startActivity启动过程

代码来自Android 7.1.1版本，简单分析startActivity的启动过程

【1.1】
启动Activity，最终都会调到startActivityForResult，requestCode默认-1，options默认null

    //Activity.java
  public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            Instrumentation.ActivityResult ar =
                //启动Activty组件，以便它可以监听交互过程和生命周期，看【1.2】
                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) {
                // If this start is requesting a result, we can avoid making
                // the activity visible until the result is received.  Setting
                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
                // activity hidden during this time, to avoid flickering.
                // This can only be done when a result is requested because
                // that guarantees we will get information back when the
                // activity is finished, no matter what happens to it.
                mStartedActivity = true;
            }

            cancelInputsAndStartExitTransition(options);
            // TODO Consider clearing/flushing other event sources and events for child windows.
        } else {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                // Note we want to go through this method for compatibility with
                // existing applications that may have overridden it.
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

【1.2】
Instrumentation

//token是一个Binder代理对象，
public void execStartActivitiesAsUser(Context who, IBinder contextThread,
            IBinder token, Activity target, Intent[] intents, Bundle options,
            int userId) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    if (am.match(who, null, intents[0])) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return;
                        }
                        break;
                    }
                }
            }
        }
        try {
            String[] resolvedTypes = new String[intents.length];
            for (int i=0; i<intents.length; i++) {
                intents[i].migrateExtraStreamToClipData();
                intents[i].prepareToLeaveProcess(who);
                resolvedTypes[i] = intents[i].resolveTypeIfNeeded(who.getContentResolver());
            }
            //获取ActivityManagerService是代理对象，再调startActivities方法启动Activty
            int result = ActivityManagerNative.getDefault()
                .startActivities(whoThread, who.getBasePackageName(), intents, resolvedTypes,
                        token, options, userId);
            checkStartActivityResult(result, intents[0]);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
    }

**【1.3】**ActivityManagerProxy，通过Binder把参数传给ActivityManagerService

public int startActivities(IApplicationThread caller, String callingPackage,
            Intent[] intents, String[] resolvedTypes, IBinder resultTo,
            Bundle options, int userId) throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IActivityManager.descriptor);
        data.writeStrongBinder(caller != null ? caller.asBinder() : null);
        data.writeString(callingPackage);
        data.writeTypedArray(intents, 0);
        data.writeStringArray(resolvedTypes);
        data.writeStrongBinder(resultTo);
        if (options != null) {
            data.writeInt(1);
            options.writeToParcel(data, 0);
        } else {
            data.writeInt(0);
        }
        data.writeInt(userId);
        mRemote.transact(START_ACTIVITIES_TRANSACTION, data, reply, 0);
        reply.readException();
        int result = reply.readInt();
        reply.recycle();
        data.recycle();
        return result;
    }

**【1.4】**ActivityManagerService 通过Binder获取参数，然后调用startActivity

    @Override
    public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
            throws RemoteException {
        switch (code) {
        case START_ACTIVITY_TRANSACTION:
        {
            data.enforceInterface(IActivityManager.descriptor);
            IBinder b = data.readStrongBinder();
            IApplicationThread app = ApplicationThreadNative.asInterface(b);
            String callingPackage = data.readString();
            Intent intent = Intent.CREATOR.createFromParcel(data);
            String resolvedType = data.readString();
            IBinder resultTo = data.readStrongBinder();
            String resultWho = data.readString();
            int requestCode = data.readInt();
            int startFlags = data.readInt();
            ProfilerInfo profilerInfo = data.readInt() != 0
                    ? ProfilerInfo.CREATOR.createFromParcel(data) : null;
            Bundle options = data.readInt() != 0
                    ? Bundle.CREATOR.createFromParcel(data) : null;
            int result = startActivity(app, callingPackage, intent, resolvedType,
                    resultTo, resultWho, requestCode, startFlags, profilerInfo, options);
            reply.writeNoException();
            reply.writeInt(result);
            return true;
        }

