ActivityRecord、TaskRecord、Activi

Start

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.
 */
final class ActivityRecord {
    final ComponentName realActivity;  // the intent component, or target of an alias.
    final String shortComponentName; // the short component name of the intent
    final String packageName; // the package implementing intent's component
    final String processName; // process where this component wants to run
    final String taskAffinity; // as per ActivityInfo.taskAffinity
    boolean 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。

final class TaskRecord {
    /** List of all activities in the task arranged in history order */
    final ArrayList<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。

final class ActivityStack {
    /**
     * The back history of all previous (and possibly still
     * running) activities.  It contains #TaskRecord objects.
     */
    private ArrayList<TaskRecord> mTaskHistory = new ArrayList<>();
}

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。

class ActivityDisplay {
    final ArrayList<ActivityStack> mStacks = new ArrayList<ActivityStack>();

    void attachActivities(ActivityStack stack) {
            if (DEBUG_STACK) Slog.v(TAG_STACK,
                    "attachActivities: attaching " + stack + " to displayId=" + mDisplayId);
            mStacks.add(stack);
     }

    void detachActivitiesLocked(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。

public final class ActivityStackSupervisor implements DisplayListener {

    /** The stack containing the launcher app. Assumed to always be attached to
     * Display.DEFAULT_DISPLAY. */
    private ActivityStack mHomeStack;

    /** The stack currently receiving input or launching the next activity. */
    private ActivityStack mFocusedStack;

    /** 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. */
    private ActivityStack mLastFocusedStack;

    /** Mapping from (ActivityStack/TaskStack).mStackId to their current state */
    private SparseArray<ActivityContainer> mActivityContainers = new SparseArray<>();

    ActivityStack createStackOnDisplay(int stackId, int displayId) {
        ……
        ActivityContainer activityContainer = new ActivityContainer(stackId);
        mActivityContainers.put(stackId, activityContainer);
        activityContainer.attachToDisplayLocked(activityDisplay);
        return activityContainer.mStack;
    }

    class ActivityContainer extends android.app.IActivityContainer.Stub {
            final ActivityStack mStack;
            ActivityDisplay mActivityDisplay;

            ActivityContainer(int stackId) {
                synchronized (mService) {
                    mStackId = stackId;
                    mStack = new ActivityStack(this, mRecentTasks);
                    }
            }

            void attachToDisplayLocked(ActivityDisplay activityDisplay) {
                    ……
                    mActivityDisplay = activityDisplay;
                    activityDisplay.attachActivities(mStack);
                }

            protected void detachLocked() {
                   ……
                    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。

最后：原文里面还有对一个APP对应一个ActivityStack的验证，感兴趣的可以去原文查看

郭霖转发地址
原作者地址

申明：开始和结束的图片来源网络，侵删

End