Activity的启动过程详解(基于Android10.0)

话说Android中有四大组件:Activity、Service、BroadcastReceiver、ContentProvider。我们最常接触也是用户直接感受到的就是Activity了,今天来就说说Android启动的执行过程和工作原理。

Activity是一种 展示型组件,用于展示给用户一个可以交互的页面。Activity是Android中最重要的组件,对用户来说,所有的Activity就是一个App的全部,而其他组件用户是不能直接感知的。在开发层面,要启动一个Activity可以使用Intent,分显式和隐式,并且还可以设置Activity的 启动模式

Android系统对四大组件都做了很大程度的封装,这样我们可以快速使用组件。Activity的启动在系统封装后,变的极为简单,显示启动activity代码如下:

Intent intent = new Intent(this, TestActivity.class);
this.startActivity(intent);
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这样就可以启动TestActivity了,那么问题来了,

  • 这个代码是如何启动一个Activity的?
  • 里面做了哪些事情?
  • onCreate这些生命周期是何时执行的?
  • Activity对象何时创建的?
  • 视图是怎么处理以及何时可见的?

那么为啥需要了解这些问题呢?不了解 ,平时开发好像也没啥问题啊。其实不然,解决这些问题后,你会对Android系统有更深层次的理解,也会学习到系统源码优秀的设计。并且对解决一些高级问题和深入的性能优化问题有很大帮助,是技术进阶的必要阶段。这就需要我们通过阅读源码来梳理这些问题,但另一方面,系统源码是很庞大繁杂的,我们需要带着问题抓住 主流程 ,不能陷入代码细节——这是阅读系统源码以及其他第三方库源码的正确姿势。

流程分析

Activity启动的发起

下面我们就来对Activity的工作流程进行梳理,达到对Activity整体流程的掌握。从startActivity方法开始,会走到startActivityForResult方法:

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }
            if (requestCode >= 0) {
                mStartedActivity = true;
            }

            cancelInputsAndStartExitTransition(options);
        } else {
            ...
        }
    }
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看到里面调用了mInstrumentation.execStartActivity方法,其中一个参数mMainThread.getApplicationThread(),它的类型是ApplicationThread, ApplicationThread是ActivityThread的内部类,继承IApplicationThread.Stub,也是个Binder对象 ,在Activity工作流程中有重要作用。而Instrumentation具有跟踪application及activity生命周期的功能,用于android 应用测试框架中代码检测。接着看下mInstrumentation.execStartActivity方法:

public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        Uri referrer = target != null ? target.onProvideReferrer() : null;
        if (referrer != null) {
            intent.putExtra(Intent.EXTRA_REFERRER, referrer);
        }
        ...
        
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            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);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }
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这里看到Activity的启动又交给了ActivityManager.getService(),这是啥?跟进去看看:

public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }
    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };
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看到这里你应该明白了:这里是获取一个 跨进程 的服务。然后我们看下著名的ActivityManagerService类

public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback
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ActivityManagerService继承IActivityManager.Stub,实际也就是继承了Binder并且实现了IActivityManager这个Binder接口,AMS也就是一个Binder,是IActivityManager的集体实现。所以ActivityManager.getService()获取的Binder对象,具体实现是ActivityManagerService(AMS),并且是通过单例提供服务的。

然后ActivityManager.getService().startActivity有个返回值result,且调用了checkStartActivityResult(result, intent):

public static void checkStartActivityResult(int res, Object intent) {
        if (!ActivityManager.isStartResultFatalError(res)) {
            return;
        }

        switch (res) {
            case ActivityManager.START_INTENT_NOT_RESOLVED:
            case ActivityManager.START_CLASS_NOT_FOUND:
                if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
                    throw new ActivityNotFoundException(
                            "Unable to find explicit activity class "
                            + ((Intent)intent).getComponent().toShortString()
                            + "; have you declared this activity in your AndroidManifest.xml?");
                throw new ActivityNotFoundException(
                        "No Activity found to handle " + intent);
            case ActivityManager.START_PERMISSION_DENIED:
                throw new SecurityException("Not allowed to start activity "
                        + intent);
            ...
            
