如何使用AMS

这篇文章主要讲解了"如何使用AMS"，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着小编的思路慢慢深入，一起来研究和学习"如何使用AMS"吧！

服务的启动

之前在SystemServer章节说过，系统的服务一般都是通过SystemServer进程启动的，AMS也不例外。

//SystemServer.java     private void startBootstrapServices() {         //...          // Activity manager runs the show.         traceBeginAndSlog("StartActivityManager");         mActivityManagerService = mSystemServiceManager.startService(                 ActivityManagerService.Lifecycle.class).getService();         mActivityManagerService.setSystemServiceManager(mSystemServiceManager);         mActivityManagerService.setInstaller(installer);         traceEnd();     }      //中间用到了反射，之前说过。       public void startService(@NonNull final SystemService service) {         // Register it.         mServices.add(service);         // Start it.         long time = SystemClock.elapsedRealtime();         try {             service.onStart();         } catch (RuntimeException ex) {             throw new RuntimeException("Failed to start service " + service.getClass().getName()                     + ": onStart threw an exception", ex);         }     }       //ActivityManagerService.java     public static final class Lifecycle extends SystemService {         private final ActivityManagerService mService;          public Lifecycle(Context context) {             super(context);             mService = new ActivityManagerService(context);         }          @Override         public void onStart() {             mService.start();         }          @Override         public void onBootPhase(int phase) {             mService.mBootPhase = phase;             if (phase == PHASE_SYSTEM_SERVICES_READY) {                 mService.mBatteryStatsService.systemServicesReady();                 mService.mServices.systemServicesReady();             }         }          @Override         public void onCleanupUser(int userId) {             mService.mBatteryStatsService.onCleanupUser(userId);         }          public ActivityManagerService getService() {             return mService;         }     }

可以看到，通过调用了ActivityManagerService.Lifecycle这个内部类中的onStart方法，启动了AMS，并调用了AMS的start方法。

再简单看看AMS的实例化方法和start方法：

public ActivityManagerService(Context systemContext) {        mContext = systemContext;         mFactoryTest = FactoryTest.getMode();        mSystemThread = ActivityThread.currentActivityThread();        mUiContext = mSystemThread.getSystemUiContext();          mHandlerThread = new ServiceThread(TAG,                THREAD_PRIORITY_FOREGROUND, false /*allowIo*/);        mHandlerThread.start();        mHandler = new MainHandler(mHandlerThread.getLooper());        mUiHandler = mInjector.getUiHandler(this);         //...         mServices = new ActiveServices(this);        mProviderMap = new ProviderMap(this);        mAppErrors = new AppErrors(mUiContext, this);             // TODO: Move creation of battery stats service outside of activity manager service.        mBatteryStatsService = new BatteryStatsService(systemContext, systemDir, mHandler);        mBatteryStatsService.getActiveStatistics().readLocked();        mBatteryStatsService.scheduleWriteToDisk();        mOnBattery = DEBUG_POWER ? true                : mBatteryStatsService.getActiveStatistics().getIsOnBattery();        mBatteryStatsService.getActiveStatistics().setCallback(this);          mStackSupervisor = createStackSupervisor();        mStackSupervisor.onConfigurationChanged(mTempConfig);                mActivityStartController = new ActivityStartController(this);        mRecentTasks = createRecentTasks();        mStackSupervisor.setRecentTasks(mRecentTasks);        mLockTaskController = new LockTaskController(mContext, mStackSupervisor, mHandler);        mLifecycleManager = new ClientLifecycleManager();         mProcessCpuThread = new Thread("CpuTracker")        //...     }      private void start() {        removeAllProcessGroups();        mProcessCpuThread.start();         mBatteryStatsService.publish();        mAppOpsService.publish(mContext);        Slog.d("AppOps", "AppOpsService published");        LocalServices.addService(ActivityManagerInternal.class, new LocalService());        // Wait for the synchronized block started in mProcessCpuThread,        // so that any other acccess to mProcessCpuTracker from main thread        // will be blocked during mProcessCpuTracker initialization.        try {            mProcessCpuInitLatch.await();        } catch (InterruptedException e) {            Slog.wtf(TAG, "Interrupted wait during start", e);            Thread.currentThread().interrupt();            throw new IllegalStateException("Interrupted wait during start");        }    }

代码很长，我只截取了一部分。

在构造函数中，主要初始化了一些对象，比如Context、ActivityThrad、Handler、CPU监控线程，还有一些后文要用到的ActivityStackSupervisor、ActivityStarter等对象，

