在[UI]抽屜菜單DrawerLayout分析（一）和[UI]抽屜菜單DrawerLayout分析（二）中分別介紹了DrawerLayout得基本框架結構和ViewDragerHelper的作用以及手勢分發，本文一起來分析其中的Scroller的使用情況。
在ViewDragerHelper中可以發現private ScrollerCompat mScroller;說明抽屜菜單的具體滑動也是依賴於Scroller的使用，檢索一下mScroller的引用，定位到forceSettleCapturedViewAt，這個方法回調用Scroller的startScroll來計算位移，它本身適用於計算和保存位移在特定時間的變化情況，最終的在繪制view時我可以獲取其保存的x，y坐標值。
/**
* Settle the captured view at the given (left, top) position.
*
* @param finalLeft Target left position for the captured view
* @param finalTop Target top position for the captured view
* @param xvel Horizontal velocity
* @param yvel Vertical velocity
* @return true if animation should continue through {@link #continueSettling(boolean)} calls
*/
private boolean forceSettleCapturedViewAt(int finalLeft, int finalTop, int xvel, int yvel) {
final int startLeft = mCapturedView.getLeft();
final int startTop = mCapturedView.getTop();
final int dx = finalLeft - startLeft;
final int dy = finalTop - startTop;
if (dx == 0 && dy == 0) {
// Nothing to do. Send callbacks, be done.
mScroller.abortAnimation();
setDragState(STATE_IDLE);
returnfalse;
}
final int duration = computeSettleDuration(mCapturedView, dx, dy, xvel, yvel);
mScroller.startScroll(startLeft, startTop, dx, dy, duration);
setDragState(STATE_SETTLING);
returntrue;
}
這里用的是v4擴展包里的ScrollerCompat用於低版本兼容，它繼承自ScrollerCompatImpl，可以看到里面主要的方法聲明：
interface ScrollerCompatImpl{
Object createScroller(Context context, Interpolator interpolator);
boolean isFinished(Object scroller);
int getCurrX(Object scroller);
int getCurrY(Object scroller);
float getCurrVelocity(Object scroller);
boolean computeScrollOffset(Object scroller);
void startScroll(Object scroller, int startX, int startY, int dx, int dy);
void startScroll(Object scroller, int startX, int startY, int dx, int dy, int duration);
void fling(Object scroller, int startX, int startY, int velX, int velY,
int minX, int maxX, int minY, int maxY);
void fling(Object scroller, int startX, int startY, int velX, int velY,
int minX, int maxX, int minY, int maxY, int overX, int overY);
void abortAnimation(Object scroller);
void notifyHorizontalEdgeReached(Object scroller, int startX, int finalX, int overX);
void notifyVerticalEdgeReached(Object scroller, int startY, int finalY, int overY);
boolean isOverScrolled(Object scroller);
int getFinalX(Object scroller);
int getFinalY(Object scroller);
}
從Scroller一直往上追溯，可以得到如圖的調用流程。
當滑動屏幕時，DrawerLayout中的手勢分發被觸發，先執行onInterceptTouchEvent根據返回結果確定是否執行onTouchEvent,之后就是一些和ViewDragHelper之間的回調接口處理。
接下來追蹤一下什么時候從Scroller中取出x，y來使用：
在View里面有一個實現為空的computeScroll，DrawerLayout對它進行重寫，這個方法應該是在view自動重繪是會被調用，回到continueSettling：
/**
* Move the captured settling view by the appropriate amount for the current time.
* If <code>continueSettling</code> returns true, the caller should call it again
* on the next frame to continue.
*
* @param deferCallbacks true if state callbacks should be deferred via posted message.
* Set this to true if you are calling this method from
* {@link android.view.View#computeScroll()} or similar methods
* invoked as part of layout or drawing.
* @return true if settle is still in progress
*/
public boolean continueSettling(boolean deferCallbacks) {
if (mDragState == STATE_SETTLING) {
boolean keepGoing = mScroller.computeScrollOffset();
final int x = mScroller.getCurrX();
final int y = mScroller.getCurrY();
final int dx = x - mCapturedView.getLeft();
final int dy = y - mCapturedView.getTop();
if (dx != 0) {
mCapturedView.offsetLeftAndRight(dx);
}
if (dy != 0) {
mCapturedView.offsetTopAndBottom(dy);
}
if (dx != 0 || dy != 0) {
mCallback.onViewPositionChanged(mCapturedView, x, y, dx, dy);
}
if (keepGoing && x == mScroller.getFinalX() && y == mScroller.getFinalY()) {
// Close enough. The interpolator/scroller might think we're still moving
// but the user sure doesn't.
mScroller.abortAnimation();
keepGoing = mScroller.isFinished();
}
if (!keepGoing) {
if (deferCallbacks) {
mParentView.post(mSetIdleRunnable);
} else {
setDragState(STATE_IDLE);
}
}
}
return mDragState == STATE_SETTLING;
}
當狀態處於STATE_SETTLING時開始獲取Scroller中的x，y值，結合當前運動view的left，top位置，計算出偏移量，通過offsetLeftAndRight設置，里面是一些具體的位置改變，挺復雜的。
/**
* Offset this view's horizontal location by the specified amount of pixels.
*
* @param offset the number of pixels to offset the view by
*/
public void offsetLeftAndRight(int offset) {
if (offset != 0) {
updateMatrix();
final boolean matrixIsIdentity = mTransformationInfo == null
|| mTransformationInfo.mMatrixIsIdentity;
if (matrixIsIdentity) {
if (mDisplayList != null) {
invalidateViewProperty(false, false);
} else {
final ViewParent p = mParent;
if (p != null && mAttachInfo != null) {
final Rect r = mAttachInfo.mTmpInvalRect;
int minLeft;
int maxRight;
if (offset < 0) {
minLeft = mLeft + offset;
maxRight = mRight;
} else {
minLeft = mLeft;
maxRight = mRight + offset;
}
r.set(0, 0, maxRight - minLeft, mBottom - mTop);
p.invalidateChild(this, r);
}
}
} else {
invalidateViewProperty(false, false);
}
mLeft += offset;
mRight += offset;
if (mDisplayList != null) {
mDisplayList.offsetLeftAndRight(offset);
invalidateViewProperty(false, false);
} else {
if (!matrixIsIdentity) {
invalidateViewProperty(false, true);
}
invalidateParentIfNeeded();
}
}
}
小結
至此DrawerLayout的基本工作流程分析完畢，簡單做一個總結，v4包提供了ViewDragHelper類，里面封裝了對scroller合view的位移操作，和Callback接口，通過DrawerLayout內的onInterceptTouchEvent和onTouchEvent的重載，觸發ViewDragHelper內的相關方法，同時在DrawerLayout內實現ViewDragHelp.Callback.