Android可见APP的不可见任务栈（TaskRecord）销毁分析

 private void attach(boolean system) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        if (!system) {
           ...
            final IActivityManager mgr = ActivityManagerNative.getDefault();
             ...
            // Watch for getting close to heap limit.
            <!--关键点1，添加监测工具-->
            BinderInternal.addGcWatcher(new Runnable() {
                @Override public void run() {
                    if (!mSomeActivitiesChanged) {
                        return;
                    }
                    Runtime runtime = Runtime.getRuntime();
                    long dalvikMax = runtime.maxMemory();
                    long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
                     <!--关键点2 ：如果已经可用的内存不足1/4着手处理杀死Activity，并且这个时候，没有缓存进程-->
                    if (dalvikUsed > ((3*dalvikMax)/4)) {
                        mSomeActivitiesChanged = false;
                        try {
                            mgr.releaseSomeActivities(mAppThread);
                        } catch (RemoteException e) {
                    ...
                }

  public class BinderInternal {
  <!--关键点1 弱引用-->
    static WeakReference<GcWatcher> sGcWatcher
            = new WeakReference<GcWatcher>(new GcWatcher());
    static ArrayList<Runnable> sGcWatchers = new ArrayList<>();
    static Runnable[] sTmpWatchers = new Runnable[1];
    static long sLastGcTime;
    static final class GcWatcher {
        @Override
        protected void finalize() throws Throwable {
            handleGc();
            sLastGcTime = SystemClock.uptimeMillis();
            synchronized (sGcWatchers) {
                sTmpWatchers = sGcWatchers.toArray(sTmpWatchers);
            }
            <!--关键点2 执行之前添加的回调-->
            for (int i=0; i<sTmpWatchers.length; i++) {
                if (sTmpWatchers[i] != null) {
                    sTmpWatchers[i].run();
                }
            }
            <!--关键点3 下一次轮回-->
            sGcWatcher = new WeakReference<GcWatcher>(new GcWatcher());
        }
    }
    public static void addGcWatcher(Runnable watcher) {
        synchronized (sGcWatchers) {
            sGcWatchers.add(watcher);
        }   
    }
 ...
}

@Override
public void releaseSomeActivities(IApplicationThread appInt) {
    synchronized(this) {
        final long origId = Binder.clearCallingIdentity();
        try {
            ProcessRecord app = getRecordForAppLocked(appInt);
            mStackSupervisor.releaseSomeActivitiesLocked(app, "low-mem");
        } finally {
            Binder.restoreCallingIdentity(origId);
        }
    }
}
void releaseSomeActivitiesLocked(ProcessRecord app, String reason) {
    TaskRecord firstTask = null;
    ArraySet<TaskRecord> tasks = null;
    for (int i = 0; i < app.activities.size(); i++) {
        ActivityRecord r = app.activities.get(i);
        <!--如果已经有一个进行，则不再继续-->
        if (r.finishing || r.state == DESTROYING || r.state == DESTROYED) {
            return;
        }
        <!--过滤-->
        if (r.visible || !r.stopped || !r.haveState || r.state == RESUMED || r.state == PAUSING
                || r.state == PAUSED || r.state == STOPPING) {
            continue;
        }
        if (r.task != null) {
            if (firstTask == null) {
                firstTask = r.task;
         <!--关键点1 只要要多余一个TaskRecord才有机会走这一步，-->
            } else if (firstTask != r.task) {
                if (tasks == null) {
                    tasks = new ArraySet<>();
                    tasks.add(firstTask);
                }
                tasks.add(r.task);
            }
        }
    }
    <!--注释很明显，-->
    if (tasks == null) {
        if (DEBUG_RELEASE) Slog.d(TAG_RELEASE, "Didn't find two or more tasks to release");
        return;
    }
    // If we have activities in multiple tasks that are in a position to be destroyed,
    // let's iterate through the tasks and release the oldest one.
    final int numDisplays = mActivityDisplays.size();
    for (int displayNdx = 0; displayNdx < numDisplays; ++displayNdx) {
        final ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks;
        // Step through all stacks starting from behind, to hit the oldest things first.
        for (int stackNdx = 0; stackNdx < stacks.size(); stackNdx++) {
            final ActivityStack stack = stacks.get(stackNdx);
            // Try to release activities in this stack; if we manage to, we are done.
            if (stack.releaseSomeActivitiesLocked(app, tasks, reason) > 0) {
                return;
            }
        }
    }
}

 final int releaseSomeActivitiesLocked(ProcessRecord app, ArraySet<TaskRecord> tasks,
        String reason) {
    <!--maxTasks 保证最多清理- tasks.size() / 4有效个，最少清理一个 同时最少保留一个前台TaskRecord->
    int maxTasks = tasks.size() / 4;
    if (maxTasks < 1) {
    <!--至少清理一个-->
        maxTasks = 1;
    }
    int numReleased = 0;
    for (int taskNdx = 0; taskNdx < mTaskHistory.size() && maxTasks > 0; taskNdx++) {
        final TaskRecord task = mTaskHistory.get(taskNdx);
        if (!tasks.contains(task)) {
            continue;
        }
        int curNum = 0;
        final ArrayList<ActivityRecord> activities = task.mActivities;
        for (int actNdx = 0; actNdx < activities.size(); actNdx++) {
            final ActivityRecord activity = activities.get(actNdx);
            if (activity.app == app && activity.isDestroyable()) {
                destroyActivityLocked(activity, true, reason);
                if (activities.get(actNdx) != activity) {
                    actNdx--;
                }
                curNum++;
            }
        }
        if (curNum > 0) {
            numReleased += curNum;
            maxTasks--;
            if (mTaskHistory.get(taskNdx) != task) {
                // The entire task got removed, back up so we don't miss the next one.
                taskNdx--;
            }
        }
    }
    return numReleased;
}