四大组件之Activity的工作过程学习

Activity的启动流程

• 让我们从Activity的启动开始看起，当我们在程序中显式启动一个Activity的时候，我们通常使用如下两句代码

Intent intent = new Intent(this,SecondActivity.class);
startActivity(intent);

• 直接进startActivity（）方法看看，发现他重载了好多次，但最终调用的都是startActivityForResult（）带三个参数的方法

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

• 这个方法我们只需要关注if (mParent == null) 这一部分，这里的MParent表示的是ActivityGroup，在api13之后，Activity被推荐使用Fragment
• 其中mMainThread.getApplicationThread()获得的是ApplicationThread
• 接下来看看mInstrumentation.execStartActivity（）这个方法

public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        Uri referrer = target != null ? target.onProvideReferrer() : null;
        if (referrer != null) {
            intent.putExtra(Intent.EXTRA_REFERRER, referrer);
        }
        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;
    }

• 可以看到，方法的前面依旧是各种情况的判断，到最后的ActivityManager.getService().startActivity才是真正的启动Activity的过程
• 猜想，ActivityManager.getService()此方法返回一个ActivityManagerService对象，并且根据getservice方法

public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };

• 得到的ActivityManagerService对象应继承自IActivityManager.Stub，事实确实如此，并且返回的对象应与Context.ACTIVITY_SERVICE对应的IBinder相关联，
• 这里具体的逻辑也没搞清楚，有点迷，以后再说
• 从上面可知，Activity由ActivityManager.getService()来启动，而ActivityManager.getService()实际上是ActivityManagerService，因此我们需要分析ActivityManagerService的startActivity方法来查看他的具体启动流程
• 再看这个方法之前，我们先来看一下Instrumentation的execStartActivity方法的其他部分
• 在ActivityManagerService的startActivity完之后，我们看到有这么一个方法checkStartActivityResult(result, intent);看他的名字好像是检查启动的结果，来看一下

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

        switch (res) {
            case ActivityManager.START_INTENT_NOT_RESOLVED:
            case ActivityManager.START_CLASS_NOT_FOUND:
                if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
                    throw new ActivityNotFoundException(
                            "Unable to find explicit activity class "
                            + ((Intent)intent).getComponent().toShortString()
                            + "; have you declared this activity in your AndroidManifest.xml?");
                throw new ActivityNotFoundException(
                        "No Activity found to handle " + intent);
            case ActivityManager.START_PERMISSION_DENIED:
                throw new SecurityException("Not allowed to start activity "
                        + intent);
            case ActivityManager.START_FORWARD_AND_REQUEST_CONFLICT:
                throw new AndroidRuntimeException(
                        "FORWARD_RESULT_FLAG used while also requesting a result");
            case ActivityManager.START_NOT_ACTIVITY:
                throw new IllegalArgumentException(
                        "PendingIntent is not an activity");
            case ActivityManager.START_NOT_VOICE_COMPATIBLE:
                throw new SecurityException(
                        "Starting under voice control not allowed for: " + intent);
            case ActivityManager.START_VOICE_NOT_ACTIVE_SESSION:
                throw new IllegalStateException(
                        "Session calling startVoiceActivity does not match active session");
            case ActivityManager.START_VOICE_HIDDEN_SESSION:
                throw new IllegalStateException(
                        "Cannot start voice activity on a hidden session");
            case ActivityManager.START_ASSISTANT_NOT_ACTIVE_SESSION:
                throw new IllegalStateException(
                        "Session calling startAssistantActivity does not match active session");
            case ActivityManager.START_ASSISTANT_HIDDEN_SESSION:
                throw new IllegalStateException(
                        "Cannot start assistant activity on a hidden session");
            case ActivityManager.START_CANCELED:
                throw new AndroidRuntimeException("Activity could not be started for "
                        + intent);
            default:
                throw new AndroidRuntimeException("Unknown error code "
                        + res + " when starting " + intent);
        }
    }

• 可以看到，这个方法就是对启动的结果进行一系列的判断，如果正常启动，就不做处理，如果没有正常启动，就进行错误的判断，并抛出异常
• 接下来我们继续分许ActivityManagerService的startActivity方法