            case ActivityManager.START_CANCELED:
                throw new AndroidRuntimeException("Activity could not be started for "
                        + intent);
            default:
                throw new AndroidRuntimeException("Unknown error code "
                        + res + " when starting " + intent);
        }
    }
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这是用来检查Activity启动的结果,如果发生致命错误,就会抛出对应的异常。看到第一个case中就抛出了 have you declared this activity in your AndroidManifest.xml?——如果Activity没在Manifest中注册就会有这个错误。

Activity的管理——AMS

好了,到这里,Activity的启动就转移到系统进程提供的服务AMS中了,接着看AMS的startActivity:

@Override
    public int startActivity(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
        return mActivityTaskManager.startActivity(caller, callingPackage, intent, resolvedType,
                resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions);
    }
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AMS把启动转移到了ActivityTaskManagerService(ATMS)中了,ATMS用于管理Activity及其容器(任务、堆栈、显示等)的系统服务。接着看:

//ActivityTaskManagerService
    @Override
    public final int startActivity(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions,
                UserHandle.getCallingUserId());
    }
    
    @Override
    public int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
                true /*validateIncomingUser*/);
    }

    int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId,
            boolean validateIncomingUser) {
        enforceNotIsolatedCaller("startActivityAsUser");

        userId = getActivityStartController().checkTargetUser(userId, validateIncomingUser,
                Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");

        // TODO: Switch to user app stacks here.
        return getActivityStartController().obtainStarter(intent, "startActivityAsUser")
                .setCaller(caller)
                .setCallingPackage(callingPackage)
                .setResolvedType(resolvedType)
                .setResultTo(resultTo)
                .setResultWho(resultWho)
                .setRequestCode(requestCode)
                .setStartFlags(startFlags)
                .setProfilerInfo(profilerInfo)
                .setActivityOptions(bOptions)
                .setMayWait(userId)
                .execute();

    }
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跟到startActivityAsUser中,通过getActivityStartController().obtainStarter方法获取ActivityStarter实例 然后调用一系列方法,最后的execute()方法是开始启动activity:

int execute() {
        try {
            // TODO(b/64750076): Look into passing request directly to these methods to allow
            // for transactional diffs and preprocessing.
            if (mRequest.mayWait) {
                return startActivityMayWait(mRequest.caller, mRequest.callingUid,
                        mRequest.callingPackage, mRequest.realCallingPid, mRequest.realCallingUid,
                        mRequest.intent, mRequest.resolvedType,
                        mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
                        mRequest.resultWho, mRequest.requestCode, mRequest.startFlags,
                        mRequest.profilerInfo, mRequest.waitResult, mRequest.globalConfig,
                        mRequest.activityOptions, mRequest.ignoreTargetSecurity, mRequest.userId,
                        mRequest.inTask, mRequest.reason,
                        mRequest.allowPendingRemoteAnimationRegistryLookup,
                        mRequest.originatingPendingIntent, mRequest.allowBackgroundActivityStart);
            } else {
                return startActivity(mRequest.caller, mRequest.intent, mRequest.ephemeralIntent,
                        mRequest.resolvedType, mRequest.activityInfo, mRequest.resolveInfo,
                        mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
                        mRequest.resultWho, mRequest.requestCode, mRequest.callingPid,
                        mRequest.callingUid, mRequest.callingPackage, mRequest.realCallingPid,
                        mRequest.realCallingUid, mRequest.startFlags, mRequest.activityOptions,
                        mRequest.ignoreTargetSecurity, mRequest.componentSpecified,
                        mRequest.outActivity, mRequest.inTask, mRequest.reason,
                        mRequest.allowPendingRemoteAnimationRegistryLookup,
                        mRequest.originatingPendingIntent, mRequest.allowBackgroundActivityStart);
            }
        } finally {
            onExecutionComplete();
        }
    }
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分了两种情况,不过 不论startActivityMayWait还是startActivity最终都是走到下面这个startActivity方法:

private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
                IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
                int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
                ActivityRecord[] outActivity, boolean restrictedBgActivity) {
        int result = START_CANCELED;
        final ActivityStack startedActivityStack;
        try {
            mService.mWindowManager.deferSurfaceLayout();
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity, restrictedBgActivity);
        } finally {
            final ActivityStack currentStack = r.getActivityStack();
            startedActivityStack = currentStack != null ? currentStack : mTargetStack;

