Android 10.0 Activity的启动流程

本文主要学习记录，基于Android 10的源码，有错误欢迎指正，主要目的是梳理流程图。

Activity启动的主要流程

以进程为单位的调用栈图如下：

activity

1.activity中的startActivity方法最终都会通过拿到ATSM的代理IActivityTaskManager调用的startActivity；

2.之后进入system server进程中的ATMS startActivity，ATMS 经过收集Intent信息，然后使用ActivityStackSupervisor.startSpecificActivityLocked，如果进程已经存在，则直接使用realStartActivityLocked，通过App的binder客户端的代理ApplicationThread调用回到bindApplication，走入Activity的启动流程；如果进程不存在则通过socket链接Zygote，请求fork新的进程；

3.App进程创建完成后，进程启动会调用ActivityThread.main方法，初始化主线程Handler,接着走入attach方法，然后通过AMS的代理调用AMS的attachApplication方法，并将App进程的通信代理ApplicationThread传入AMS；

4.AMS获取到ATMS调用ApplicationThread的bindApplication回到App进程的ActivityThread.ApplicationThread.bindApplication方法中，然后使用Handler切换到主线程执行handleBindApplication，这里初始化了App的进程名字、时间，用户的硬件配置，包括App的文件系统，创建了App的Context实例，Instrumentation实例，调用App的onCreate回调方法，同时告诉AMS APP初始化工作完毕；

5.AMS接着会调用ATMS的attachApplication，最后调用ClientLifecycleManager的scheduleTransaction方法，通过App的Binder代理ApplicationThread回到ActivityThread；

6.进入ActivityThread.ApplicationThread.scheduleTransaction方法之后就进入了Activity的onStart、onResume回调

创建进程之前的过程主要是AMS的内部信息收集的判断的过程，下面主要看一下App进程启动的源码流程

APP进程起点

从应用进程被创建开始，ActivityThread.main被执行

public static void main(String[] args) {
    ActivityThread thread = new ActivityThread();
    thread.attach(false, startSeq);

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }

    if (false) {
        Looper.myLooper().setMessageLogging(new
                LogPrinter(Log.DEBUG, "ActivityThread"));
    }

    // End of event ActivityThreadMain.
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    Looper.loop();

    throw new RuntimeException("Main thread loop unexpectedly exited");
}

调用ActivityThread的attach方法，然后将activity和AMS通信的Binder代理IApplicationThread实例传入AMS

@UnsupportedAppUsage
private void attach(boolean system, long startSeq) {
    sCurrentActivityThread = this;
    mSystemThread = system;
    if (!system) {
        android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
                                                UserHandle.myUserId());
        RuntimeInit.setApplicationObject(mAppThread.asBinder());
        final IActivityManager mgr = ActivityManager.getService();
        try {
            mgr.attachApplication(mAppThread, startSeq);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }
        // Watch for getting close to heap limit.
        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();
                if (dalvikUsed > ((3*dalvikMax)/4)) {
                    if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
                            + " total=" + (runtime.totalMemory()/1024)
                            + " used=" + (dalvikUsed/1024));
                    mSomeActivitiesChanged = false;
                    try {
                        ActivityTaskManager.getService().releaseSomeActivities(mAppThread);
                    } catch (RemoteException e) {
                        throw e.rethrowFromSystemServer();
                    }
                }
            }
        });
    } 
}

AMS的attachApplication

接着进入AMS进程，ActivityManagerService.attachApplicationLocked

   private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid, int callingUid, long startSeq) {