在start方法中，主要就是启动了CPU监控线程，然后注册了电池状态服务和权限管理服务。

初始工作

AMS被启动之后，还会在SystemServer启动三大服务的时候偷偷干一些工作，我们搜索下mActivityManagerService变量就可以看到：

private void startBootstrapServices() {    //1、初始化电源管理器       mActivityManagerService.initPowerManagement();       //2、为系统进程设置应用程序实例并启动。       mActivityManagerService.setSystemProcess();   }    private void startCoreServices() {       // 启动UsageStatsManager，用于查询应用的使用情况       mSystemServiceManager.startService(UsageStatsService.class);       mActivityManagerService.setUsageStatsManager(               LocalServices.getService(UsageStatsManagerInternal.class));       traceEnd();   }    private void startOtherServices() {     //安装系统的Providers       mActivityManagerService.installSystemProviders();        //启动WMS，并为AMS设置WMS关系       wm = WindowManagerService.main(context, inputManager,                   mFactoryTestMode != FactoryTest.FACTORY_TEST_LOW_LEVEL,                   !mFirstBoot, mOnlyCore, new PhoneWindowManager());       mActivityManagerService.setWindowManager(wm);        //...   }      public void setSystemProcess() {       try {           ServiceManager.addService(Context.ACTIVITY_SERVICE, this, /* allowIsolated= */ true,                   DUMP_FLAG_PRIORITY_CRITICAL | DUMP_FLAG_PRIORITY_NORMAL | DUMP_FLAG_PROTO);       }   }

其中第二步setSystemProcess方法中，会注册AMS到ServiceManager中，这样后续如果需要用到AMS的时候就可以通过ServiceManager进行获取，下面马上就要讲到。

启动就说这么多，都是比较枯燥的内容，所以也没有深入下去，有个印象就行，以后如果需要用到相关知识就知道去哪里找了。

从启动流程看AMS工作内容

为了了解AMS的具体工作，我们就从Activity的启动过程看起。

上文app启动流程中说过，startActivityForResult方法会转到mInstrumentation.execStartActivity方法:

//mInstrumentation.execStartActivity     int result = ActivityManager.getService()                 .startActivity(whoThread, who.getBasePackageName(), intent,                         intent.resolveTypeIfNeeded(who.getContentResolver()),                         token, target != null ? target.mEmbeddedID : null,                         requestCode, 0, null, options);     checkStartActivityResult(result, intent);       public static IActivityManager getService() {         return IActivityManagerSingleton.get();     }      private static final Singleton IActivityManagerSingleton =             new Singleton() {                 @Override                 protected IActivityManager create() {                     final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);                     final IActivityManager am = IActivityManager.Stub.asInterface(b);                     return am;                 }             };

可以看到，最终要拿到AMS的IBinder类型引用，这里的ServiceManager.getService(Context.ACTIVITY_SERVICE)是不是有点熟悉，没错，就是刚才专门调用了setSystemProcess方法对AMS进行了注册在ServiceManager中。然后我们要使用相关服务的方法的时候，就通过Servermanager拿到对应服务的引用。

这里也就是拿到了IActivityManager对象，IActivityManager其实就是AMS在当前进程的代理，这里的逻辑就是通过AIDL做了一个进程间的通信。因为这些服务，包括我们今天说的AMS都是在SystemServer进程中的，而我们实际用到的时候是在我们自己的应用进程中，所以就涉及到进程间通信了，这里是用的Binder机制进行通信。

Binder,ServiceManager,这是Binder通信一整套流程，不光是AMS，包括其他的WMS等服务基本上都是通过Binder机制进行进程间通信的，具体内容可以期待下后面说到的Binder章节。

接着看启动流程，通过Binder调用到了AMS的startActivity方法，然后会调用到ActivityStarter的startActivity方法，在这个方法中，我们发现一个新的类：

//ActivityStarter.java private int startActivity(...){         ActivityRecord r = new ActivityRecord(mService, callerApp, callingPid, callingUid,                 callingPackage, intent, resolvedType, aInfo, mService.getGlobalConfiguration(),                 resultRecord, resultWho, requestCode, componentSpecified, voiceSession != null,                 mSupervisor, checkedOptions, sourceRecord);         if (outActivity != null) {             outActivity[0] = r;         }          //...          return startActivity(r, sourceRecord, voiceSession, voiceInteractor, startFlags,                 true /* doResume */, checkedOptions, inTask, outActivity);     }

ActivityRecord

这个类翻译过来是Activity的记录，所以猜测是和Activity有关，我们点进去看看它里面包含了什么：

final ActivityManagerService service; // owner     final IApplicationToken.Stub appToken; // window manager token       final ActivityInfo info; // all about me     ApplicationInfo appInfo; // information about activity's app     final int userId;          // Which user is this running for?     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      private int icon;               // resource identifier of activity's icon.     private int logo;               // resource identifier of activity's logo.     private int theme;              // resource identifier of activity's theme.     int launchMode;         // the launch mode activity attribute.

我保留了一些比较常用的属性，大家应该都看得出来是什么了吧，比如当前Activity的主题——theme，当前Activity的token——apptoken，当前Activity的包名——packageName。

所以这个ActivityRecord其实就是保存记录了Activity的所有信息。

接着看流程，后续会执行到startActivityUnchecked方法，这个方法中，我们又可以看到一个新的类——TaskRecord.