           ...
        }

        postStartActivityProcessing(r, result, startedActivityStack);
        return result;
    }
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里面有调用了startActivityUnchecked方法,startActivityUnchecked内部调用了ActivityStack的startActivityLocked方法,startActivityLocked内部调用ensureActivitiesVisibleLocked方法,ensureActivitiesVisibleLocked又调用makeVisibleAndRestartIfNeeded方法,来看下:

private boolean makeVisibleAndRestartIfNeeded(ActivityRecord starting, int configChanges,
            boolean isTop, boolean andResume, ActivityRecord r) {
        // We need to make sure the app is running if it's the top, or it is just made visible from
        // invisible. If the app is already visible, it must have died while it was visible. In this
        // case, we'll show the dead window but will not restart the app. Otherwise we could end up
        // thrashing.
        if (isTop || !r.visible) {
            // This activity needs to be visible, but isn't even running...
            // get it started and resume if no other stack in this stack is resumed.
            if (DEBUG_VISIBILITY) Slog.v(TAG_VISIBILITY, "Start and freeze screen for " + r);
            if (r != starting) {
                r.startFreezingScreenLocked(r.app, configChanges);
            }
            if (!r.visible || r.mLaunchTaskBehind) {
                if (DEBUG_VISIBILITY) Slog.v(TAG_VISIBILITY, "Starting and making visible: " + r);
                r.setVisible(true);
            }
            if (r != starting) {
                // We should not resume activities that being launched behind because these
                // activities are actually behind other fullscreen activities, but still required
                // to be visible (such as performing Recents animation).
                mStackSupervisor.startSpecificActivityLocked(r, andResume && !r.mLaunchTaskBehind,
                        true /* checkConfig */);
                return true;
            }
        }
        return false;
    }
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看到最后调用了ActivityStackSupervisor的startSpecificActivityLocked方法:

void startSpecificActivityLocked(ActivityRecord r, boolean andResume, boolean checkConfig) {
        // Is this activity's application already running?
        final WindowProcessController wpc =
                mService.getProcessController(r.processName, r.info.applicationInfo.uid);

        boolean knownToBeDead = false;
        if (wpc != null && wpc.hasThread()) {
            try {
                realStartActivityLocked(r, wpc, andResume, checkConfig);
                return;
            } catch (RemoteException e) {
                Slog.w(TAG, "Exception when starting activity "
                        + r.intent.getComponent().flattenToShortString(), e);
            }

            // If a dead object exception was thrown -- fall through to
            // restart the application.
            knownToBeDead = true;
        }

        ...
    }
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接着调用了realStartActivityLocked方法:

boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
            boolean andResume, boolean checkConfig) throws RemoteException {
	
			...

                // Create activity launch transaction.
                final ClientTransaction clientTransaction = ClientTransaction.obtain(
                        proc.getThread(), r.appToken);

                final DisplayContent dc = r.getDisplay().mDisplayContent;
                clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
                        System.identityHashCode(r), r.info,
                        // TODO: Have this take the merged configuration instead of separate global
                        // and override configs.
                        mergedConfiguration.getGlobalConfiguration(),
                        mergedConfiguration.getOverrideConfiguration(), r.compat,
                        r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
                        r.icicle, r.persistentState, results, newIntents,
                        dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
                                r.assistToken));

