View針對屏幕在豎值方向上的偏移量。
從以下代碼我們可以得知，scrollTo就是把View移動到屏幕的X和Y位置，也就是絕對位置。而scrollBy其實就是調用的scrollTo，但是參數是當前mScrollX和mScrollY加上X和Y的位置，所以ScrollBy調用的是相對於mScrollX和mScrollY的位置。我們在上面的代碼中可以看到當我們手指不放移動屏幕時，就會調用scrollBy來移動一段相對的距離。而當我們手指鬆開後，會調用mScroller.startScroll(mUnboundedScrollX,
0, delta, 0, duration);來產生一段動畫來移動到相應的頁面，在這個過程中系統回不斷調用computeScroll()，我們再使用scrollTo來把View移動到當前Scroller所在的絕對位置。
/**
* Set the scrolled position of your view. This will cause a call to
* {@link #onScrollChanged(int, int, int, int)} and the view will be
* invalidated.
* @param x the x position to scroll to
* @param y the y position to scroll to
*/
public void scrollTo(int x, int y) {
if (mScrollX != x || mScrollY != y) {
int oldX = mScrollX;
int oldY = mScrollY;
mScrollX = x;
mScrollY = y;
invalidateParentCaches();
onScrollChanged(mScrollX, mScrollY, oldX, oldY);
if (!awakenScrollBars()) {
invalidate(true);
}
}
}
/**
* Move the scrolled position of your view. This will cause a call to
* {@link #onScrollChanged(int, int, int, int)} and the view will be
* invalidated.
* @param x the amount of pixels to scroll by horizontally
* @param y the amount of pixels to scroll by vertically
*/
public void scrollBy(int x, int y) {
scrollTo(mScrollX + x, mScrollY + y);
}