废话不说，开搞。

一、StartActivity

最终是调的startActivityForResult

    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
       ……
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
      ……
    }

成员变量Instrumentation的execStartActivity

    public ActivityResult execStartActivity(
        ……
        int result = ActivityManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
        ……

ActivityManager.getService()

    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;
                }
            };

Binder 机制 调用系统服务 ActivityManagerService,所以直接看 AMS（简写） 的startActivity

二、 来到系统进程ActivityManagerService

startActivity——startActivityAsUser

    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");

        // TODO: Switch to user app stacks here.
        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();

    }

mActivityStartController—— obtainStarter—— execute
点两下等于
ActivityStarter ——execute

    int execute() {
        try {
            // TODO(b/64750076): Look into passing request directly to these methods to allow
            // for transactional diffs and preprocessing.
            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);
            } 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);
            }
        } finally {
            onExecutionComplete();
        }
    }

ActivityStarter ——startActivityMayWait

    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) {

            ……

            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);
          ……

ActivityStarter ——startActivity 注意这里有好几层

    private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
                IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
                int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
                ActivityRecord[] outActivity) {
            ……
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity);
            ……
        return result;
    }

ActivityStarter ——startActivityUnchecked 这里面不知道怎么找了 百度了一下别人的

    private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
            ActivityRecord[] outActivity) {
……
             mSupervisor.resumeFocusedStackTopActivityLocked();
……

ActivityStackSupervisor ——resumeFocusedStackTopActivityLocked

    boolean resumeFocusedStackTopActivityLocked() {
        return resumeFocusedStackTopActivityLocked(null, null, null);
    }

    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;
    }

ActivityStack ——resumeTopActivityUncheckedLocked

 result = resumeTopActivityInnerLocked(prev, options);

ActivityStack ——resumeTopActivityInnerLocked 真长

mStackSupervisor.startSpecificActivityLocked(next, true, true);

ActivityStackSupervisor ——startSpecificActivityLocked

realStartActivityLocked(r, app, andResume, checkConfig);

ActivityStackSupervisor ——realStartActivityLocked

然后这里秀了我一脸，大爷的差点没 neng 晕我！！！！

因为 api-28 之前 直接就是app.thread.scheduleLaunchActivity 进入 ApplicationThread，然而 api-28好像用了某种 我目前还不太了解的设计模式，什么Transaction记录后期学习下。

    final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {
            ……
            //已屏蔽 无关代码 和参数
             final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
                        r.appToken);
              clientTransaction.addCallback(LaunchActivityItem.obtain(……),……）；

               // 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);
      

比较重要的ClientTransaction，LaunchActivityItem，ResumeActivityItem，mService就是ActivityManagerService。

这行代码：
mService.getLifecycleManager().scheduleTransaction(clientTransaction);

ActivityManagerService的getLifecycleManager

    ClientLifecycleManager getLifecycleManager() {
        return mLifecycleManager;
    }

mLifecycleManager是ClientLifecycleManager

class ClientLifecycleManager {
    
    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        transaction.schedule();
        if (!(client instanceof Binder)) {
            transaction.recycle();
        }

ClientTransaction——schedule， ClientTransaction在上面realStartActivityLocked初始化的

public class ClientTransaction implements Parcelable, ObjectPoolItem {
    private IApplicationThread mClient;
    public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);
    }

mClient就是ClientTransaction在上面realStartActivityLocked初始化时候 app.thread。
变量app是一个ProcessRecord对象，它的成员变量thread是IApplicationThread类型，因此app.thread是IApplicationThread，一猜就是某个服务的代理对象，又要用 Binder 机制了；

IApplicationThread 服务端实体是 ApplicationThread类。
所以这里又一次 通过 Binder 机制，交给了ApplicationThread的scheduleTransaction方法。

三、交给ApplicationThread

ApplicationThread原来是 ActivityThread 的内部类，我说怎么搜了半天也没有。
看继承关系

public final class ActivityThread extends ClientTransactionHandler {
        private class ApplicationThread extends IApplicationThread.Stub {
               @Override
              public void scheduleTransaction(ClientTransaction transaction) throws         
                     RemoteException {
                  ActivityThread.this.scheduleTransaction(transaction);
              }
        }
}

ActivityThread里没有scheduleTransaction，原来在他爹ClientTransactionHandler里

public abstract class ClientTransactionHandler {

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

用 Handler讲transaction发给了ActivityThread的 H:

class H extends Handler {
      public void handleMessage(Message msg) {
            case EXECUTE_TRANSACTION:
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;
                    mTransactionExecutor.execute(transaction);
                    if (isSystem()) {                      
                         transaction.recycle();
                    }
                    break;
      }
}

mTransactionExecutor是TransactionExecutor类
TransactionExecutor——execute

 public void execute(ClientTransaction transaction) {
        final IBinder token = transaction.getActivityToken();

        executeCallbacks(transaction);

        executeLifecycleState(transaction);
        mPendingActions.clear();
    }

  public void executeCallbacks(ClientTransaction transaction) {
      final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
      ……
      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) {
        final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();
        ……
        lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
        lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
   }