                // Set desired final state.
                final ActivityLifecycleItem lifecycleItem;
                if (andResume) {
                    lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
                } else {
                    lifecycleItem = PauseActivityItem.obtain();
                }
                clientTransaction.setLifecycleStateRequest(lifecycleItem);

                // Schedule transaction.
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);

                ...

        return true;
    }
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中间有段代码如上,通过 ClientTransaction.obtain( proc.getThread(), r.appToken)获取了clientTransaction,其中参数proc.getThread()是IApplicationThread,就是前面提到的ApplicationThread在系统进程的代理。

ClientTransaction是包含一系列的message的容器,message用于 发送到客户端 ,包含回调方法和生命周期状态。

接着看,使用clientTransaction.addCallback添加了LaunchActivityItem实例:

//都是用来发送到客户端的
	private List<ClientTransactionItem> mActivityCallbacks;
	
    public void addCallback(ClientTransactionItem activityCallback) {
        if (mActivityCallbacks == null) {
            mActivityCallbacks = new ArrayList<>();
        }
        mActivityCallbacks.add(activityCallback);
    }
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看下LaunchActivityItem实例的获取:

/** Obtain an instance initialized with provided params. */
    public static LaunchActivityItem obtain(Intent intent, int ident, ActivityInfo info,
            Configuration curConfig, Configuration overrideConfig, CompatibilityInfo compatInfo,
            String referrer, IVoiceInteractor voiceInteractor, int procState, Bundle state,
            PersistableBundle persistentState, List<ResultInfo> pendingResults,
            List<ReferrerIntent> pendingNewIntents, boolean isForward, ProfilerInfo profilerInfo,
            IBinder assistToken) {
        LaunchActivityItem instance = ObjectPool.obtain(LaunchActivityItem.class);
        if (instance == null) {
            instance = new LaunchActivityItem();
        }
        setValues(instance, intent, ident, info, curConfig, overrideConfig, compatInfo, referrer,
                voiceInteractor, procState, state, persistentState, pendingResults,
                pendingNewIntents, isForward, profilerInfo, assistToken);

        return instance;
    }
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new了一个LaunchActivityItem然后设置各种值。我们从名字就能看出,它就是用来启动activity的。它是怎么发挥作用的呢?接着看:

回到realStartActivityLocked方法,接着调用了mService.getLifecycleManager().scheduleTransaction(clientTransaction),mService是前面提到了ActivityTaskManagerService,getLifecycleManager()方法获取的是ClientLifecycleManager实例,它的scheduleTransaction方法如下:

void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        transaction.schedule();
        if (!(client instanceof Binder)) {
            transaction.recycle();
        }
    }
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就是调用ClientTransaction的schedule方法,那就看看:

public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);
    }
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很简单,就是调用IApplicationThread的scheduleTransaction方法。由于IApplicationThread是ApplicationThread在系统进程的代理,所以真正执行的地方就是 客户端的ApplicationThread中了。也就是说, Activity启动的操作又跨进程的还给了客户端

好了,到这里我们稍稍梳理下:启动Activity的操作从客户端 跨进程 转移到 AMS,AMS通过ActivityTaskManagerService、ActivityStarter、ActivityStack、ActivityStackSupervisor 对 Activity任务、activity栈、Activity记录 管理后,又用过跨进程把正在启动过程又转移到了客户端。

线程切换及消息处理——mH

接着上面的分析,我们找到ApplicationThread的scheduleTransaction方法:

@Override
        public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
            ActivityThread.this.scheduleTransaction(transaction);
        }
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那就再看ActivityThread的scheduleTransaction方法,实际在其父类ClientTransactionHandler中:

void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
    }
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使用sendMessage发送消息,参数是ActivityThread.H.EXECUTE_TRANSACTION和transaction,接着看:

void sendMessage(int what, Object obj) {
        sendMessage(what, obj, 0, 0, false);
    }
    private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
        if (DEBUG_MESSAGES) {
            Slog.v(TAG,
                    "SCHEDULE " + what + " " + mH.codeToString(what) + ": " + arg1 + " / " + obj);
        }
        Message msg = Message.obtain();
        msg.what = what;
        msg.obj = obj;
        msg.arg1 = arg1;
        msg.arg2 = arg2;
        if (async) {
            msg.setAsynchronous(true);
        }
        mH.sendMessage(msg);
    }
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最后调用了mH.sendMessage(msg),mH是个啥?我们看看:

//ActivityThread
final H mH = new H();

    class H extends Handler {
        public static final int BIND_APPLICATION        = 110;
        @UnsupportedAppUsage
        public static final int EXIT_APPLICATION        = 111;
        @UnsupportedAppUsage
        public static final int RECEIVER                = 113;
        @UnsupportedAppUsage
        public static final int CREATE_SERVICE          = 114;
        @UnsupportedAppUsage
        public static final int SERVICE_ARGS            = 115;
        @UnsupportedAppUsage
        public static final int STOP_SERVICE            = 116;

        public static final int CONFIGURATION_CHANGED   = 118;
        ...
        @UnsupportedAppUsage
        public static final int BIND_SERVICE            = 121;
        @UnsupportedAppUsage
        public static final int UNBIND_SERVICE          = 122;
        ...
        
        public static final int EXECUTE_TRANSACTION = 159;
        
        public static final int RELAUNCH_ACTIVITY = 160;
        public static final int PURGE_RESOURCES = 161;

        public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
            switch (msg.what) {
                case BIND_APPLICATION:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
                    AppBindData data = (AppBindData)msg.obj;
                    handleBindApplication(data);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case EXIT_APPLICATION:
                    if (mInitialApplication != null) {
                        mInitialApplication.onTerminate();
                    }
                    Looper.myLooper().quit();
                    break;
                case RECEIVER:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastReceiveComp");
                    handleReceiver((ReceiverData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CREATE_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
                    handleCreateService((CreateServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case BIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
                    handleBindService((BindServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case UNBIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceUnbind");
                    handleUnbindService((BindServiceData)msg.obj);
                    schedulePurgeIdler();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SERVICE_ARGS:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceStart: " + String.valueOf(msg.obj)));
                    handleServiceArgs((ServiceArgsData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case STOP_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStop");
                    handleStopService((IBinder)msg.obj);
                    schedulePurgeIdler();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CONFIGURATION_CHANGED:
                    handleConfigurationChanged((Configuration) msg.obj);
                    break;
                    
                ...
                
                case EXECUTE_TRANSACTION:
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;
                    mTransactionExecutor.execute(transaction);
                    if (isSystem()) {
                        // Client transactions inside system process are recycled on the client side
                        // instead of ClientLifecycleManager to avoid being cleared before this
                        // message is handled.
                        transaction.recycle();
                    }
                    // TODO(lifecycler): Recycle locally scheduled transactions.
                    break;
                case RELAUNCH_ACTIVITY:
                    handleRelaunchActivityLocally((IBinder) msg.obj);
                    break;
                case PURGE_RESOURCES:
                    schedulePurgeIdler();
                    break;
            }
            Object obj = msg.obj;
            if (obj instanceof SomeArgs) {
                ((SomeArgs) obj).recycle();
            }
            if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
        }
    }
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mH是在创建ActivityThread实例时赋值的,是自定义 Handler 子类H的实例,也就是在ActivityThread的main方法中,并且初始化是已经主线程已经有了mainLooper,所以, 使用这个mH来sendMessage就把消息发送到了主线程

那么是从哪个线程发送的呢?那就要看看ApplicationThread的scheduleTransaction方法是执行在哪个线程了。根据 IPC 知识,我们知道,服务器的Binder方法运行在Binder的线程池中,也就是说系统进行跨进程调用ApplicationThread的scheduleTransaction就是执行在Binder的线程池中的了。