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());
    }

• 可以看到，再加上一个参数后，来到了这个方法

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,
                "startActivityAsUser");
    }

• 来到ActivityStarter的startActivityMayWait方法

final 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, Bundle bOptions, boolean ignoreTargetSecurity, int userId,
            IActivityContainer iContainer, TaskRecord inTask, String reason) {
        // Refuse possible leaked file descriptors
        if (intent != null && intent.hasFileDescriptors()) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }
        mSupervisor.mActivityMetricsLogger.notifyActivityLaunching();
        boolean componentSpecified = intent.getComponent() != null;

        // Save a copy in case ephemeral needs it
        final Intent ephemeralIntent = new Intent(intent);
        // Don't modify the client's object!
        intent = new Intent(intent);
        if (componentSpecified
                && intent.getData() != null
                && Intent.ACTION_VIEW.equals(intent.getAction())
                && mService.getPackageManagerInternalLocked()
                        .isInstantAppInstallerComponent(intent.getComponent())) {
            // intercept intents targeted directly to the ephemeral installer the
            // ephemeral installer should never be started with a raw URL; instead
            // adjust the intent so it looks like a "normal" instant app launch
            intent.setComponent(null /*component*/);
            componentSpecified = false;
        }

        ResolveInfo rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId);
        if (rInfo == null) {
            UserInfo userInfo = mSupervisor.getUserInfo(userId);
            if (userInfo != null && userInfo.isManagedProfile()) {
                // Special case for managed profiles, if attempting to launch non-cryto aware
                // app in a locked managed profile from an unlocked parent allow it to resolve
                // as user will be sent via confirm credentials to unlock the profile.
                UserManager userManager = UserManager.get(mService.mContext);
                boolean profileLockedAndParentUnlockingOrUnlocked = false;
                long token = Binder.clearCallingIdentity();
                try {
                    UserInfo parent = userManager.getProfileParent(userId);
                    profileLockedAndParentUnlockingOrUnlocked = (parent != null)
                            && userManager.isUserUnlockingOrUnlocked(parent.id)
                            && !userManager.isUserUnlockingOrUnlocked(userId);
                } finally {
                    Binder.restoreCallingIdentity(token);
                }
                if (profileLockedAndParentUnlockingOrUnlocked) {
                    rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId,
                            PackageManager.MATCH_DIRECT_BOOT_AWARE
                                    | PackageManager.MATCH_DIRECT_BOOT_UNAWARE);
                }
            }
        }
        // Collect information about the target of the Intent.
        ActivityInfo aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags, profilerInfo);

        ActivityOptions options = ActivityOptions.fromBundle(bOptions);
        ActivityStackSupervisor.ActivityContainer container =
                (ActivityStackSupervisor.ActivityContainer)iContainer;
        synchronized (mService) {
            if (container != null && container.mParentActivity != null &&
                    container.mParentActivity.state != RESUMED) {
                // Cannot start a child activity if the parent is not resumed.
                return ActivityManager.START_CANCELED;
            }
            final int realCallingPid = Binder.getCallingPid();
            final int realCallingUid = Binder.getCallingUid();
            int callingPid;
            if (callingUid >= 0) {
                callingPid = -1;
            } else if (caller == null) {
                callingPid = realCallingPid;
                callingUid = realCallingUid;
            } else {
                callingPid = callingUid = -1;
            }

            final ActivityStack stack;
            if (container == null || container.mStack.isOnHomeDisplay()) {
                stack = mSupervisor.mFocusedStack;
            } else {
                stack = container.mStack;
            }
            stack.mConfigWillChange = globalConfig != null
                    && mService.getGlobalConfiguration().diff(globalConfig) != 0;
            if (DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
                    "Starting activity when config will change = " + stack.mConfigWillChange);

            final long origId = Binder.clearCallingIdentity();