            if (app.isolatedEntryPoint != null) {
                // This is an isolated process which should just call an entry point instead of
                // being bound to an application.
                thread.runIsolatedEntryPoint(app.isolatedEntryPoint, app.isolatedEntryPointArgs);
            } else if (instr2 != null) {
                thread.bindApplication(processName, appInfo, providers,
                        instr2.mClass,
                        profilerInfo, instr2.mArguments,
                        instr2.mWatcher,
                        instr2.mUiAutomationConnection, testMode,
                        mBinderTransactionTrackingEnabled, enableTrackAllocation,
                        isRestrictedBackupMode || !normalMode, app.isPersistent(),
                        new Configuration(app.getWindowProcessController().getConfiguration()),
                        app.compat, getCommonServicesLocked(app.isolated),
                        mCoreSettingsObserver.getCoreSettingsLocked(),
                        buildSerial, autofillOptions, contentCaptureOptions);
            } else {
                thread.bindApplication(processName, appInfo, providers, null, profilerInfo,
                        null, null, null, testMode,
                        mBinderTransactionTrackingEnabled, enableTrackAllocation,
                        isRestrictedBackupMode || !normalMode, app.isPersistent(),
                        new Configuration(app.getWindowProcessController().getConfiguration()),
                        app.compat, getCommonServicesLocked(app.isolated),
                        mCoreSettingsObserver.getCoreSettingsLocked(),
                        buildSerial, autofillOptions, contentCaptureOptions);
            }

        } catch (Exception e) {
            
        }


        // See if the top visible activity is waiting to run in this process...
        if (normalMode) {
            try {
                didSomething = mAtmInternal.attachApplication(app.getWindowProcessController());
            } catch (Exception e) {
                Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
                badApp = true;
            }
        }

        // Find any services that should be running in this process...
        if (!badApp) {
            try {
                didSomething |= mServices.attachApplicationLocked(app, processName);
                checkTime(startTime, "attachApplicationLocked: after mServices.attachApplicationLocked");
            } catch (Exception e) {
                Slog.wtf(TAG, "Exception thrown starting services in " + app, e);
                badApp = true;
            }
        }

      
        return true;
    }

1.thread.bindApplication：该方法主要讲App进程的配置信息通过IApplicationThread Binder通信回传到ActivityThread中

2.mAtmInternal.attachApplication：mAtmInternal实际就是ActivityTaskManager的实例，通过LocalServices加载

mAtmInternal = LocalServices.getService(ActivityTaskManagerInternal.class);

那么这里相当于走到了ActivityTaskManagerServer的attachApplication中

APP的bindApplication

先看第一条：

注意：ActivityThread中存在于Binder通信的代理--》ApplicationThread extends IApplicationThread.Stub

ActivityThread--》ApplicationThread--》bindApplication

@Override
        public final void bindApplication(String processName, ApplicationInfo appInfo,
                ProviderInfoList providerList, ComponentName instrumentationName,
                ProfilerInfo profilerInfo, Bundle instrumentationArgs,
                IInstrumentationWatcher instrumentationWatcher,
                IUiAutomationConnection instrumentationUiConnection, int debugMode,
                boolean enableBinderTracking, boolean trackAllocation,
                boolean isRestrictedBackupMode, boolean persistent, Configuration config,
                CompatibilityInfo compatInfo, Map services, Bundle coreSettings,
                String buildSerial, AutofillOptions autofillOptions,
                ContentCaptureOptions contentCaptureOptions, long[] disabledCompatChanges) {

            setCoreSettings(coreSettings);

            AppBindData data = new AppBindData();
            data.processName = processName;
            .....
            sendMessage(H.BIND_APPLICATION, data);
        }

这里的bindApplication主要初始化了AppBindData，然后发送BIND_APPLICATION给APP的主线程BIND_APPLICATION，最后执行了handleBindApplication

handleBindApplication如下：

ActivityThread--》class H extends Handler

 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;

Application的onCreate方法

private void handleBindApplication(AppBindData data) {
          mBoundApplication = data;
        mConfiguration = new Configuration(data.config);
        mCompatConfiguration = new Configuration(data.config);
         if (data.initProfilerInfo != null) {
            mProfiler.profileFile = data.initProfilerInfo.profileFile;
            mProfiler.profileFd = data.initProfilerInfo.profileFd;
            mProfiler.samplingInterval = data.initProfilerInfo.samplingInterval;
            mProfiler.autoStopProfiler = data.initProfilerInfo.autoStopProfiler;
            mProfiler.streamingOutput = data.initProfilerInfo.streamingOutput;
            if (data.initProfilerInfo.attachAgentDuringBind) {
                agent = data.initProfilerInfo.agent;
            }
        }
mInstrumentationPackageName = ii.packageName;
            mInstrumentationAppDir = ii.sourceDir;
            mInstrumentationSplitAppDirs = ii.splitSourceDirs;
            mInstrumentationLibDir = getInstrumentationLibrary(data.appInfo, ii);
            mInstrumentedAppDir = data.info.getAppDir();
            mInstrumentedSplitAppDirs = data.info.getSplitAppDirs();
            mInstrumentedLibDir = data.info.getLibDir();
             mInstrumentation.callApplicationOnCreate(app);

