我們上一章分析了由Launcher啟動應用MainActivity時的源碼课竣，涉及到MainActivity的三個生命周期onCreate()暗挑，onStart()劈猿，onResume()沮榜，現(xiàn)在我們繼續(xù)走完剩下的生命周期尚胞，我們新建一個AActivity签钩，在MainActivity中做跳轉掏呼。

startActivity(new Intent(this,AActivity.class));

追蹤startActivity()方法的源碼。

    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 {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    }

此時我們沒設置Bundle 參數(shù)铅檩，所以options 為null憎夷。

 public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
        startActivityForResult(intent, requestCode, null);
    }

 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);
            // 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);
            }
        }
    }

此時mParent為null，所以調(diào)用了mInstrumentation.execStartActivity()方法昧旨。

 private Instrumentation mInstrumentation;

所有我們在Instrumentation類中找到execStartActivity()方法拾给。

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);
        }
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    ActivityResult result = null;
                    if (am.ignoreMatchingSpecificIntents()) {
                        result = am.onStartActivity(intent);
                    }
                    if (result != null) {
                        am.mHits++;
                        return result;
                    } else if (am.match(who, null, intent)) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return requestCode >= 0 ? am.getResult() : null;
                        }
                        break;
                    }
                }
            }
        }
        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;
    }

這里我們只看ActivityManager.getService().startActivity()方法祥得。這里就是調(diào)用ActivityManagerService的startActivity()方法，這就由回到了我們上一章的流程蒋得，新啟動的Activity也就是AActivity調(diào)用onCreate(),onStart(),onResume()级及，那么是什么時候我們調(diào)用了MainActivity的onPause()和onStop()方法的呢，我們來看ActivityStack類的resumeTopActivityInnerLocked()方法额衙，此時AActivity
還沒有走到onCreate()方法饮焦。

private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
        if (!mService.mBooting && !mService.mBooted) {
            // Not ready yet!
            return false;
        }

        // Find the next top-most activity to resume in this stack that is not finishing and is
        // focusable. If it is not focusable, we will fall into the case below to resume the
        // top activity in the next focusable task.
        final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
        ...
        boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, next, false);
        if (mResumedActivity != null) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Pausing " + mResumedActivity);
            pausing |= startPausingLocked(userLeaving, false, next, false);
        }
        ...
        //next表示我們當前的Activity
        if (next.app != null && next.app.thread != null) {
           ...
        } else {
            ...
            mStackSupervisor.startSpecificActivityLocked(next, true, true);
        }

        if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
        return true;
    }

這次我們不看mStackSupervisor.startSpecificActivityLocked(next, true, true)方法，我們來觀察startPausingLocked(userLeaving, false, next, false)方法窍侧。mResumedActivity 參數(shù)是ActivityRecord的對象县踢，此時ActivityRecord是存在的，在resumeTopActivityInnerLocked方法中加入過疏之，我們不去追究這里殿雪，繼續(xù)看startPausingLocked(userLeaving, false, next, false)方法。

final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
            ActivityRecord resuming, boolean pauseImmediately) {
        ...
        ActivityRecord prev = mResumedActivity;
        ...
        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) {
             ...
            }
        } else {
          ...
        }
        ...
    }

這里我們已知mResumedActivity不為null锋爪，調(diào)用了 mService.getLifecycleManager().scheduleTransaction方法丙曙，也就是ClientLifecycleManager的scheduleTransaction()方法。

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();
        }
    }

繼續(xù)來看ClientTransaction的getClient()方法其骄。

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

IApplicationThread我們在上邊曾經(jīng)遇到過亏镰，他的服務端在ActivityThread中，以下流程我們在上篇文章中都經(jīng)歷過拯爽，不過多解釋索抓。

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

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

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);
    }

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;

 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");
    }

我們還是分別看一下executeCallbacks()和executeLifecycleState()。

public void executeCallbacks(ClientTransaction transaction) {
        final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
        if (callbacks == null) {
            // No callbacks to execute, return early.
            return;
        }
        log("Resolving callbacks");