            if (aInfo != null &&
                    (aInfo.applicationInfo.privateFlags
                            & ApplicationInfo.PRIVATE_FLAG_CANT_SAVE_STATE) != 0) {
                // This may be a heavy-weight process!  Check to see if we already
                // have another, different heavy-weight process running.
                if (aInfo.processName.equals(aInfo.applicationInfo.packageName)) {
                    final ProcessRecord heavy = mService.mHeavyWeightProcess;
                    if (heavy != null && (heavy.info.uid != aInfo.applicationInfo.uid
                            || !heavy.processName.equals(aInfo.processName))) {
                        int appCallingUid = callingUid;
                        if (caller != null) {
                            ProcessRecord callerApp = mService.getRecordForAppLocked(caller);
                            if (callerApp != null) {
                                appCallingUid = callerApp.info.uid;
                            } else {
                                Slog.w(TAG, "Unable to find app for caller " + caller
                                        + " (pid=" + callingPid + ") when starting: "
                                        + intent.toString());
                                ActivityOptions.abort(options);
                                return ActivityManager.START_PERMISSION_DENIED;
                            }
                        }

                        IIntentSender target = mService.getIntentSenderLocked(
                                ActivityManager.INTENT_SENDER_ACTIVITY, "android",
                                appCallingUid, userId, null, null, 0, new Intent[] { intent },
                                new String[] { resolvedType }, PendingIntent.FLAG_CANCEL_CURRENT
                                        | PendingIntent.FLAG_ONE_SHOT, null);

                        Intent newIntent = new Intent();
                        if (requestCode >= 0) {
                            // Caller is requesting a result.
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_HAS_RESULT, true);
                        }
                        newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_INTENT,
                                new IntentSender(target));
                        if (heavy.activities.size() > 0) {
                            ActivityRecord hist = heavy.activities.get(0);
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_CUR_APP,
                                    hist.packageName);
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_CUR_TASK,
                                    hist.getTask().taskId);
                        }
                        newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_NEW_APP,
                                aInfo.packageName);
                        newIntent.setFlags(intent.getFlags());
                        newIntent.setClassName("android",
                                HeavyWeightSwitcherActivity.class.getName());
                        intent = newIntent;
                        resolvedType = null;
                        caller = null;
                        callingUid = Binder.getCallingUid();
                        callingPid = Binder.getCallingPid();
                        componentSpecified = true;
                        rInfo = mSupervisor.resolveIntent(intent, null /*resolvedType*/, userId);
                        aInfo = rInfo != null ? rInfo.activityInfo : null;
                        if (aInfo != null) {
                            aInfo = mService.getActivityInfoForUser(aInfo, userId);
                        }
                    }
                }
            }

            final ActivityRecord[] outRecord = new ActivityRecord[1];
            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, reason);

            Binder.restoreCallingIdentity(origId);

            if (stack.mConfigWillChange) {
                // If the caller also wants to switch to a new configuration,
                // do so now.  This allows a clean switch, as we are waiting
                // for the current activity to pause (so we will not destroy
                // it), and have not yet started the next activity.
                mService.enforceCallingPermission(android.Manifest.permission.CHANGE_CONFIGURATION,
                        "updateConfiguration()");
                stack.mConfigWillChange = false;
                if (DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
                        "Updating to new configuration after starting activity.");
                mService.updateConfigurationLocked(globalConfig, null, false);
            }

            if (outResult != null) {
                outResult.result = res;
                if (res == ActivityManager.START_SUCCESS) {
                    mSupervisor.mWaitingActivityLaunched.add(outResult);
                    do {
                        try {
                            mService.wait();
                        } catch (InterruptedException e) {
                        }
                    } while (outResult.result != START_TASK_TO_FRONT
                            && !outResult.timeout && outResult.who == null);
                    if (outResult.result == START_TASK_TO_FRONT) {
                        res = START_TASK_TO_FRONT;
                    }
                }
                if (res == START_TASK_TO_FRONT) {
                    final ActivityRecord r = outRecord[0];

                    // ActivityRecord may represent a different activity, but it should not be in
                    // the resumed state.
                    if (r.nowVisible && r.state == RESUMED) {
                        outResult.timeout = false;
                        outResult.who = r.realActivity;
                        outResult.totalTime = 0;
                        outResult.thisTime = 0;
                    } else {
                        outResult.thisTime = SystemClock.uptimeMillis();
                        mSupervisor.waitActivityVisible(r.realActivity, outResult);
                        // Note: the timeout variable is not currently not ever set.
                        do {
                            try {
                                mService.wait();
                            } catch (InterruptedException e) {
                            }
                        } while (!outResult.timeout && outResult.who == null);
                    }
                }
            }

            mSupervisor.mActivityMetricsLogger.notifyActivityLaunched(res, outRecord[0]);
            return res;
        }
    }

