Andorid N 最近任务管理器流程详解（四）

4. Task的移除

本章节主要讲讲Task移除的流程

4.1 removeTask

文件：SystemServicesProxy.java

    public void removeTask(final int taskId) {
        if (mAm == null) return;
        if (RecentsDebugFlags.Static.EnableMockTasks) return;

        // Remove the task.
        BackgroundThread.getHandler().post(new Runnable() {
            @Override
            public void run() {
                // 见4.2 
                mAm.removeTask(taskId);
            }
        });
    }

调用AM的removeTask方法来移除Task

4.2 removeTask

文件：ActivityManagerService.java

    @Override
    public boolean removeTask(int taskId) {
        // 检查是否有移除栈的权限
        enforceCallingPermission(android.Manifest.permission.REMOVE_TASKS, "removeTask()");
        synchronized (this) {
            final long ident = Binder.clearCallingIdentity();
            try {
                // 见4.3
                return removeTaskByIdLocked(taskId, true, REMOVE_FROM_RECENTS);
            } finally {
                Binder.restoreCallingIdentity(ident);
            }
        }
    }

根据任务的taskId来移除

4.3 removeTaskByIdLocked

文件：ActivityManagerService.java

    private boolean removeTaskByIdLocked(int taskId, boolean killProcess,
            boolean removeFromRecents) {
            // 根据taskId找到对应的栈
        final TaskRecord tr = mStackSupervisor.anyTaskForIdLocked(
                taskId, !RESTORE_FROM_RECENTS, INVALID_STACK_ID);

        if (tr != null) {
            // 见4.3.1 调用TaskRecord的removeTaskActivitiesLocked
            tr.removeTaskActivitiesLocked();
            // 见4.3.2
            cleanUpRemovedTaskLocked(tr, killProcess, removeFromRecents);
            if (tr.isPersistable) {
                notifyTaskPersisterLocked(null, true);
            }
            return true;
        }
        Slog.w(TAG, "Request to remove task ignored for non-existent task " + taskId);
        return false;
    }

这三个参数的意思分别是：
taskId：需要被移除的task的Id
killProcess：如果可能，杀掉所有和该task相关的进程
removeFromRecents：是否将该任务从Recents中移除
这个方法调用了两个比较关键的方法，分别是removeTaskActivitiesLocked和cleanUpRemovedTaskLocked

4.3.1 removeTaskActivitiesLocked

文件：TaskRecord.java

    public void removeTaskActivitiesLocked() {
        // 见4.3.1.1仅仅移除整个任务
        performClearTaskAtIndexLocked(0);
    }

4.3.1.1 performClearTaskAtIndexLocked

文件：TaskRecord.java

final void performClearTaskAtIndexLocked(int activityNdx) {
        int numActivities = mActivities.size();
        for ( ; activityNdx < numActivities; ++activityNdx) {
            final ActivityRecord r = mActivities.get(activityNdx);
            if (r.finishing) {
                continue;
            }
            if (stack == null) {
                // Task was restored from persistent storage.
                r.takeFromHistory();
                mActivities.remove(activityNdx);
                --activityNdx;
                --numActivities;
            } else if (stack.finishActivityLocked( // 移除该TaskRecord中的ActivityRecord对象
                    r, Activity.RESULT_CANCELED, null, "clear-task-index", false)) {
                --activityNdx;
                --numActivities;
            }
        }
    }

这个方法是真正执行移除栈，移除所有和该栈相关连的activity。参数activityNdx为0，代表要把TaskRecord内的所有Activity移除。
方法中mActivities是该TaskRecord中保存的所有的ActivityRecord对象；stack是该TaskRecord所在的ActivityStack，这里不为空（ActivityStack是一个管理者的角色，负责管理ActivityRecord和TaskRecord）

4.3.2 cleanUpRemovedTaskLocked

文件：ActivityManagerService.java

    private void cleanUpRemovedTaskLocked(TaskRecord tr, boolean killProcess,
            boolean removeFromRecents) {
        // 把该TaskRecord从mRecentTasks中移除
        if (removeFromRecents) {
            mRecentTasks.remove(tr);
            tr.removedFromRecents();
        }
        ComponentName component = tr.getBaseIntent().getComponent();
        if (component == null) {
            Slog.w(TAG, "No component for base intent of task: " + tr);
            return;
        }

        // 如果需要，找到和这个app相关并正在运行的services，将它们停止
        mServices.cleanUpRemovedTaskLocked(tr, component, new Intent(tr.getBaseIntent()));
        // 如果不需要杀进程，则返回
        if (!killProcess) {
            return;
        }

        // Determine if the process(es) for this task should be killed.
        final String pkg = component.getPackageName();
        ArrayList procsToKill = new ArrayList<>();
        ArrayMap> pmap = mProcessNames.getMap();
        for (int i = 0; i < pmap.size(); i++) {

            SparseArray uids = pmap.valueAt(i);
            for (int j = 0; j < uids.size(); j++) {
                ProcessRecord proc = uids.valueAt(j);
                if (proc.userId != tr.userId) {
                    // 不杀不同用户的进程
                    continue;
                }
                if (proc == mHomeProcess) {
                    // Don't kill the home process along with tasks from the same package.
                    continue;
                }
                if (!proc.pkgList.containsKey(pkg)) {
                    // 不杀与该task无关的进程
                    continue;
                }

                for (int k = 0; k < proc.activities.size(); k++) {
                    TaskRecord otherTask = proc.activities.get(k).task;
                    if (tr.taskId != otherTask.taskId && otherTask.inRecents) {
                        // 如果进程中有activity属于recents中另一个不同的task，那么该进程也不杀
                        return;
                    }
                }

                if (proc.foregroundServices) {
                    // 有前台service的不杀
                    return;
                }

                // 添加进程到kill list
                // 如下情况都不杀：进程的userid和TaskRecord不同；进程是HomeProcess;和该TaskRecord无关的进程；
                // 进程有Activity处于其他的任务栈，并且也在recents里面的;除了上述的，其他的都加入procsToKill列表
                procsToKill.add(proc);
            }
        }

        // 杀掉运行的进程
        for (int i = 0; i < procsToKill.size(); i++) {
            ProcessRecord pr = procsToKill.get(i);
            if (pr.setSchedGroup == ProcessList.SCHED_GROUP_BACKGROUND
                    && pr.curReceiver == null) {
                // 立刻杀
                pr.kill("remove task", true);
            } else {
                // 如果没有运行在后台，或者运行了一个receiver，就延迟杀
                pr.waitingToKill = "remove task";
            }
        }
    }

这个方法主要的作用就是清除进程。这里removeFromRecents为true，会先TaskRecord从mRecntTasks中移除，然后将不能杀的进程过滤掉，剩下的加入到kill list里面。最后杀进程也分为两种：
- 一种是SHED_GROUP_BACKGROUND类型并且没有正执行reciver，那就会调用kill立即去杀掉
- 另一种是简单将procsToKill中ProcessRecord的waitingToKill字段设置为remove task