TaskRecord

private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,             IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,             int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,             ActivityRecord[] outActivity) {          if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask                 && (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {             newTask = true;             result = setTaskFromReuseOrCreateNewTask(taskToAffiliate, topStack);         } else if (mSourceRecord != null) {             result = setTaskFromSourceRecord();         } else if (mInTask != null) {             result = setTaskFromInTask();         } else {             // This not being started from an existing activity, and not part of a new task...             // just put it in the top task, though these days this case should never happen.             setTaskToCurrentTopOrCreateNewTask();         }  }  // 新建一个任务栈     private void setTaskToCurrentTopOrCreateNewTask() {         //...         final ActivityRecord prev = mTargetStack.getTopActivity();         final TaskRecord task = (prev != null) ? prev.getTask() : mTargetStack.createTaskRecord(                 mSupervisor.getNextTaskIdForUserLocked(mStartActivity.userId), mStartActivity.info,                 mIntent, null, null, true, mStartActivity, mSourceRecord, mOptions);         addOrReparentStartingActivity(task, "setTaskToCurrentTopOrCreateNewTask");         mTargetStack.positionChildWindowContainerAtTop(task);     }      //添加Ac到栈顶     private void addOrReparentStartingActivity(TaskRecord parent, String reason) {         if (mStartActivity.getTask() == null || mStartActivity.getTask() == parent) {             parent.addActivityToTop(mStartActivity);         } else {             mStartActivity.reparent(parent, parent.mActivities.size() /* top */, reason);         }     }

从代码中可知，当我们启动的Activity需要一个新的任务栈的时候(比如启动模式为FLAG_ACTIVITY_NEW_TASK)，我们会走到setTaskToCurrentTopOrCreateNewTask方法中，新建一个TaskRecord类，并且把当前的Activity通过addActivityToTop方法添加到栈顶。

所以这个TaskRecord类就是一个任务栈类了，它的作用就是维护栈内的所有Activity，进去看看这个类有哪些变量：

final int taskId;       // Unique identifier for this task.     /** List of all activities in the task arranged in history order */    final ArrayList mActivities;     /** Current stack. Setter must always be used to update the value. */    private ActivityStack mStack;

这里截取了一些，可以发现有任务id——taskId，任务栈的所有ActivityRecord——mActivities，以及这个还不知道是什么的但是我知道是用来管理所有Activity和任务栈的大管家——ActivityStack。

ActivityStack

启动流程再往后面走，就会走到的ActivityStackSupervisor的resumeFocusedStackTopActivityLocked方法：

//ActivityStackSupervisor.java      /** The stack containing the launcher app. Assumed to always be attached to      * Display.DEFAULT_DISPLAY. */     ActivityStack mHomeStack;      /** The stack currently receiving input or launching the next activity. */     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;       public ActivityStackSupervisor(ActivityManagerService service, Looper looper) {         mService = service;         mLooper = looper;         mHandler = new ActivityStackSupervisorHandler(looper);     }       boolean resumeFocusedStackTopActivityLocked(             ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {           if (targetStack != null && isFocusedStack(targetStack)) {             return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);         }          final ActivityRecord r = mFocusedStack.topRunningActivityLocked();         if (r == null || !r.isState(RESUMED)) {             mFocusedStack.resumeTopActivityUncheckedLocked(null, null);         } else if (r.isState(RESUMED)) {             // Kick off any lingering app transitions form the MoveTaskToFront operation.             mFocusedStack.executeAppTransition(targetOptions);         }          return false;     }

ActivityStackSupervisor是一个管理ActivityStack的类，在AMS的构造方法中被创建，这个类中可以看到有一些任务栈，比如mHomeStack——包含了Launcher APP的Activity。

然后再看看ActivityStack这个大管家家里存储了什么好东西：

enum ActivityState {      INITIALIZING,      RESUMED,      PAUSING,      PAUSED,      STOPPING,      STOPPED,      FINISHING,      DESTROYING,      DESTROYED  }     private final ArrayList mTaskHistory = new ArrayList<>();   final ArrayList mLRUActivities = new ArrayList<>();   ActivityRecord mPausingActivity = null;   ActivityRecord mLastPausedActivity = null;

可以看到，在ActivityStack中：

• 有一个枚举ActivityState，存储了Activity的所有状态。

• 有一些TaskRecord和ActivityRecord的列表，比如mTaskHistory——没有被销毁的任务栈列表，mLRUActivities——通过LRU计算的列表头目是最近最少使用的Activity的ActivityRecord列表。

• 还有一些特殊状态的Activity对应的ActivityRecord，比如正在暂停的Activity，上一个暂停过的Activity。

最后，启动流程会走到AMS的startProcessLocked方法，然后跟Zygote进程通信，fork进程。后续就不说了。

感谢各位的阅读，以上就是"如何使用AMS"的内容了，经过本文的学习后，相信大家对如何使用AMS这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是，小编将为大家推送更多相关知识点的文章，欢迎关注！