到这里,消息就在主线程处理了,那么是怎么处理Activity的启动的呢?接着看。我们找到ActivityThread.H.EXECUTE_TRANSACTION这个消息的处理,就在handleMessage方法的倒数第三个case(就在上面代码):取出ClientTransaction实例,调用TransactionExecutor的execute方法,那就看看:

public void execute(ClientTransaction transaction) {
        if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Start resolving transaction");

        final IBinder token = transaction.getActivityToken();
        ...
        executeCallbacks(transaction);

        executeLifecycleState(transaction);
        ...
    }
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继续跟进executeCallbacks方法:

public void executeCallbacks(ClientTransaction transaction) {
        final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
        if (callbacks == null || callbacks.isEmpty()) {
            // No callbacks to execute, return early.
            return;
        }
        if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Resolving callbacks in transaction");

        final IBinder token = transaction.getActivityToken();
        ActivityClientRecord r = mTransactionHandler.getActivityClient(token);

        // In case when post-execution state of the last callback matches the final state requested
        // for the activity in this transaction, we won't do the last transition here and do it when
        // moving to final state instead (because it may contain additional parameters from server).
        final ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();
        final int finalState = finalStateRequest != null ? finalStateRequest.getTargetState()
                : UNDEFINED;
        // Index of the last callback that requests some post-execution state.
        final int lastCallbackRequestingState = lastCallbackRequestingState(transaction);

        final int size = callbacks.size();
        for (int i = 0; i < size; ++i) {
            final ClientTransactionItem item = callbacks.get(i);
            ...
            item.execute(mTransactionHandler, token, mPendingActions);
            item.postExecute(mTransactionHandler, token, mPendingActions);
            ...
        }
    }
复制代码

遍历callbacks,调用ClientTransactionItem的execute方法,而我们这里要关注的是ClientTransactionItem的子类LaunchActivityItem,看下它的execute方法:

public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
        ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
                mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
                mPendingResults, mPendingNewIntents, mIsForward,
                mProfilerInfo, client, mAssistToken);
        client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
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里面调用了client.handleLaunchActivity方法,client是ClientTransactionHandler的实例,是在TransactionExecutor构造方法传入的,TransactionExecutor创建是在ActivityThread中:

//ActivityThread
private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);
复制代码

所以,client.handleLaunchActivity方法就是ActivityThread的handleLaunchActivity方法。

好了,到这里 ApplicationThread把启动Activity的操作,通过mH切到了主线程,走到了ActivityThread的handleLaunchActivity方法

Activity启动核心实现——初始化及生命周期

那就接着看:

public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
        ...
        final Activity a = performLaunchActivity(r, customIntent);
        ...
        return a;
    }
复制代码

继续跟 performLaunchActivity方法,这里就是activity 启动的核心实现 了:

/**  activity 启动的核心实现. */
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    	//1、从ActivityClientRecord获取待启动的Activity的组件信息
        ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

        ComponentName component = r.intent.getComponent();
        if (component == null) {
            component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
            r.intent.setComponent(component);
        }

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }
		//创建ContextImpl对象
        ContextImpl appContext = createBaseContextForActivity(r);
        Activity activity = null;
        try {
        	//2、创建activity实例
            java.lang.ClassLoader cl = appContext.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            StrictMode.incrementExpectedActivityCount(activity.getClass());
            r.intent.setExtrasClassLoader(cl);
            r.intent.prepareToEnterProcess();
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
            ..
        }
        try {
        	//3、创建Application对象(如果没有的话)
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);
            ...
            if (activity != null) {
                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
                Configuration config = new Configuration(mCompatConfiguration);
                if (r.overrideConfig != null) {
                    config.updateFrom(r.overrideConfig);
                }
              
                Window window = null;
                if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                    window = r.mPendingRemoveWindow;
                    r.mPendingRemoveWindow = null;
                    r.mPendingRemoveWindowManager = null;
                }
                appContext.setOuterContext(activity);
                