}

该方法主要在App进程中对App的一些硬件资源配置申请的属性、App的文件夹等完成App基本信息的初始化

ATMS的attachApplication

接着看第二条：mAtmInternal.attachApplication

mAtmInternal.attachApplication最终会调用mRootActivityContainer.attachApplication(wpc)

@HotPath(caller = HotPath.PROCESS_CHANGE)
        @Override
        public boolean attachApplication(WindowProcessController wpc) throws RemoteException {
            synchronized (mGlobalLockWithoutBoost) {
                return mRootActivityContainer.attachApplication(wpc);
            }
        }

RootActivityContainer.attachApplication

boolean attachApplication(WindowProcessController app) throws RemoteException {
    final String processName = app.mName;
    boolean didSomething = false;
    for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
        final ActivityDisplay display = mActivityDisplays.get(displayNdx);
        final ActivityStack stack = display.getFocusedStack();
        if (stack != null) {
            stack.getAllRunningVisibleActivitiesLocked(mTmpActivityList);
            //获取前台栈顶第一个非finishing的Activity
            final ActivityRecord top = stack.topRunningActivityLocked();
            final int size = mTmpActivityList.size();
            for (int i = 0; i < size; i++) {
                final ActivityRecord activity = mTmpActivityList.get(i);
                if (activity.app == null && app.mUid == activity.info.applicationInfo.uid
                        && processName.equals(activity.processName)) {
                    try {
                    //真正启动Activity
                        if (mStackSupervisor.realStartActivityLocked(activity, app,
                                top == activity /* andResume */, true /* checkConfig */)) {
                            didSomething = true;
                        }
                    } catch (RemoteException e) {
                        Slog.w(TAG, "Exception in new application when starting activity "
                                + top.intent.getComponent().flattenToShortString(), e);
                        throw e;
                    }
                }
            }
        }
    }
    if (!didSomething) {
        ensureActivitiesVisible(null, 0, false /* preserve_windows */);
    }
    return didSomething;
}

接着调用ActivityStackSupervisor.realStartActivityLocked开始创建Activity

ASS的realStartActivityLocked

ActivityStackSupervisor.realStartActivityLocked

final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {

        try {
            r.startFreezingScreenLocked(app, 0);
            //启动tick，收集应用启动慢的信息
            // schedule launch ticks to collect information about slow apps.
            r.startLaunchTickingLocked();

            r.setProcess(app);

            
            try {
                

                //创建Activity启动事务
                // Create activity launch transaction.
                final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
                        r.appToken);
                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, app.repProcState, r.icicle,
                        r.persistentState, results, newIntents, mService.isNextTransitionForward(),
                        profilerInfo));

                //设置目标事务的状态为onResume
                // Set desired final state.
                final ActivityLifecycleItem lifecycleItem;
                if (andResume) {
                    lifecycleItem = ResumeActivityItem.obtain(mService.isNextTransitionForward());
                } else {
                    lifecycleItem = PauseActivityItem.obtain();
                }
                clientTransaction.setLifecycleStateRequest(lifecycleItem);

                //通过transaciton方式开始activity生命周期，onCreate,onStart,onResume
                // Schedule transaction.
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);


                
            } catch (RemoteException e) {
               
            }
        } finally {
            endDeferResume();
        }

     
        return true;
    }

ClientLifecycleManager

创建ClientLifecycleManager和ClientTransactionHandler来辅助管理Activity的生命周期

注意

clientTransaction.addCallback是LaunchActivityItem

lifecycleItem是ResumeActivityItem

ClientLifecycleManager.scheduleTransaction最终会调用ClientTransaction的schedule方法

public void schedule() throws RemoteException {
    mClient.scheduleTransaction(this);
}