• 这个方法看起来有点长，不过仔细分析一下不难发现，前大半部分都是对intent的判断，以便及时返回错误信息，在判断完成之后会进入ActivityStarter的startActivityLocked这个方法

int startActivityLocked(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,
            ActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
            ActivityRecord[] outActivity, ActivityStackSupervisor.ActivityContainer container,
            TaskRecord inTask, String reason) {

        if (TextUtils.isEmpty(reason)) {
            throw new IllegalArgumentException("Need to specify a reason.");
        }
        mLastStartReason = reason;
        mLastStartActivityTimeMs = System.currentTimeMillis();
        mLastStartActivityRecord[0] = null;

        mLastStartActivityResult = startActivity(caller, intent, ephemeralIntent, resolvedType,
                aInfo, rInfo, voiceSession, voiceInteractor, resultTo, resultWho, requestCode,
                callingPid, callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
                options, ignoreTargetSecurity, componentSpecified, mLastStartActivityRecord,
                container, inTask);

        if (outActivity != null) {
            // mLastStartActivityRecord[0] is set in the call to startActivity above.
            outActivity[0] = mLastStartActivityRecord[0];
        }
        return mLastStartActivityResult;
    }

• 又调用了ActivityStarter的startActivity方法
• 这个方法实在太长，这里就不贴代码了，通过分析这个方法的大概流程，发现他还是做了一部分启动状态的判断，并且做了一些相关数据的记录，比如说返回数据，启动次数等，最终又进入了一个九个参数的startActivity方法

private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
            ActivityRecord[] outActivity) {
        int result = START_CANCELED;
        try {
            mService.mWindowManager.deferSurfaceLayout();
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity);
        } finally {
            // If we are not able to proceed, disassociate the activity from the task. Leaving an
            // activity in an incomplete state can lead to issues, such as performing operations
            // without a window container.
            if (!ActivityManager.isStartResultSuccessful(result)
                    && mStartActivity.getTask() != null) {
                mStartActivity.getTask().removeActivity(mStartActivity);
            }
            mService.mWindowManager.continueSurfaceLayout();
        }

        postStartActivityProcessing(r, result, mSupervisor.getLastStack().mStackId,  mSourceRecord,
                mTargetStack);

        return result;
    }

• 继续进入startActivityUnchecked方法
• 方法还是太长，慢慢看吧

private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask) {

         //1. 初始化环境和lunchModeFlags   
        setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
                voiceInteractor);
        computeLaunchingTaskFlags();
        computeSourceStack();
        mIntent.setFlags(mLaunchFlags);

        //2. 复用activity逻辑
        mReusedActivity = getReusableIntentActivity();
        ．．．．．．
        if (mReusedActivity != null) {
            ．．．．．．
            mReusedActivity = setTargetStackAndMoveToFrontIfNeeded(mReusedActivity);
　　　　　   ．．．．．．
            setTaskFromIntentActivity(mReusedActivity);

            if (!mAddingToTask && mReuseTask == null) {
                resumeTargetStackIfNeeded();
                return START_TASK_TO_FRONT;
            }
        }
　　　　 ．．．．．．．

     　　//singleTop 或者singleInstance的处理
        if (dontStart) {
            ActivityStack.logStartActivity(AM_NEW_INTENT, top, top.task);
            // For paranoia, make sure we have correctly resumed the top activity.
            if (mDoResume) {
                mSupervisor.resumeFocusedStackTopActivityLocked();
            }
            ．．．．．．
            top.deliverNewIntentLocked(
                    mCallingUid, mStartActivity.intent, mStartActivity.launchedFromPackage);
　　    　   ．．．．．．
            return START_DELIVERED_TO_TOP;
        }

