關於）View.onScrollChanged來源（

Question

這裏是protected void View.onScrollChanged(int l, int t, int oldl, int oldt)來源：

/** 
* This is called in response to an internal scroll in this view (i.e., the 
* view scrolled its own contents). This is typically as a result of 
* {@link #scrollBy(int, int)} or {@link #scrollTo(int, int)} having been 
* called. 
* 
* @param l Current horizontal scroll origin. 
* @param t Current vertical scroll origin. 
* @param oldl Previous horizontal scroll origin. 
* @param oldt Previous vertical scroll origin. 
*/ 
protected void onScrollChanged(int l, int t, int oldl, int oldt) { 
    if (AccessibilityManager.getInstance(mContext).isEnabled()) { 
     postSendViewScrolledAccessibilityEventCallback(); 
    } 

    mBackgroundSizeChanged = true; 

    final AttachInfo ai = mAttachInfo; 
    if (ai != null) { 
     ai.mViewScrollChanged = true; 
    } 
} 

的問題是關於這一行：final AttachInfo ai = mAttachInfo;。爲什麼ai被引入，爲什麼它被製成final？

Answer 1

最有可能的是，mAttachInfo是volatile;那麼ai的目的是爲了避免NullPOinterException當兩個線程同時訪問該對象和mAttachInfo正在檢查mAttachInfo!=null之後，但在執行mAttachInfo.mViewScrollChanged = true;