那么这个mClient是IApplicationThread的实例，那么此时也就回到了ActivityThread的ApplicationThread中

IApplicationThread getClient() {
    return mClient;
}

Activity的onCreate方法

APP的scheduleTransaction

ActivityThread的ApplicationThread中

 @Override
  public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        ActivityThread.this.scheduleTransaction(transaction);
  }

因为ActivityThread继承ClientTransactionHandler，所以到了ClientTransactionHandler中

 void scheduleTransaction(ClientTransaction transaction) {
      transaction.preExecute(this);
      sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
 }

通过Handler发送消息EXECUTE_TRANSACTION到H中

 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;

接着TransactionExecutor的execute方法

public void execute(ClientTransaction transaction) {
        final IBinder token = transaction.getActivityToken();
        log("Start resolving transaction for client: " + mTransactionHandler + ", token: " + token);
        
        executeCallbacks(transaction);
        
        executeLifecycleState(transaction);
        mPendingActions.clear();
        log("End resolving transaction");
    }

LaunchActivityItem.execute方法

 @Override
    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);
        
        client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }

client其实是在ActivityThread的实例，那么就回到了ActivityThread的handleLaunchActivity

 @Override
    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
            final Activity a = performLaunchActivity(r, customIntent);
            }

接着调用performLaunchActivity

 private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        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Activity的context实例
        ContextImpl appContext = createBaseContextForActivity(r);
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
            //创建Activity的实例
            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 {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);

            if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
            if (localLOGV) Slog.v(
                    TAG, r + ": app=" + app
                    + ", appName=" + app.getPackageName()
                    + ", pkg=" + r.packageInfo.getPackageName()
                    + ", comp=" + r.intent.getComponent().toShortString()
                    + ", dir=" + r.packageInfo.getAppDir());

            if (activity != null) {
               //获取Resources资源
                appContext.getResources().addLoaders(
                        app.getResources().getLoaders().toArray(new ResourcesLoader[0]));

                appContext.setOuterContext(activity);
                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 (r.isPersistable()) {
                //回调Activity的onCreat
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
              
               
            }
          

        } catch (SuperNotCalledException e) {
            throw e;

        } catch (Exception e) {
          
        }

        return activity;
    }

在performLaunchActivity中，主要是加载App的资源包，然后创建了Activity的context实例，并创建了Activity的实例，接着调用activity.attach方法，attach执行完之后调用了onCreate方法。

APP的activity.attach

activity.attach

final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor,
            Window window, ActivityConfigCallback activityConfigCallback, IBinder assistToken) {
        attachBaseContext(context);

        mFragments.attachHost(null /*parent*/);
        //创建了PhoneWindow实例
        mWindow = new PhoneWindow(this, window, activityConfigCallback);
        //设置了WindowControllerCallback监听
        mWindow.setWindowControllerCallback(this);
        //设置了WindowCallback监听
        mWindow.setCallback(this);
        
        mWindow.setOnWindowDismissedCallback(this);
        mWindow.getLayoutInflater().setPrivateFactory(this);
        if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
            mWindow.setSoftInputMode(info.softInputMode);
        }
        if (info.uiOptions != 0) {
            mWindow.setUiOptions(info.uiOptions);
        }
        //线程对象赋值，即ActivityThread main方法所在的线程
        mUiThread = Thread.currentThread();

