ActivityRecord、TaskRecord、Activi

前言：最近一直在看关于Activity的文章，刚好郭神推了这篇文章，以前一直没关注过这几个概念，今天学习一下。

提问：ActivityRecord、TaskRecord、ActivityStack、ActivityDisplay、ActivityStackSupervisor。它们是什么？可以用来干什么？以及怎么干的？

常规回答：ActivityStackSupervisor管理ActivityDisplay，ActivityDisplay管理ActivityStack、ActivityStack管理TaskRecord、TaskRecord管理ActivityRecord。

ActivityRecord

ActivityRecord是Activity在system_server进程中的镜像，Activity实例与ActivityRecord实例一一对应。ActivityRecord用来存储Activity的信息，如所在的进程名称，应用的包名，所在的任务栈的taskAffinity等。

/**

* An entry in the history stack, representing an activity.

*/finalclassActivityRecord{finalComponentName realActivity;// the intent component, or target of an alias.finalString shortComponentName;// the short component name of the intentfinalString packageName;// the package implementing intent's componentfinalString processName;// process where this component wants to runfinalString taskAffinity;// as per ActivityInfo.taskAffinityboolean fullscreen;// covers the full screen?}

TaskRecord

TaskRecord表示任务栈，用于记录activity开启的先后顺序。其所存放的Activity是不支持重新排序的，只能根据压栈和出栈操作更改Activity的顺序。有了TaskRecord，Android系统才能知道当一个Activity退出时，接下来该显示哪一个Activity。

finalclassTaskRecord{/** List of all activities in the task arranged in history order */finalArrayList<ActivityRecord>mActivities;}

ActivityStack

ActivityStack这个类在名字上给人很大的误解，Stack是栈的意思（Heap是堆的意思），那ActivityStack就表示“Activity的栈”？其实不是。从下面的代码中可以看出ActivityStack维护的是TaskRecord的列表。而且该列表也不是栈结构，列表中的TaskRecord可以重排顺序。

finalclassActivityStack{/**

    * The back history of all previous (and possibly still

    * running) activities.  It contains #TaskRecord objects.

    */privateArrayList<TaskRecord>mTaskHistory=newArrayList<>();}

ActivityDisplay

ActivityDisplay表示一个屏幕，Android支持三种屏幕：主屏幕，外接屏幕（HDMI等），虚拟屏幕（投屏）。一般情况下，即只有主屏幕时，ActivityStackSupervisor与ActivityDisplay都是系统唯一。

ActivityDisplay是ActivityStackSupervisor的内部类，它相当于一个工具类，封装了移除和添加ActivityStack的方法。

classActivityDisplay{final ArrayList<ActivityStack>mStacks=newArrayList<ActivityStack>();voidattachActivities(ActivityStack stack){if(DEBUG_STACK)Slog.v(TAG_STACK,"attachActivities: attaching "+stack+" to displayId="+mDisplayId);mStacks.add(stack);}voiddetachActivitiesLocked(ActivityStack stack){if(DEBUG_STACK)Slog.v(TAG_STACK,"detachActivitiesLocked: detaching "+stack+" from displayId="+mDisplayId);mStacks.remove(stack);}}

ActivityStackSupervisor

ActivityStackSupervisor是ActivityStack的管理者，内部管理了mHomeStack、mFocusedStack和mLastFocusedStack三个ActivityStack。其中，mHomeStack管理的是Launcher相关的Activity栈， stackId为0；mFocusedStack管理的是当前显示在前台Activity的Activity栈；mLastFocusedStack管理的是上一次显示在前台Activity的Activity栈。

publicfinalclassActivityStackSupervisorimplementsDisplayListener{/** The stack containing the launcher app. Assumed to always be attached to

    * Display.DEFAULT_DISPLAY. */privateActivityStackmHomeStack;/** The stack currently receiving input or launching the next activity. */privateActivityStackmFocusedStack;/** If this is the same as mFocusedStack then the activity on the top of the focused stack has

    * been resumed. If stacks are changing position this will hold the old stack until the new

    * stack becomes resumed after which it will be set to mFocusedStack. */privateActivityStackmLastFocusedStack;/** Mapping from (ActivityStack/TaskStack).mStackId to their current state */privateSparseArray<ActivityContainer>mActivityContainers=newSparseArray<>();ActivityStackcreateStackOnDisplay(intstackId,intdisplayId){……ActivityContaineractivityContainer=newActivityContainer(stackId);mActivityContainers.put(stackId,activityContainer);activityContainer.attachToDisplayLocked(activityDisplay);returnactivityContainer.mStack;}classActivityContainerextendsandroid.app.IActivityContainer.Stub{finalActivityStackmStack;ActivityDisplaymActivityDisplay;ActivityContainer(intstackId){synchronized(mService){mStackId=stackId;mStack=newActivityStack(this,mRecentTasks);}}voidattachToDisplayLocked(ActivityDisplayactivityDisplay){……                    mActivityDisplay=activityDisplay;activityDisplay.attachActivities(mStack);}protectedvoiddetachLocked(){……if(mActivityDisplay!=null){mActivityDisplay.detachActivitiesLocked(mStack);}}}}

ActivityDisplay的添加和移除ActivityStack的方法又被封装进了ActivityContainer类中，ActivityStackSupervisor调用ActivityContainer的attachToDisplayLocked与detachLocked对ActivityStack列表进行重排序，将任务栈从后台切换至前台。

前台任务栈与后台任务栈

如上图所示，只有Task #5才是前台任务栈，其余的都是后台任务栈。后台任务栈里面的Activity全部处于onStop状态。