                //4、attach方法为activity关联上下文环境
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor, window, r.configCallback,
                        r.assistToken);

                if (customIntent != null) {
                    activity.mIntent = customIntent;
                }
                r.lastNonConfigurationInstances = null;
                checkAndBlockForNetworkAccess();
                activity.mStartedActivity = false;
                int theme = r.activityInfo.getThemeResource();
                if (theme != 0) {
                    activity.setTheme(theme);
                }

                activity.mCalled = false;
                
                //5、调用生命周期onCreate
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                        " did not call through to super.onCreate()");
                }
                r.activity = activity;
            }
            r.setState(ON_CREATE);
            
            synchronized (mResourcesManager) {
                mActivities.put(r.token, r);
            }

        } 
        ...

        return activity;
    }
复制代码

performLaunchActivity主要完成以下事情:

  1. 从ActivityClientRecord获取待启动的Activity的组件信息
  2. 通过mInstrumentation.newActivity方法使用类加载器创建activity实例
  3. 通过LoadedApk的makeApplication方法创建Application对象,内部也是通过mInstrumentation使用类加载器,创建后就调用了instrumentation.callApplicationOnCreate方法,也就是Application的onCreate方法。
  4. 创建ContextImpl对象并通过activity.attach方法对重要数据初始化,关联了Context的具体实现ContextImpl,attach方法内部还完成了window创建,这样Window接收到外部事件后就能传递给Activity了。
  5. 调用Activity的onCreate方法,是通过 mInstrumentation.callActivityOnCreate方法完成。

到这里Activity的onCreate方法执行完,那么onStart、onResume呢?

上面看到LaunchActivityItem,是用来启动Activity的,也就是走到Activity的onCreate,那么是不是有 "XXXActivityItem"呢? 有的:

  • LaunchActivityItem 远程App端的onCreate生命周期事务
  • ResumeActivityItem 远程App端的onResume生命周期事务
  • PauseActivityItem 远程App端的onPause生命周期事务
  • StopActivityItem 远程App端的onStop生命周期事务
  • DestroyActivityItem 远程App端onDestroy生命周期事务

另外梳理过程中涉及的几个类:

  • ClientTransaction 客户端事务控制者
  • ClientLifecycleManager 客户端的生命周期事务控制者
  • TransactionExecutor 远程通信事务执行者

那么我们再来看看ResumeActivityItem吧。

我们再来重新看看在ActivityStackSupervisor的realStartActivityLocked方法:

boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
            boolean andResume, boolean checkConfig) throws RemoteException {
			...
                // Create activity launch transaction.
                final ClientTransaction clientTransaction = ClientTransaction.obtain(
                        proc.getThread(), r.appToken);

                final DisplayContent dc = r.getDisplay().mDisplayContent;
                clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
                        System.identityHashCode(r), r.info,
                        mergedConfiguration.getGlobalConfiguration(),
                        mergedConfiguration.getOverrideConfiguration(), r.compat,
                        r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
                        r.icicle, r.persistentState, results, newIntents,
                        dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
                                r.assistToken));

                // Set desired final state.
                final ActivityLifecycleItem lifecycleItem;
                //这里ResumeActivityItem
                if (andResume) {
                    lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
                } else {
                    lifecycleItem = PauseActivityItem.obtain();
                }
                clientTransaction.setLifecycleStateRequest(lifecycleItem);

                // Schedule transaction.
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);

                ...

        return true;
    }
复制代码

前面只说了通过clientTransaction.addCallback添加LaunchActivityItem实例,在注意下面接着调用了clientTransaction.setLifecycleStateRequest(lifecycleItem)方法,lifecycleItem是ResumeActivityItem或PauseActivityItem实例,这里我们关注ResumeActivityItem,先看下setLifecycleStateRequest方法:

/**
     * Final lifecycle state in which the client activity should be after the transaction is
     * executed.
     */
	private ActivityLifecycleItem mLifecycleStateRequest;
	
    public void setLifecycleStateRequest(ActivityLifecycleItem stateRequest) {
        mLifecycleStateRequest = stateRequest;
    }
复制代码

mLifecycleStateRequest表示执行transaction后的最终的生命周期状态。

继续看处理ActivityThread.H.EXECUTE_TRANSACTION这个消息的处理,即TransactionExecutor的execute方法:

public void execute(ClientTransaction transaction) {
        if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Start resolving transaction");

        final IBinder token = transaction.getActivityToken();
        ...
        executeCallbacks(transaction);

        executeLifecycleState(transaction);
        ...
    }
复制代码

前面我们关注的是executeCallbacks方法,现在看看executeLifecycleState方法:

/** Transition to the final state if requested by the transaction. */
    private void executeLifecycleState(ClientTransaction transaction) {
        final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();
        if (lifecycleItem == null) {
            // No lifecycle request, return early.
            return;
        }

        final IBinder token = transaction.getActivityToken();
        final ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
        ...

        // Execute the final transition with proper parameters.
        lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
        lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
    }
复制代码

这里取出了ActivityLifecycleItem并且调用了它的execute方法,实际就是ResumeActivityItem的方法:

@Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
        client.handleResumeActivity(token, true /* finalStateRequest */, mIsForward,
                "RESUME_ACTIVITY");
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
复制代码

经过上面分析实际是走到ActivityThread的handleResumeActivity方法:

@Override
    public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
            String reason) {
        ...
        // performResumeActivity内部会走onStart、onResume
        final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
        if (r == null) {
            // We didn't actually resume the activity, so skipping any follow-up actions.
            return;
        }
        ...
        
        if (r.window == null && !a.mFinished && willBeVisible) {
            r.window = r.activity.getWindow();
            View decor = r.window.getDecorView();
            decor.setVisibility(View.INVISIBLE);
            ViewManager wm = a.getWindowManager();
            WindowManager.LayoutParams l = r.window.getAttributes();
            a.mDecor = decor;
            l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
            l.softInputMode |= forwardBit;
            if (r.mPreserveWindow) {
                a.mWindowAdded = true;
                r.mPreserveWindow = false;
                
                ViewRootImpl impl = decor.getViewRootImpl();
                if (impl != null) {
                    impl.notifyChildRebuilt();
                }
            }
            ...
            
        if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {
            if (r.newConfig != null) {
                performConfigurationChangedForActivity(r, r.newConfig);
                if (DEBUG_CONFIGURATION) {
                    Slog.v(TAG, "Resuming activity " + r.activityInfo.name + " with newConfig "
                            + r.activity.mCurrentConfig);
                }
                r.newConfig = null;
            }
            if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward=" + isForward);
            WindowManager.LayoutParams l = r.window.getAttributes();
            if ((l.softInputMode
                    & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
                    != forwardBit) {
                l.softInputMode = (l.softInputMode
                        & (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
                        | forwardBit;
                if (r.activity.mVisibleFromClient) {
                    ViewManager wm = a.getWindowManager();
                    View decor = r.window.getDecorView();
                    wm.updateViewLayout(decor, l);
                }
            }

            r.activity.mVisibleFromServer = true;
            mNumVisibleActivities++;
            if (r.activity.mVisibleFromClient) {
            	//添加window、设置可见
                r.activity.makeVisible();
            }
        }

        r.nextIdle = mNewActivities;
        mNewActivities = r;
        if (localLOGV) Slog.v(TAG, "Scheduling idle handler for " + r);
        Looper.myQueue().addIdleHandler(new Idler());
    }
复制代码

handleResumeActivity做了以下事情:

  1. 通过performResumeActivity方法,内部调用生命周期onStart、onResume(可以自行查看,这里不再扩展)
  2. 通过activity.makeVisible方法,添加window、设置可见。(所以视图的真正可见是在onResume方法之后)

好了,到这里就是真正创建完成并且可见了。

总结

关于Activity启动的流程的讲解,我们分成了几个阶段:启动的发起、AMS的管理、线程切换、启动核心实现,知道了启动过程经历了两次IPC,客户端到AMS、AMS到客户端,以及Activity创建和生命周期的执行。

.

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