        //３. 设置对应task并带到前台
        if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask
                && (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {
            newTask = true;
            setTaskFromReuseOrCreateNewTask(taskToAffiliate);
　　　　     ．．．．．．
        } else if (mSourceRecord != null) {
            ．．．．．．
            final int result = setTaskFromSourceRecord();
            ．．．．．．
        } else if (mInTask != null) {
            ．．．．．．
            final int result = setTaskFromInTask();
            if (result != START_SUCCESS) {
                return result;
            }
        } else {
            setTaskToCurrentTopOrCreateNewTask();
        }
    //  这个方法的主要作用是Task和Activity的进栈处理
        mTargetStack.startActivityLocked(mStartActivity, newTask, mKeepCurTransition, mOptions);

   //   5. 使Activity可见
        if (mDoResume) {
            if (!mLaunchTaskBehind) {
                mService.setFocusedActivityLocked(mStartActivity, "startedActivity");
            }
            final ActivityRecord topTaskActivity = mStartActivity.task.topRunningActivityLocked();
            if (!mTargetStack.isFocusable()
                    || (topTaskActivity != null && topTaskActivity.mTaskOverlay
                    && mStartActivity != topTaskActivity)) {
              　．．．. . .
                mTargetStack.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
                mWindowManager.executeAppTransition();
            } else {
                mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
                        mOptions);
            }
        } else {
            mTargetStack.addRecentActivityLocked(mStartActivity);
        }
        ．．．．．．
        return START_SUCCESS;
    }

• 继续来到mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack,mStartActivity,mOptions)

boolean resumeFocusedStackTopActivityLocked(
            ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
        if (targetStack != null && isFocusedStack(targetStack)) {
            return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
        }
        final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
        if (r == null || r.state != RESUMED) {
            mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
        } else if (r.state == RESUMED) {
            // Kick off any lingering app transitions form the MoveTaskToFront operation.
            mFocusedStack.executeAppTransition(targetOptions);
        }
        return false;
    }

• 进入ActivityStack的resumeTopActivityUncheckedLocked

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);
        } finally {
            mStackSupervisor.inResumeTopActivity = false;
        }
        // When resuming the top activity, it may be necessary to pause the top activity (for
        // example, returning to the lock screen. We suppress the normal pause logic in
        // {@link #resumeTopActivityUncheckedLocked}, since the top activity is resumed at the end.
        // We call the {@link ActivityStackSupervisor#checkReadyForSleepLocked} again here to ensure
        // any necessary pause logic occurs.
        mStackSupervisor.checkReadyForSleepLocked();

        return result;
    }

• 进入ActivityStack的resumeTopActivityInnerLocked方法
• 太长，不贴
• 这个方法主要做的就是启动前的最后判断，对上一个活动的状态的判断，以及对即将启动的Activity的状态的更新
• 来到这里mStackSupervisor.startSpecificActivityLocked(next, true, true);

-void startSpecificActivityLocked(ActivityRecord r,
            boolean andResume, boolean checkConfig) {
        // Is this activity's application already running?
        ProcessRecord app = mService.getProcessRecordLocked(r.processName,
                r.info.applicationInfo.uid, true);

        r.getStack().setLaunchTime(r);

        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.versionCode,
                            mService.mProcessStats);
                }
                realStartActivityLocked(r, app, andResume, checkConfig);
                return;
            } catch (RemoteException e) {
                Slog.w(TAG, "Exception when starting activity "
                        + r.intent.getComponent().flattenToShortString(), e);
            }

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

        mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
                "activity", r.intent.getComponent(), false, false, true);
    }

• 这里realStartActivityLocked(r, app, andResume, checkConfig);
• 这里只截取这个方法的一部分

app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
                    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, !andResume,
                    mService.isNextTransitionForward(), profilerInfo);

• 上面这段代码很重要，app.thread的类型为IApplicationThread，IApplicationThread继承了IInterface接口，所以它是一个Binder类型的接口，从IApplicationThread的声明可以看出，其内部包含了大量启动，停止Activity的接口，此外还包括了启动和停止服务的接口，这个BInder接口的IApplicationThread的实现者实现了大量的Activity和Service启动和停止的相关功能代码
• 而IApplicationThread的实现者是ActivityThread的内部类，ApplicationThread
• 去ApplicationThread的scheduleLaunchActivity方法

public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
                ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
                CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
                int procState, Bundle state, PersistableBundle persistentState,
                List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
                boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {

            updateProcessState(procState, false);