        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);
            log("Resolving callback: " + item);
            final int postExecutionState = item.getPostExecutionState();
            final int closestPreExecutionState = mHelper.getClosestPreExecutionState(r,
                    item.getPostExecutionState());
            if (closestPreExecutionState != UNDEFINED) {
                cycleToPath(r, closestPreExecutionState);
            }

            item.execute(mTransactionHandler, token, mPendingActions);
            item.postExecute(mTransactionHandler, token, mPendingActions);
            if (r == null) {
                // Launch activity request will create an activity record.
                r = mTransactionHandler.getActivityClient(token);
            }

            if (postExecutionState != UNDEFINED && r != null) {
                // Skip the very last transition and perform it by explicit state request instead.
                final boolean shouldExcludeLastTransition =
                        i == lastCallbackRequestingState && finalState == postExecutionState;
                cycleToPath(r, postExecutionState, shouldExcludeLastTransition);
            }
        }
    }

這次我們的callbacks為空毯炮，這個方法整體沒有調(diào)用逼肯，那我們看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);
    }

依然是調(diào)用了
1桃煎、cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */)
2篮幢、lifecycleItem.execute(mTransactionHandler, token, mPendingActions)
3、lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions)
按順序分析为迈。

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);
    }

根據(jù)startPausingLocked()方法中的傳入我們已知此時的lifecycleItem是PauseActivityItem的實例三椿，我們還是計算一下start和finish的值。
我們已知在MainActivity創(chuàng)建的過程中start的值已經(jīng)被設置為ON_RESUME葫辐，而finish的值

public int getTargetState() {
        return ON_PAUSE;
    }

故而為ON_PAUSE搜锰，excludeLastState為true，而

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);
            }
        } else { // finish < start, can't just cycle down
            if (start == ON_PAUSE && finish == ON_RESUME) {
                // Special case when we can just directly go to resumed state.
                mLifecycleSequence.add(ON_RESUME);
            } else if (start <= ON_STOP && finish >= ON_START) {
                // Restart and go to required state.

                // Go to stopped state first.
                for (int i = start + 1; i <= ON_STOP; i++) {
                    mLifecycleSequence.add(i);
                }
                // Restart
                mLifecycleSequence.add(ON_RESTART);
                // Go to required state
                for (int i = ON_START; i <= finish; i++) {
                    mLifecycleSequence.add(i);
                }
            } else {
                // Relaunch and go to required state

                // Go to destroyed state first.
                for (int i = start + 1; i <= ON_DESTROY; i++) {
                    mLifecycleSequence.add(i);
                }
                // Go to required state
                for (int i = ON_CREATE; i <= finish; i++) {
                    mLifecycleSequence.add(i);
                }
            }
        }

        // Remove last transition in case we want to perform it with some specific params.
        if (excludeLastState && mLifecycleSequence.size() != 0) {
            mLifecycleSequence.remove(mLifecycleSequence.size() - 1);
        }

        return mLifecycleSequence;
    }

故mLifecycleSequence為空集耿战，cycleToPath()方法無實現(xiàn)蛋叼，再來看lifecycleItem.execute()。

    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);
    }

ClientTransactionHandler是ActivityThread的父類剂陡，只是定義了handlePauseActivity的抽象方法鸦列，具體實現(xiàn)還在ActivityThread中租冠。

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;
        }
    }

繼續(xù)看performPauseActivity()方法。

private Bundle performPauseActivity(ActivityClientRecord r, boolean finished, String reason,
            PendingTransactionActions pendingActions) {
        ...
        // Pre-Honeycomb apps always save their state before pausing
        final boolean shouldSaveState = !r.activity.mFinished && r.isPreHoneycomb();
        if (shouldSaveState) {
            callActivityOnSaveInstanceState(r);
        }

        performPauseActivityIfNeeded(r, reason);
        ...

        return shouldSaveState ? r.state : null;
    }

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);
            if (!r.activity.mCalled) {
                throw new SuperNotCalledException("Activity " + safeToComponentShortString(r.intent)
                        + " did not call through to super.onPause()");
            }
        } catch (SuperNotCalledException e) {
            throw e;
        } catch (Exception e) {
            if (!mInstrumentation.onException(r.activity, e)) {
                throw new RuntimeException("Unable to pause activity "
                        + safeToComponentShortString(r.intent) + ": " + e.toString(), e);
            }
        }
        r.setState(ON_PAUSE);
    }