//初始化Activity的成员变量，包括windowmanager
        mMainThread = aThread;
        mInstrumentation = instr;
        mToken = token;
        mAssistToken = assistToken;
        mIdent = ident;
        mApplication = application;
        mIntent = intent;
        mReferrer = referrer;
        mComponent = intent.getComponent();
        mActivityInfo = info;
        mTitle = title;
        mParent = parent;
        mEmbeddedID = id;
        mLastNonConfigurationInstances = lastNonConfigurationInstances;
        if (voiceInteractor != null) {
            if (lastNonConfigurationInstances != null) {
                mVoiceInteractor = lastNonConfigurationInstances.voiceInteractor;
            } else {
                mVoiceInteractor = new VoiceInteractor(voiceInteractor, this, this,
                        Looper.myLooper());
            }
        }

        mWindow.setWindowManager(
                (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        mWindowManager = mWindow.getWindowManager();
        mCurrentConfig = config;

        mWindow.setColorMode(info.colorMode);

        setAutofillOptions(application.getAutofillOptions());
        setContentCaptureOptions(application.getContentCaptureOptions());
    }

activity.attach中主要

1.创建了PhoneWindow实例

2.设置了Window接口的监听

3.初始化了成员变量，包括线程和WindowManager

Activity的onStart方法

到此Oncreate已经完成，那么OnStart和OnResume去哪了？

TransactionExecutor的execute方法

public void execute(ClientTransaction transaction) {
        final IBinder token = transaction.getActivityToken();
        log("Start resolving transaction for client: " + mTransactionHandler + ", token: " + token);
        
        executeCallbacks(transaction);
        
        executeLifecycleState(transaction);
        mPendingActions.clear();
        log("End resolving transaction");
    }

之前们只分析了executeCallbacks，接着executeLifecycleState方法

TransactionExecutor的executeLifecycleState方法

 private void executeLifecycleState(ClientTransaction transaction) {
        final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();
        if (lifecycleItem == null) {
            // No lifecycle request, return early.
            return;
        }
        log("Resolving lifecycle state: " + lifecycleItem);
 
        final IBinder token = transaction.getActivityToken();
        final ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
 
        if (r == null) {
            // Ignore requests for non-existent client records for now.
            return;
        }
 
        // Cycle to the state right before the final requested state.
        cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);
 
        // Execute the final transition with proper parameters.
        lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
        lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
}

cycleToPath

cycleToPath：lifecycleItem即为ResumeActivityItem

private void cycleToPath(ActivityClientRecord r, int finish,
            boolean excludeLastState) {
        final int start = r.getLifecycleState();
        log("Cycle from: " + start + " to: " + finish + " excludeLastState:" + excludeLastState);
        final IntArray path = mHelper.getLifecyclePath(start, finish, excludeLastState);
        performLifecycleSequence(r, path);
}

第一点：

int finish = lifecycleItem.getTargetState()

lifecycleItem对应ResumeActivityItem，如下：

ResumeActivityItem的getTargetState方法

@Override
public int getTargetState() {
    return ON_RESUME;
}

对应ActivityLifecycleItem中的枚举类型：

public static final int ON_RESUME = 3;

第二点：ActivityClientRecord中的mLifecycleState，由于在前面已经执行了handleLaunchActivity所以mLifecycleState=1

  @LifecycleState
   public static final int ON_CREATE = 1;

对应ActivityLifecycleItem中的枚举类型：

public abstract class ActivityLifecycleItem extends ClientTransactionItem {
    public static final int PRE_ON_CREATE = 0;
}

PRE_ON_CREATE = 0

所以final int star = 1

接着看getLifecyclePath，此时start=1，finish=3

 @VisibleForTesting
    public IntArray getLifecyclePath(int start, int finish, boolean excludeLastState) {
        if (start == UNDEFINED || finish == UNDEFINED) {
            throw new IllegalArgumentException("Can't resolve lifecycle path for undefined state");
        }
        if (start == ON_RESTART || finish == ON_RESTART) {
            throw new IllegalArgumentException(
                    "Can't start or finish in intermittent RESTART state");
        }
        if (finish == PRE_ON_CREATE && start != finish) {
            throw new IllegalArgumentException("Can only start in pre-onCreate state");
        }

        mLifecycleSequence.clear();
        if (finish >= start) {
            // just go there
            for (int i = start + 1; i <= finish; i++) {
                mLifecycleSequence.add(i);
            }
        } 
        if (excludeLastState && mLifecycleSequence.size() != 0) {
            mLifecycleSequence.remove(mLifecycleSequence.size() - 1);
        }

