深入理解Activity——Token之旅

    Token是ActivityRecord的内部静态类，我们先来看下Token的继承关系，Token extends IApplicationToken.Stub，从IApplicationToken.Stub类进行继承，根据Binder的机制可以知道Token是一个匿名Binder实体类，这个匿名Binder实体会传递给其他进程，其他进程会拿到Token的代理端。

    我们知道匿名Binder有两个比较重要的用途，一个是拿到Binder代理端后可跨Binder调用实体端的函数接口，另一个作用便是在多个进程中标识同一个对象。往往这两个作用是同时存在的，比如我们这里研究的Token就同时存在这两个作用，但最重要的便是后者，Token标识了一个ActivityRecord对象，即间接标识了一个Activity。

    下面这个图是Token的传递，首先会传递到WMS中，接着会传递到应用进程ActivityThread中，下面来具体分析这个传递流程。

1、Token对象的创建

    ActivityRecord(ActivityManagerService _service, ProcessRecord _caller,            int _launchedFromUid, String _launchedFromPackage, Intent _intent, String _resolvedType,            ActivityInfo aInfo, Configuration _configuration,            ActivityRecord _resultTo, String _resultWho, int _reqCode,            boolean _componentSpecified, ActivityStackSupervisor supervisor,            ActivityContainer container, Bundle options) {        service = _service;        appToken = new Token(this);        ........}

在ActivityRecord的构造函数中创建，标识着当前这个ActivityRecord，即间接代表着一个Activity。

2、AMS调用WMS的addAPPToken()接口

在启动一个Activity时，会调用startActivityLocked()来在WMS中添加一个AppWindowToken对象；

    final void startActivityLocked(ActivityRecord r, boolean newTask,            boolean doResume, boolean keepCurTransition, Bundle options) {            ......            mWindowManager.addAppToken(task.mActivities.indexOf(r), r.appToken,                    r.task.taskId, mStackId, r.info.screenOrientation, r.fullscreen,                    (r.info.flags & ActivityInfo.FLAG_SHOW_ON_LOCK_SCREEN) != 0, r.userId,                    r.info.configChanges, task.voiceSession != null, r.mLaunchTaskBehind);            ......    }

    public void addAppToken(int addPos, IApplicationToken token, int taskId, int stackId,            int requestedOrientation, boolean fullscreen, boolean showWhenLocked, int userId,            int configChanges, boolean voiceInteraction, boolean launchTaskBehind) {            ......        synchronized(mWindowMap) {            AppWindowToken atoken = findAppWindowToken(token.asBinder());            if (atoken != null) {                Slog.w(TAG, "Attempted to add existing app token: " + token);                return;            }            atoken = new AppWindowToken(this, token, voiceInteraction);            ......            Task task = mTaskIdToTask.get(taskId);            if (task == null) {                createTask(taskId, stackId, userId, atoken);            } else {                task.addAppToken(addPos, atoken);            }            mTokenMap.put(token.asBinder(), atoken);            // Application tokens start out hidden.            atoken.hidden = true;            atoken.hiddenRequested = true;            //dump();        }    }

3、AMS跨Binder调用应用进程的scheduleLaunchActivity()将Token传递给上层应用进程

    final boolean realStartActivityLocked(ActivityRecord r,            ProcessRecord app, boolean andResume, boolean checkConfig)            throws RemoteException {            ......            app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,                    System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),                    r.compat, r.launchedFromPackage, r.task.voiceInteractor, app.repProcState,                    r.icicle, r.persistentState, results, newIntents, !andResume,                    mService.isNextTransitionForward(), profilerInfo);            ......    }

这个是通过调用app.thread.scheduleLaunchActivity()完成的，可以知道对端接收到的便是Token的代理对象。

我们来看下ApplicationThread中scheduleLaunchActivity()的实现：

        public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,                ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,                String referrer, IVoiceInteractor voiceInteractor, int procState, Bundle state,                PersistableBundle persistentState, List<ResultInfo> pendingResults,                List<ReferrerIntent> pendingNewIntents, boolean notResumed, boolean isForward,                ProfilerInfo profilerInfo) {            updateProcessState(procState, false);            ActivityClientRecord r = new ActivityClientRecord();            r.token = token;            r.ident = ident;            r.intent = intent;            r.referrer = referrer;            r.voiceInteractor = voiceInteractor;            r.activityInfo = info;            r.compatInfo = compatInfo;            r.state = state;            r.persistentState = persistentState;            r.pendingResults = pendingResults;            r.pendingIntents = pendingNewIntents;            r.startsNotResumed = notResumed;            r.isForward = isForward;            r.profilerInfo = profilerInfo;            updatePendingConfiguration(curConfig);            sendMessage(H.LAUNCH_ACTIVITY, r);        }

函数中创建一个ActivityClientRecord对象，然后将Token的代理对象保存在ActivityClientRecord.token中。ActivityClientRecord也代表着一个Activity，不过是在应用进程中，而ActivityRecord是在ActivityManagerService中代表一个Activity。

4、Activity窗口添加

ViewRootImpl.setView()函数中添加Activity窗口时在参数mWindowAttributes中携带Token代理对象。

    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {        synchronized (this) {            if (mView == null) {                mView = view;                ......                mWindowAttributes.copyFrom(attrs);                ......                try {                    ......                    res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,                            getHostVisibility(), mDisplay.getDisplayId(),                            mAttachInfo.mContentInsets, mAttachInfo.mStableInsets, mInputChannel);                } catch (RemoteException e) {                    ......                } finally {                    ......                }            ......            }        }    }

    public int addWindow(Session session, IWindow client, int seq,            WindowManager.LayoutParams attrs, int viewVisibility, int displayId,            Rect outContentInsets, Rect outStableInsets, InputChannel outInputChannel) {            ......            boolean addToken = false;            WindowToken token = mTokenMap.get(attrs.token);            ......            win = new WindowState(this, session, client, token,                    attachedWindow, appOp[0], seq, attrs, viewVisibility, displayContent);            mWindowMap.put(client.asBinder(), win);            ......    }

根据Binder机制可以知道从上层应用传递过来的Token代理对象会转换成SystemServer进程中的Token本地对象，后者与第2步中从Token对象是同一个对象，所以上面调用mTokenMap.get(attrs.token)时便能返回正确返回一个WindowToken（这个WindowToken其实是一个APPWindowToken），这样添加的窗口也就跟Activity关联上了。