            ActivityClientRecord r = new ActivityClientRecord();

            r.token = token;
            r.ident = ident;
            r.intent = intent;
            r.referrer = referrer;
            r.voiceInteractor = voiceInteractor;
            r.activityInfo = info;
            r.compatInfo = compatInfo;
            r.state = state;
            r.persistentState = persistentState;

            r.pendingResults = pendingResults;
            r.pendingIntents = pendingNewIntents;

            r.startsNotResumed = notResumed;
            r.isForward = isForward;

            r.profilerInfo = profilerInfo;

            r.overrideConfig = overrideConfig;
            updatePendingConfiguration(curConfig);

            sendMessage(H.LAUNCH_ACTIVITY, r);
        }

• 那么最终的结果通过自己内部的Handler实现类发送到了这里

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);
                } break;

• 这里我只截取到这个case处理的地方
• 来到handleLaunchActivity这个方法

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
        ...........
        if (localLOGV) Slog.v(
            TAG, "Handling launch of " + r);

        // Initialize before creating the activity
        WindowManagerGlobal.initialize();
        // 完成了Activity的创建
        Activity a = performLaunchActivity(r, customIntent);

        if (a != null) {
            r.createdConfig = new Configuration(mConfiguration);
            reportSizeConfigurations(r);
            Bundle oldState = r.state;
            handleResumeActivity(r.token, false, r.isForward,
                    !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

            if (!r.activity.mFinished && r.startsNotResumed) {
                // The activity manager actually wants this one to start out paused, because it
                // needs to be visible but isn't in the foreground. We accomplish this by going
                // through the normal startup (because activities expect to go through onResume()
                // the first time they run, before their window is displayed), and then pausing it.
                // However, in this case we do -not- need to do the full pause cycle (of freezing
                // and such) because the activity manager assumes it can just retain the current
                // state it has.
                performPauseActivityIfNeeded(r, reason);

                // We need to keep around the original state, in case we need to be created again.
                // But we only do this for pre-Honeycomb apps, which always save their state when
                // pausing, so we can not have them save their state when restarting from a paused
                // state. For HC and later, we want to (and can) let the state be saved as the
                // normal part of stopping the activity.
                if (r.isPreHoneycomb()) {
                    r.state = oldState;
                }
            }
        } else {
            // If there was an error, for any reason, tell the activity manager to stop us.
            try {
                ActivityManager.getService()
                    .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

• 这里，performLaunchActivity方法完成了Activity对象的创建和启动工作
• 并且ActivityThread通过handleResumeActivity方法来调用被启动的Activity的onResume方法
• 这里看performLaunchActivity方法

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);
        }

        //以上是通过ActivityClientRecord获取待启动的Activity的组件信息
        ContextImpl appContext = createBaseContextForActivity(r);
        //try里面  通过类加载器创建Activity对象
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            StrictMode.incrementExpectedActivityCount(activity.getClass());
            r.intent.setExtrasClassLoader(cl);
            r.intent.prepareToEnterProcess();
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to instantiate activity " + component
                    + ": " + e.toString(), e);
            }
        }

        try {
        //尝试创建Application对象
            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) {
                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
                Configuration config = new Configuration(mCompatConfiguration);
                if (r.overrideConfig != null) {
                    config.updateFrom(r.overrideConfig);
                }
                if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                        + r.activityInfo.name + " with config " + config);
                Window window = null;
                if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                    window = r.mPendingRemoveWindow;
                    r.mPendingRemoveWindow = null;
                    r.mPendingRemoveWindowManager = null;
                }
                appContext.setOuterContext(activity);
                //通过attach方法来完成activity一些数据的初始化
                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);

