Activity启动流程.jpg

一、主要类介绍

• ActivityThread：App启动的入口
• ApplicationThread：ActivityThread的内部类，继承Binder，可以进程跨进程通信。Activity启动过程中主要与ActivityManagerService进行跨进程通信。ActivityManager通过它告知应用程序将要做的事。
• Instrumentation：负责发起Activity的启动、并具体负责Activity的创建以及Activity生命周期的回调。一个应用进程只会有一个Instrumentation对象，App内的所有Activity都持有该对象的引用。
• ActivityManagerService：简称AMS，是service端对象，负责管理系统中所有的Activity
• ActivityStarter：用于解释如何启动Activity的控制器。此类收集用于确定如何将Intent和flag转变为Activity以及相关任务和堆栈的所有逻辑。
• ActivityStack： 单个活动栈的状态和管理。
• ActivityStackSupervisor：Activity栈管理。

二、发起启动请求

1. 启动一个Activity通常都是通过Activity的startActivity方法启动。

frameworks/base/core/java/android/app/Activity.java
    public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }

    public void startActivity(Intent intent, @Nullable Bundle options) {
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            startActivityForResult(intent, -1);
        }
    }
    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 {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

mParent表示调用startActivity的Activity状态，如果不为空就调用Instrumentation对象的execStartActivity方法，在调用execStartActivity方法时会传入一个mMainThread.getApplicationThread()参数，它是一个ApplicationThread对象，是一个应用程序客户端端的Binder对象，AMS会使用它来和ActivityThread进行通信。

android/app/Instrumentation.java
    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;
           ......
            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);
            ......
        return null;
    }

ActivityManager.getService().startActivity会通过Binder驱动调用到AMS的startActivity方法

com/android/server/am/ActivityManagerService.java
    @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());
    }

    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,
            boolean validateIncomingUser) {
        enforceNotIsolatedCaller("startActivity");

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

        return mActivityStartController.obtainStarter(intent, "startActivityAsUser")
                .setCaller(caller)
                .setCallingPackage(callingPackage)
                .setResolvedType(resolvedType)
                .setResultTo(resultTo)
                .setResultWho(resultWho)
                .setRequestCode(requestCode)
                .setStartFlags(startFlags)
                .setProfilerInfo(profilerInfo)
                .setActivityOptions(bOptions)
                .setMayWait(userId)
                .execute();

    }

在startActivityAsUser方法最后会通过ActivityStartController.obtainStarter方法获得一个包含所有启动信息的ActivityStarter对象并调用execute方法执行。

    int execute() {
        try {  
            if (mRequest.mayWait) {
                return startActivityMayWait(mRequest.caller, mRequest.callingUid,
                        mRequest.callingPackage, 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);
            } 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);
            }
        } finally {
            onExecutionComplete();
        }
    }

上一步创建ActivityStarter时调用了setMayWait所以mRequest.mayWait为true会继续调用startActivityMayWait方法

com/android/server/am/ActivityStarter.java
    private int startActivityMayWait(IApplicationThread caller, int callingUid,
            String callingPackage, Intent intent, String resolvedType,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            IBinder resultTo, String resultWho, int requestCode, int startFlags,
            ProfilerInfo profilerInfo, WaitResult outResult,
            Configuration globalConfig, SafeActivityOptions options, boolean ignoreTargetSecurity,
            int userId, TaskRecord inTask, String reason,
            boolean allowPendingRemoteAnimationRegistryLookup,
            PendingIntentRecord originatingPendingIntent) {
        ......
         // 这里会创建一个新的Intent，表示新Activity的intent
        // Don't modify the client's object!
        intent = new Intent(intent);
        ......
        // 对intent进行解析，得到这个activity的相关信息，比如包名、类名等
        ActivityInfo aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags, profilerInfo);
        ......
        int res = startActivity(caller, intent, ephemeralIntent, resolvedType, aInfo, rInfo,
                voiceSession, voiceInteractor, resultTo, resultWho, requestCode, callingPid,
                callingUid, callingPackage, realCallingPid, realCallingUid, startFlags, options,
                ignoreTargetSecurity, componentSpecified, outRecord, inTask, reason,
                allowPendingRemoteAnimationRegistryLookup, originatingPendingIntent);
        ......
}