循环ClientTransaction里所有的 ClientTransactionItem与ActivityLifecycleItem并执行execute。
那么这俩ClientTransactionItem与ActivityLifecycleItem啥呢，就是上面realStartActivityLocked里说的比较重要的LaunchActivityItem，ResumeActivityItem啥的。
LaunchActivityItem：

public class LaunchActivityItem extends ClientTransactionItem {
     @Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        ……
        client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
        ……
    }
}

client就是 ActivityThread 所以就执行ActivityThread的handleLaunchActivity。

ResumeActivityItem跟 Activity 创建启动没事关系，我只是后来找生命周期 onStart, onResume怎么也找不到，百度也没有讲 api-28 的源码。所以这里剧透下onStart, onResume是在ResumeActivityItem的execute执行的。

ActivityThread——handleLaunchActivity

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

ActivityThread——performLaunchActivity

    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        //获取组件信息
         ContextImpl appContext = createBaseContextForActivity(r);
                ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

        ComponentName component = r.intent.getComponent();
        if (component == null) {
            component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
            r.intent.setComponent(component);
        }

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }
        //创建 Context
        ContextImpl appContext = createBaseContextForActivity(r);
        //创建 Activity
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
           ……
        } catch (Exception e) {
           ……
        }

       try {
             //创建Application
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);
              
            //并和activity  attach
             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);

           //启动 Activity 里面最终调了 onCreate
            if (r.isPersistable()) {
                   mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
            } else {
                   mInstrumentation.callActivityOnCreate(activity, r.state);
       }
    }

ActivityThread$performLaunchActivity方法的操作：

1，获取要启动的Activity的ComponentName对象：里面包含了包名，类名相关的信息；
2，创建Activity的上下文环境：即创建了ContextImpl对象；
3，创建Activity的实例：调用Instrumentation的newActivity方法，并使用类加载器创建Activity对象；
4，创建Application对象；
5，调用Activity的attach方法：初始化Activity类里的一些数据；
6，启动Activity：调用Instrumentation$callActivityOnCreate方法；

Instrumentation——newActivity：

    public Activity newActivity(ClassLoader cl, String className,
            Intent intent)
            throws InstantiationException, IllegalAccessException,
            ClassNotFoundException {
        String pkg = intent != null && intent.getComponent() != null
                ? intent.getComponent().getPackageName() : null;
        return getFactory(pkg).instantiateActivity(cl, className, intent);
    }

至此 Activity 与其所需的 Application 、Comtext 初始化完成。

四、生命周期

接上面
mInstrumentation.callActivityOnCreate：

activity.performCreate(icicle, persistentState);

Activity——performCreate

    final void performCreate(Bundle icicle, PersistableBundle persistentState) {
        mCanEnterPictureInPicture = true;
        restoreHasCurrentPermissionRequest(icicle);
        if (persistentState != null) {
            onCreate(icicle, persistentState);
        } else {
            onCreate(icicle);
        }
       ……
    }

ok 调用了 onCreate。
然后 onStart，onResume 都在
ResumeActivityItem—— ActivityThread .handleResumeActivity——ActivityThread.performResumeActivity—— Activity.performResume:

    final void performResume(boolean followedByPause, String reason) {
        performRestart(true /* start */, reason);
          ……
        // mResumed is set by the instrumentation
        mInstrumentation.callActivityOnResume(this);

performRestart：

mInstrumentation.callActivityOnRestart(this);

Instrumentation:

    public void callActivityOnRestart(Activity activity) {
        activity.onRestart();
    }
  
    public void callActivityOnResume(Activity activity) {
        activity.mResumed = true;
        activity.onResume();
    ……
    }

ok,完毕！

五、总结

进程内 startActivity 启动 Activity
首先调用Activity的startActivityForResult；
然后是mInstrumentation.execStartActivity，Instrumentation通过 Binder 机制将任务交给ActivityManagerService；
ActivityManagerService做了一系列准备工作后，通过 Binder 机制将任务交给 ApplicationThread；
ApplicationThread为 ActivityThread 的内部类，直接把启动任务交于ActivityThread；ActivityThread通过handler 将任务发给 内部Handler H最终执行自己的performLaunchActivity，
并在此方法创建Activity ，以及其所需的 Context、组件信息、与Application 的 attach，生命周期方法 onCreate 的执行。