                if (customIntent != null) {
                    activity.mIntent = customIntent;
                }
                r.lastNonConfigurationInstances = null;
                checkAndBlockForNetworkAccess();
                activity.mStartedActivity = false;
                int theme = r.activityInfo.getThemeResource();
                if (theme != 0) {
                    activity.setTheme(theme);
                }
                //从这里开始正式启动Activity
                    activity.mCalled = false;
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                        " did not call through to super.onCreate()");
                }
                r.activity = activity;
                r.stopped = true;
                if (!r.activity.mFinished) {
                    activity.performStart();
                    r.stopped = false;
                }
                if (!r.activity.mFinished) {
                    if (r.isPersistable()) {
                        if (r.state != null || r.persistentState != null) {
                            mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                                    r.persistentState);
                        }
                    } else if (r.state != null) {
                        mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
                    }
                }
                if (!r.activity.mFinished) {
                    activity.mCalled = false;
                    if (r.isPersistable()) {
                        mInstrumentation.callActivityOnPostCreate(activity, r.state,
                                r.persistentState);
                    } else {
                        mInstrumentation.callActivityOnPostCreate(activity, r.state);
                    }
                    if (!activity.mCalled) {
                        throw new SuperNotCalledException(
                            "Activity " + r.intent.getComponent().toShortString() +
                            " did not call through to super.onPostCreate()");
                    }
                }
            }
            r.paused = true;

            mActivities.put(r.token, r);

        } catch (SuperNotCalledException e) {
            throw e;

        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to start activity " + component
                    + ": " + e.toString(), e);
            }
        }

        return activity;
    }

• 这里可以看到，显示创建Activity，再尝试创建Application，为什么是尝试呢，因为一个应用只能有一个Application
• 然后便是Activity的attach方法，它完成一些重要数据的初始化以及Activity的回调啊之类的关联，还做了Window的创建等
• 然后在方法的后半部分就来到了Activity的正式启动部分，这里我将每一部分拿出来单独看
• 第一部分，调用onCreate方法

if (r.isPersistable()) {
    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
    mInstrumentation.callActivityOnCreate(activity, r.state);
}

• 跟进Instrumentation.callActivityOnCreate

public void callActivityOnCreate(Activity activity, Bundle icicle,
            PersistableBundle persistentState) {
        prePerformCreate(activity);
        activity.performCreate(icicle, persistentState);
        postPerformCreate(activity);
    }

• 跟进activity.performCreate

final void performCreate(Bundle icicle, PersistableBundle persistentState) {
        restoreHasCurrentPermissionRequest(icicle);
        onCreate(icicle, persistentState);
        mActivityTransitionState.readState(icicle);
        performCreateCommon();
    }

• 再进

public void onCreate(@Nullable Bundle savedInstanceState,
            @Nullable PersistableBundle persistentState) {
        onCreate(savedInstanceState);
    }

• 这里通过重载方法调用了我们写程序时直接见到的那个onCreate方法
• 接下来是我们刚才performLaunchActivity方法正式启动Activity部分的第二部分

if (!r.activity.mFinished) {
    if (r.isPersistable()) {
        if (r.state != null || r.persistentState != null) {
            mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                    r.persistentState);
        }
    } else if (r.state != null) {
        mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
    }
}

• 这里就不跟进了，它主要做的是在Activity创建好之后的数据初始化工作，比方说，我们平时写的记录Activity临时数据的恢复就可以通过重写Activity的onRestoreInstanceState方法来实现
• 接下来是我们刚才performLaunchActivity方法正式启动Activity部分的第三部分

if (!r.activity.mFinished) {
    activity.mCalled = false;
    if (r.isPersistable()) {
        mInstrumentation.callActivityOnPostCreate(activity, r.state,
                r.persistentState);
    } else {
        mInstrumentation.callActivityOnPostCreate(activity, r.state);
    }
    if (!activity.mCalled) {
        throw new SuperNotCalledException(
            "Activity " + r.intent.getComponent().toShortString() +
            " did not call through to super.onPostCreate()");
    }
}

• 这个方法跟进去主要做的是当主题改变时改变主题的工作
• 到这里这个方法就完了，完了之后回到ActivityThread的handleResumeActivity方法
• 这个方法最终调用的是Activity的onResume方法
• 那么到这里整个过程也就过完了
• 可以发现，整个真正的启动过程是在ActivityThread里面执行的
• 最后再来张图