【1.5】进入AMS层处理

    @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 final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
        enforceNotIsolatedCaller("startActivity");
        userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(),
                userId, false, ALLOW_FULL_ONLY, "startActivity", null);
        // TODO: Switch to user app stacks here.
        return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
                resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
                profilerInfo, null, null, bOptions, false, userId, null, null);
    }

【1.6】
ActivityStarter.startActivityMayWait

    //收集activity的信息
  ActivityInfo aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags, profilerInfo);
  ...
  启动Activty组件
   int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
                    aInfo, rInfo, voiceSession, voiceInteractor,
                    resultTo, resultWho, requestCode, callingPid,
                    callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
                    options, ignoreTargetSecurity, componentSpecified, outRecord, container,
                    inTask);

【1.7】
startActivityLocked

创建ActivityRecord，记录Activity的信息
ActivityRecord r = new ActivityRecord(mService, callerApp, callingUid, callingPackage,
                intent, resolvedType, aInfo, mService.mConfiguration, resultRecord, resultWho,
                requestCode, componentSpecified, voiceSession != null, mSupervisor, container,
                options, sourceRecord);

【1.8】
最终调用ActivityMetricsLogger.notifyActivityLaunched，通知应用Activty启动

    void notifyActivityLaunched(int resultCode, ActivityRecord launchedActivity) {
        final ProcessRecord processRecord = launchedActivity != null
                ? mSupervisor.mService.mProcessNames.get(launchedActivity.processName,
                        launchedActivity.appInfo.uid)
                : null;
        final boolean processRunning = processRecord != null;
        final String componentName = launchedActivity != null
                ? launchedActivity.shortComponentName
                : null;

        // We consider this a "process switch" if the process of the activity that gets launched
        // didn't have an activity that was in started state. In this case, we assume that lot
        // of caches might be purged so the time until it produces the first frame is very
        // interesting.
        final boolean processSwitch = processRecord == null
                || !hasStartedActivity(processRecord, launchedActivity);

        notifyActivityLaunched(resultCode, componentName, processRunning, processSwitch);
    }

【1.9】
回到应用进程，并且通过ActivityThread的Handler H加入主线程，调用handleLaunchActivity

 public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
            switch (msg.what) {
                case LAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
                    final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

                    r.packageInfo = getPackageInfoNoCheck(
                            r.activityInfo.applicationInfo, r.compatInfo);
                    handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    ......

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();
        mSomeActivitiesChanged = true;

        if (r.profilerInfo != null) {
            mProfiler.setProfiler(r.profilerInfo);
            mProfiler.startProfiling();
        }

        // Make sure we are running with the most recent config.
        handleConfigurationChanged(null, null);

        if (localLOGV) Slog.v(
            TAG, "Handling launch of " + r);

        // Initialize before creating the activity
        WindowManagerGlobal.initialize();

        Activity a = performLaunchActivity(r, customIntent);

        ......
    }

再调用performLaunchActivity方法

 private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ......
    //使用反射创建Activity实例
   activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);

    ......
    //初始化windows
    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);
    ......
    //接下来会调用Activty的OnCreate，完成Activity的启动
    if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }            

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) {
        attachBaseContext(context);

        mFragments.attachHost(null /*parent*/);

        mWindow = new PhoneWindow(this, window);
        .....
        //创建一个新的WindowManagerImpl来控制页面
        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;
    }

      public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
            boolean hardwareAccelerated) {
        mAppToken = appToken;
        mAppName = appName;
        mHardwareAccelerated = hardwareAccelerated
                || SystemProperties.getBoolean(PROPERTY_HARDWARE_UI, false);
        if (wm == null) {
            wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
        }
        mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
    }

    public WindowManagerImpl createLocalWindowManager(Window parentWindow) {
        //一个Activty对应一个WindowManagerImpl
        return new WindowManagerImpl(mContext, parentWindow);
    }

总结

调用startActivity方法后，首先收集Activity的启动信息，然后传入ASM进行处理，主要是分配栈和记录Activity信息，方便进行交互和生命周期控制，然后合到应用进程，创建Activity实例、windows等等。