        return mLifecycleSequence;
    }

那么返回的IntArray就是2

public static final int ON_START = 2;

performLifecycleSequence

接着看performLifecycleSequence

 private void performLifecycleSequence(ActivityClientRecord r, IntArray path,
            ClientTransaction transaction) {
        final int size = path.size();
        for (int i = 0, state; i < size; i++) {
            state = path.get(i);
            if (DEBUG_RESOLVER) {
                Slog.d(TAG, tId(transaction) + "Transitioning activity: "
                        + getShortActivityName(r.token, mTransactionHandler)
                        + " to state: " + getStateName(state));
            }
            switch (state) {
                case ON_CREATE:
                    mTransactionHandler.handleLaunchActivity(r, mPendingActions,
                            null /* customIntent */);
                    break;
                case ON_START:
                    mTransactionHandler.handleStartActivity(r, mPendingActions);
                    break;
                case ON_RESUME:
                    mTransactionHandler.handleResumeActivity(r.token, false /* finalStateRequest */,
                            r.isForward, "LIFECYCLER_RESUME_ACTIVITY");
                    break;
                case ON_PAUSE:
                    mTransactionHandler.handlePauseActivity(r.token, false /* finished */,
                            false /* userLeaving */, 0 /* configChanges */, mPendingActions,
                            "LIFECYCLER_PAUSE_ACTIVITY");
                    break;
                case ON_STOP:
                    mTransactionHandler.handleStopActivity(r.token, false /* show */,
                            0 /* configChanges */, mPendingActions, false /* finalStateRequest */,
                            "LIFECYCLER_STOP_ACTIVITY");
                    break;
                case ON_DESTROY:
                    mTransactionHandler.handleDestroyActivity(r.token, false /* finishing */,
                            0 /* configChanges */, false /* getNonConfigInstance */,
                            "performLifecycleSequence. cycling to:" + path.get(size - 1));
                    break;
                case ON_RESTART:
                    mTransactionHandler.performRestartActivity(r.token, false /* start */);
                    break;
                default:
                    throw new IllegalArgumentException("Unexpected lifecycle state: " + state);
            }
        }
    }

ActivityThread.handleResumeActivity

最终执行的是handleStartActivity所以最终走到了ActivityThread的handleResumeActivity

两点：

调用activity.performStart

调用Instrumetation.callActivityOnPostCreate

Activity.performStart

performStart方法：

final void performStart(String reason) {
        mActivityTransitionState.setEnterActivityOptions(this, getActivityOptions());
        mFragments.noteStateNotSaved();
        mCalled = false;
        mFragments.execPendingActions();
        mInstrumentation.callActivityOnStart(this); //调用Instrumentation.callActivityOnStart
        writeEventLog(LOG_AM_ON_START_CALLED, reason);
 
        if (!mCalled) {
            throw new SuperNotCalledException(
                "Activity " + mComponent.toShortString() +
                " did not call through to super.onStart()");
        }
        mFragments.dispatchStart(); //Fragment的start操作
        mFragments.reportLoaderStart();
 
        boolean isAppDebuggable =
                (mApplication.getApplicationInfo().flags & ApplicationInfo.FLAG_DEBUGGABLE) != 0;
 
        // This property is set for all non-user builds except final release
        boolean isDlwarningEnabled = SystemProperties.getInt("ro.bionic.ld.warning", 0) == 1;
 
        ...
        mActivityTransitionState.enterReady(this);
    }

调用了Instrumentation.callActivityOnStart方法：

public void callActivityOnStart(Activity activity) {
        activity.onStart();

最终到了activity的onStart方法

第二点：Instrumentation.callActivityOnPostCreate

public void callActivityOnPostCreate(Activity activity, Bundle icicle) {
        activity.onPostCreate(icicle);
}

  @CallSuper
    protected void onPostCreate(@Nullable Bundle savedInstanceState) {
        if (!isChild()) {
            mTitleReady = true;
            onTitleChanged(getTitle(), getTitleColor()); //更新title
        }
 
        mCalled = true;
    }
 
    protected void onTitleChanged(CharSequence title, int color) {
        if (mTitleReady) {
            final Window win = getWindow();
            if (win != null) {
                win.setTitle(title);
                if (color != 0) {
                    win.setTitleColor(color);
                }
            }
            if (mActionBar != null) {
                mActionBar.setWindowTitle(title);
            }
        }
    }