    private int startActivity(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
            String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
            String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
            SafeActivityOptions options,
            boolean ignoreTargetSecurity, boolean componentSpecified, ActivityRecord[] outActivity,
            TaskRecord inTask, boolean allowPendingRemoteAnimationRegistryLookup,
            PendingIntentRecord originatingPendingIntent) {
        ......
        ProcessRecord callerApp = null;
        if (caller != null) {
            // caller是前面传进来的ApplicationThread
            callerApp = mService.getRecordForAppLocked(caller);
        }
        ......
        //获取launchmode
        final int launchFlags = intent.getFlags();
        ......
        // 创建ActivityRecord，一个Activity对应一个ActivityRecord
        ActivityRecord r = new ActivityRecord(mService, callerApp, callingPid, callingUid,
                callingPackage, intent, resolvedType, aInfo, mService.getGlobalConfiguration(),
                resultRecord, resultWho, requestCode, componentSpecified, voiceSession != null,
                mSupervisor, checkedOptions, sourceRecord);
        // 把前面创建的ActivityRecord放在第一个位置
        if (outActivity != null) {
            outActivity[0] = r;
        }
        ......
        return startActivity(r, sourceRecord, voiceSession, voiceInteractor, startFlags,
                true /* doResume */, checkedOptions, inTask, outActivity);

    }

    private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
                IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
                int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
                ActivityRecord[] outActivity) {
        int result = START_CANCELED;
        try {
            mService.mWindowManager.deferSurfaceLayout();
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity);
        } finally {
           
        }
        ......
        return result;
    }

ActivityStarter.startActivityMayWait方法中调用多个startActivity方法后会调用到startActivityUnchecked。startActivityUnchecked中会根据flags和Activity启动模式来决定如何启动一个Activity

com/android/server/am/ActivityStarter.java
    private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
            ActivityRecord[] outActivity) {
        ......
        // 是否应将新Activity插入到现有任务中。如果没有返回空
        ActivityRecord reusedActivity = getReusableIntentActivity();
        ......
        // 判断启动模式
        if ((mLaunchFlags & FLAG_ACTIVITY_CLEAR_TOP) != 0
                || isDocumentLaunchesIntoExisting(mLaunchFlags)
                || isLaunchModeOneOf(LAUNCH_SINGLE_INSTANCE, LAUNCH_SINGLE_TASK)) {
            final TaskRecord task = reusedActivity.getTask();

            // In this situation we want to remove all activities from the task up to the one
            // being started. In most cases this means we are resetting the task to its initial
            // state.
            final ActivityRecord top = task.performClearTaskForReuseLocked(mStartActivity,
                    mLaunchFlags);

            // The above code can remove {@code reusedActivity} from the task, leading to the
            // the {@code ActivityRecord} removing its reference to the {@code TaskRecord}. The
            // task reference is needed in the call below to
            // {@link setTargetStackAndMoveToFrontIfNeeded}.
            if (reusedActivity.getTask() == null) {
                reusedActivity.setTask(task);
            }

            if (top != null) {
                if (top.frontOfTask) {
                    // Activity aliases may mean we use different intents for the top activity,
                    // so make sure the task now has the identity of the new intent.
                    top.getTask().setIntent(mStartActivity);
                }
                deliverNewIntent(top);
            }
        }
        // 找到要添加的activity栈并MoveToFront
        reusedActivity = setTargetStackAndMoveToFrontIfNeeded(reusedActivity);

        if (mDoResume) {
        final ActivityRecord topTaskActivity =
                mStartActivity.getTask().topRunningActivityLocked();
        ......
        if (!mTargetStack.isFocusable()
                || (topTaskActivity != null && topTaskActivity.mTaskOverlay
                && mStartActivity != topTaskActivity)) {
            // If the activity is not focusable, we can't resume it, but still would like to
            // make sure it becomes visible as it starts (this will also trigger entry
            // animation). An example of this are PIP activities.
            // Also, we don't want to resume activities in a task that currently has an overlay
            // as the starting activity just needs to be in the visible paused state until the
            // over is removed.
            mTargetStack.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
            // Go ahead and tell window manager to execute app transition for this activity
            // since the app transition will not be triggered through the resume channel.
            mService.mWindowManager.executeAppTransition();
        } else {
            // If the target stack was not previously focusable (previous top running activity
            // on that stack was not visible) then any prior calls to move the stack to the
            // will not update the focused stack.  If starting the new activity now allows the
            // task stack to be focusable, then ensure that we now update the focused stack
            // accordingly.
            if (mTargetStack.isFocusable() && !mSupervisor.isFocusedStack(mTargetStack)) {
                mTargetStack.moveToFront("startActivityUnchecked");
            }
            mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
                    mOptions);
        }
    } else if (mStartActivity != null) {
        mSupervisor.mRecentTasks.add(mStartActivity.getTask());
    }
}