在這里調(diào)用了 mInstrumentation.callActivityOnPause(r.activity)方法并且 r.setState(ON_PAUSE)薯嗤；

 public void callActivityOnPause(Activity activity) {
        activity.performPause();
    }

final void performPause() {
        mDoReportFullyDrawn = false;
        mFragments.dispatchPause();
        mCalled = false;
        onPause();
        writeEventLog(LOG_AM_ON_PAUSE_CALLED, "performPause");
        mResumed = false;
        if (!mCalled && getApplicationInfo().targetSdkVersion
                >= android.os.Build.VERSION_CODES.GINGERBREAD) {
            throw new SuperNotCalledException(
                    "Activity " + mComponent.toShortString() +
                    " did not call through to super.onPause()");
        }
    }

protected void onPause() {
        if (DEBUG_LIFECYCLE) Slog.v(TAG, "onPause " + this);
        getApplication().dispatchActivityPaused(this);
        if (mAutoFillResetNeeded) {
            if (!mAutoFillIgnoreFirstResumePause) {
                if (DEBUG_LIFECYCLE) Slog.v(TAG, "autofill notifyViewExited " + this);
                View focus = getCurrentFocus();
                if (focus != null && focus.canNotifyAutofillEnterExitEvent()) {
                    getAutofillManager().notifyViewExited(focus);
                }
            } else {
                // reset after first pause()
                if (DEBUG_LIFECYCLE) Slog.v(TAG, "autofill got first pause " + this);
                mAutoFillIgnoreFirstResumePause = false;
            }
        }
        mCalled = true;
    }

到這里我們的onPause()方法被調(diào)用了顽爹。到這里還沒有結束，我們的lifecycleItem.postExecute()方法還沒有追蹤骆姐。

public void postExecute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        if (mDontReport) {
            return;
        }
        try {
            // TODO(lifecycler): Use interface callback instead of AMS.
            ActivityManager.getService().activityPaused(token);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }
    }

ActivityManager.getService().activityPaused(token)是調(diào)用了ActivityManagerService的activityPaused()方法镜粤。

public final void activityPaused(IBinder token) {
        final long origId = Binder.clearCallingIdentity();
        synchronized(this) {
            ActivityStack stack = ActivityRecord.getStackLocked(token);
            if (stack != null) {
                stack.activityPausedLocked(token, false);
            }
        }
        Binder.restoreCallingIdentity(origId);
    }

調(diào)用了ActivityStack.activityPausedLocked()方法。

final void activityPausedLocked(IBinder token, boolean timeout) {
        if (DEBUG_PAUSE) Slog.v(TAG_PAUSE,
            "Activity paused: token=" + token + ", timeout=" + timeout);

        final ActivityRecord r = isInStackLocked(token);
        if (r != null) {
            mHandler.removeMessages(PAUSE_TIMEOUT_MSG, r);
            if (mPausingActivity == r) {
                if (DEBUG_STATES) Slog.v(TAG_STATES, "Moving to PAUSED: " + r
                        + (timeout ? " (due to timeout)" : " (pause complete)"));
                mService.mWindowManager.deferSurfaceLayout();
                try {
                    completePauseLocked(true /* resumeNext */, null /* resumingActivity */);
                } finally {
                    mService.mWindowManager.continueSurfaceLayout();
                }
                return;
            } else {
                EventLog.writeEvent(EventLogTags.AM_FAILED_TO_PAUSE,
                        r.userId, System.identityHashCode(r), r.shortComponentName,
                        mPausingActivity != null
                            ? mPausingActivity.shortComponentName : "(none)");
                if (r.isState(PAUSING)) {
                    r.setState(PAUSED, "activityPausedLocked");
                    if (r.finishing) {
                        if (DEBUG_PAUSE) Slog.v(TAG,
                                "Executing finish of failed to pause activity: " + r);
                        finishCurrentActivityLocked(r, FINISH_AFTER_VISIBLE, false,
                                "activityPausedLocked");
                    }
                }
            }
        }
        mStackSupervisor.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
    }

這里我們已知mPausingActivity == r玻褪，所以調(diào)用了 completePauseLocked(true , null)方法肉渴。

private void completePauseLocked(boolean resumeNext, ActivityRecord resuming) {
        ActivityRecord prev = mPausingActivity;
        ...
        if (resumeNext) {
            final ActivityStack topStack = mStackSupervisor.getFocusedStack();
            if (!topStack.shouldSleepOrShutDownActivities()) {
                mStackSupervisor.resumeFocusedStackTopActivityLocked(topStack, prev, null);
            } else {
                ...
            }
        }

       ...
    }

這里最終調(diào)用了ActivityStackSupervisor類的resumeFocusedStackTopActivityLocked()方法。接下來的流程就是重復第一章中Activity啟動的流程带射，我們就不再寫一遍了同规。到這里我們走完了MainActivity的onPause()到AActivity的onCreate()、onStart()和onResume()窟社。