Activity的onResume方法

上面主要走了cycleToPath，接着ResumeActivityItem.execute

ResumeActivityItem.execute

 @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);
}

调用了handleResumeActivity方法

ActivityThread.performResumeActivity

handleResumeActivity最终调用performResumeActivity

 @VisibleForTesting
    public ActivityClientRecord performResumeActivity(IBinder token, boolean finalStateRequest,
            String reason) {
        final ActivityClientRecord r = mActivities.get(token);
        if (localLOGV) {
            Slog.v(TAG, "Performing resume of " + r + " finished=" + r.activity.mFinished);
        }
        if (r == null || r.activity.mFinished) {
            return null;
        }
        if (r.getLifecycleState() == ON_RESUME) {
            if (!finalStateRequest) {
                final RuntimeException e = new IllegalStateException(
                        "Trying to resume activity which is already resumed");
                Slog.e(TAG, e.getMessage(), e);
                Slog.e(TAG, r.getStateString());
                // TODO(lifecycler): A double resume request is possible when an activity
                // receives two consequent transactions with relaunch requests and "resumed"
                // final state requests and the second relaunch is omitted. We still try to
                // handle two resume requests for the final state. For cases other than this
                // one, we don't expect it to happen.
            }
            return null;
        }
        if (finalStateRequest) {
            r.hideForNow = false;
            r.activity.mStartedActivity = false;
        }
        try {
            r.activity.onStateNotSaved();
            r.activity.mFragments.noteStateNotSaved();
            checkAndBlockForNetworkAccess();
            if (r.pendingIntents != null) {
                deliverNewIntents(r, r.pendingIntents);
                r.pendingIntents = null;
            }
            if (r.pendingResults != null) {
                deliverResults(r, r.pendingResults, reason);
                r.pendingResults = null;
            }
            r.activity.performResume(r.startsNotResumed, reason);

            r.state = null;
            r.persistentState = null;
            r.setState(ON_RESUME);

            reportTopResumedActivityChanged(r, r.isTopResumedActivity, "topWhenResuming");
        } catch (Exception e) {
            if (!mInstrumentation.onException(r.activity, e)) {
                throw new RuntimeException("Unable to resume activity "
                        + r.intent.getComponent().toShortString() + ": " + e.toString(), e);
            }
        }
        return r;
    }

Activity.performResume

final void performResume(boolean followedByPause, String reason) {
        performRestart(true /* start */, reason); //该函数中因为mStopped = false，所以实际没有什么操作
 
        mFragments.execPendingActions(); //Fragment相关的操作
 
        mLastNonConfigurationInstances = null;
 
        if (mAutoFillResetNeeded) { //自动填充
            // When Activity is destroyed in paused state, and relaunch activity, there will be
            // extra onResume and onPause event,  ignore the first onResume and onPause.
            // see ActivityThread.handleRelaunchActivity()
            mAutoFillIgnoreFirstResumePause = followedByPause;
            if (mAutoFillIgnoreFirstResumePause && DEBUG_LIFECYCLE) {
                Slog.v(TAG, "autofill will ignore first pause when relaunching " + this);
            }
        }
 
        mCalled = false;
        // mResumed is set by the instrumentation
        mInstrumentation.callActivityOnResume(this);  //【调用Instrumentation.callActivityOnResume执行onResume流程】
        
        ...
 
        mFragments.dispatchResume(); //Fragment相关的操作
        mFragments.execPendingActions();//Fragment相关的操作
 
        onPostResume();
        
        ...
}

调用了Instrumentation.callActivityOnResume，

public void callActivityOnResume(Activity activity) {
        activity.mResumed = true;
        activity.onResume();  //调用Activity的onResume函数
 
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    am.match(activity, activity, activity.getIntent());
                }
            }
        }
}

到了activity.onResume()方法