无论以何种模式启动最终都会调用ActivityStackSupervisor.resumeFocusedStackTopActivityLocked方法。

com/android/server/am/ActivityStackSupervisor.java
    boolean resumeFocusedStackTopActivityLocked(
            ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {

        if (!readyToResume()) {
            return false;
        }

        if (targetStack != null && isFocusedStack(targetStack)) {
            return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
        }

        final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
        if (r == null || !r.isState(RESUMED)) {
            mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
        } else if (r.isState(RESUMED)) {
            // Kick off any lingering app transitions form the MoveTaskToFront operation.
            mFocusedStack.executeAppTransition(targetOptions);
        }

        return false;
    }

com/android/server/am/ActivityStack.java
    boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
        if (mStackSupervisor.inResumeTopActivity) {
            // Don't even start recursing.
            return false;
        }

        boolean result = false;
        try {
            // Protect against recursion.
            mStackSupervisor.inResumeTopActivity = true;
            result = resumeTopActivityInnerLocked(prev, options);
            final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
            if (next == null || !next.canTurnScreenOn()) {
                checkReadyForSleep();
            }
        } finally {
            mStackSupervisor.inResumeTopActivity = false;
        }

        return result;
    }
    private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
        ...
        boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, next, false);
        if (mResumedActivity != null) {
            pausing |= startPausingLocked(userLeaving, false, next, false);
        }
        ...
        mStackSupervisor.startSpecificActivityLocked(next, true, true);
        ...
        return true;
    }

在ActivityStack.resumeTopActivityInnerLocked方法中会去判断是否有Activity处于Resume状态，如果有的话会先让这个Activity执行startPausingLocked，然后再执行startSpecificActivityLocked方法启动要启动Activity。

三、第一个Activity的onPause过程

com/android/server/am/ActivityStack.java
    final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
            ActivityRecord resuming, boolean pauseImmediately) {

        if (prev.app != null && prev.app.thread != null) {
            if (DEBUG_PAUSE) Slog.v(TAG_PAUSE, "Enqueueing pending pause: " + prev);
            try {
                EventLogTags.writeAmPauseActivity(prev.userId, System.identityHashCode(prev),
                        prev.shortComponentName, "userLeaving=" + userLeaving);
                mService.updateUsageStats(prev, false);

                mService.getLifecycleManager().scheduleTransaction(prev.app.thread, prev.appToken,
                        PauseActivityItem.obtain(prev.finishing, userLeaving,
                                prev.configChangeFlags, pauseImmediately));
            } catch (Exception e) {
                // Ignore exception, if process died other code will cleanup.
                Slog.w(TAG, "Exception thrown during pause", e);
                mPausingActivity = null;
                mLastPausedActivity = null;
                mLastNoHistoryActivity = null;
            }
        }
}

Android 9.0在这里引入了ClientLifecycleManager和ClientTransactionHandler来辅助管理Activity生命周期，他会发送EXECUTE_TRANSACTION消息到ActivityThread里面继续处理
,通过ClientLifecycleManager的scheduleTransaction方法把PauseActivityItem事件加入到执行计划中，开始栈顶的pausing过程。

com/android/server/am/ClientLifecycleManager.java
    void scheduleTransaction(@NonNull IApplicationThread client, @NonNull IBinder activityToken,
            @NonNull ActivityLifecycleItem stateRequest) throws RemoteException {
        final ClientTransaction clientTransaction = transactionWithState(client, activityToken,
                stateRequest);
        scheduleTransaction(clientTransaction);
    }
    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        transaction.schedule();
        if (!(client instanceof Binder)) {
            // If client is not an instance of Binder - it's a remote call and at this point it is
            // safe to recycle the object. All objects used for local calls will be recycled after
            // the transaction is executed on client in ActivityThread.
            transaction.recycle();
        }
    }
android/app/ActivityThread.java
    void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
    }

android/app/servertransaction/TransactionExecutor.java
    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");
    }


    public void executeCallbacks(ClientTransaction transaction) {
        if (callbacks == null) {
            // No callbacks to execute, return early.
            return;
        }
        ...
        for (int i = 0; i < size; ++i) {
            final ClientTransactionItem item = callbacks.get(i);
            ...
            item.execute(mTransactionHandler, token, mPendingActions);
            item.postExecute(mTransactionHandler, token, mPendingActions);
            ...
        }
    }

    private void executeLifecycleState(ClientTransaction transaction) {
        ...

        cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);

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

Handler H的实例接收到EXECUTE_TRANSACTION消息后调用TransactionExecutor.execute方法,ClientLifecycleManager.scheduleTransaction中没有传callbacks参数所以executeCallbacks会直接return，接着执行executeLifecycleState。
lifecycleItem 在ActivityStack的startPausingLocked方法中设置的是PauseActivityItem，因此这里对调用PauseActivityItem的execute和postExecute

android/app/servertransaction/PauseActivityItem.java
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
        client.handlePauseActivity(token, mFinished, mUserLeaving, mConfigChanges, pendingActions,
                "PAUSE_ACTIVITY_ITEM");
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }

PauseActivityItem.execute方法中调用ActivityThread.handlePauseActivity方法

android/app/ActivityThread.java
    public void handlePauseActivity(IBinder token, boolean finished, boolean userLeaving,
            int configChanges, PendingTransactionActions pendingActions, String reason) {
        ActivityClientRecord r = mActivities.get(token);
        if (r != null) {
            if (userLeaving) {
                performUserLeavingActivity(r);
            }

            r.activity.mConfigChangeFlags |= configChanges;
            performPauseActivity(r, finished, reason, pendingActions);

            // Make sure any pending writes are now committed.
            if (r.isPreHoneycomb()) {
                QueuedWork.waitToFinish();
            }
            mSomeActivitiesChanged = true;
        }
    }

    private Bundle performPauseActivity(ActivityClientRecord r, boolean finished, String reason,
            PendingTransactionActions pendingActions) {
        .....
        if (shouldSaveState) {
            callActivityOnSaveInstanceState(r);
        }

        performPauseActivityIfNeeded(r, reason);

        .....
    }
    private void performPauseActivityIfNeeded(ActivityClientRecord r, String reason) {
        if (r.paused) {
            // You are already paused silly...
            return;
        }

        try {
            r.activity.mCalled = false;
            mInstrumentation.callActivityOnPause(r.activity);
            ......
        } catch (SuperNotCalledException e) {
            throw e;
        } catch (Exception e) {
           
        }
        r.setState(ON_PAUSE);
    }

Instrumentation.callActivityOnPause方法中直接调用Activity.performPause，在performPause方法中我们终于看到了熟悉的身影Activity生命周期的onPause方法，至此栈顶Activity的Pausing流程全部完毕

四、新Activity create过程

ActivityStack.resumeTopActivityInnerLocked方法中会先让这个Activity执行startPausingLocked，然后再执行startSpecificActivityLocked方法启动要启动Activity。

    void startSpecificActivityLocked(ActivityRecord r,
            boolean andResume, boolean checkConfig) {
        // 判断运行Activity的进程是否窜在
        ProcessRecord app = mService.getProcessRecordLocked(r.processName,
                r.info.applicationInfo.uid, true);

        if (app != null && app.thread != null) {
            try {
                if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
                        || !"android".equals(r.info.packageName)) {
                    // Don't add this if it is a platform component that is marked
                    // to run in multiple processes, because this is actually
                    // part of the framework so doesn't make sense to track as a
                    // separate apk in the process.
                    app.addPackage(r.info.packageName, r.info.applicationInfo.longVersionCode,
                            mService.mProcessStats);
                }
                // 进程存在启动activity
                realStartActivityLocked(r, app, andResume, checkConfig);
                return;
            } catch (RemoteException e) {
             
            }
        }
       // 调用ActivityManagerService通过Zygote孵化进程
        mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
                "activity", r.intent.getComponent(), false, false, true);
    }

realStartActivityLocked启动activity

    final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {
                ......
            try {
                ......
                // 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,
                        mergedConfiguration.getGlobalConfiguration(),
                        mergedConfiguration.getOverrideConfiguration(), r.compat,
                        r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
                        r.persistentState, results, newIntents, mService.isNextTransitionForward(),
                        profilerInfo));

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

                // Schedule transaction.
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);
                ......
            } catch (RemoteException e) {
            }
}

这里会为ClientTransaction添加的对象是LaunchActivityItem，并且添加了callback回调

调用ClientLifecycleManager.scheduleTransaction方法之后具体是如何执行的前面已经分析过了，这里就不再分析了，最后会执行到LaunchActivityItem， 基本流程同pause，LaunchActivityItem调用到ActivityThread的handleLaunchActivity->performLaunchActivity->Instrumentation.newActivity->Instrumentation.callActivityOnCreate->activity.performCreate

至此executeCallbacks执行完毕，开始执行executeLifecycleState方法。先执行cycleToPath方法，生命周期状态是从ON_CREATE状态到ON_RESUME状态，中间有一个ON_START状态，所以会执行ActivityThread.handleStartActivity方法。

android/app/servertransaction/TransactionExecutor.java
    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);
    }
    private void performLifecycleSequence(ActivityClientRecord r, IntArray path) {
        final int size = path.size();
        for (int i = 0, state; i < size; i++) {
            state = path.get(i);
            log("Transitioning to state: " + 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);
            }
        }
    }