一、流程

Activity.startActivity [这里中间会经过多个Activity流程]
->Activity.startActivityForResult
->Instrumentation.execStartActivity
->ActivityTaskManagerService.startActivity [这是通过ActivityTaskManager.getService()]
->ActivityTaskManagerService.startActivityAsUser
->ActivityStartController.obtainStarter [这是为了获取ActivityStarter]
->ActivityStarter.execute [setMayWait设置mRequest.mayWait为true]
->ActivityStarter.startActivityMayWait [初始化rInfo和aInfo]
->ActivityStarter.startActivity
->ActivityStarter.startActivity [初始化err和ActivityRecord]
->ActivityStarter.startActivity
->ActivityStarter.startActivityUnchecked [初始化TaskRecord]
->RootActivityContainer.resumeFocusedStacksTopActivities
->ActivityStack.resumeTopActivityUncheckedLocked
->ActivityStack.resumeTopActivityInnerLocked
 ->ActivityStack.startPausingLocked [开始暂停当前恢复的活动]
 ->PauseActivityItem.obtain
 ->ActivityTaskManagerService.getLifecycleManager
 ->ClientLifecycleManager.scheduleTransaction
 ->ClientLifecycleManager.scheduleTransaction
 ->ClientTransaction.getClient
 ->ClientTransaction.schedule
 ->IApplicationThread.scheduleTransaction
 ->ActivityThread.scheduleTransaction
 ->ClientTransactionHandler.scheduleTransaction
 ->ActivityThread.sendMessage [H.EXECUTE_TRANSACTION]
 ->TransactionExecutor.execute
 ->TransactionExecutor.executeCallbacks [这个流程的item是PauseActivityItem]
 ->TransactionExecutor.cycleToPath
  ->TransactionExecutor.performLifecycleSequence
 ->PauseActivityItem.execute
  ->ActivityThread.handlePauseActivity
  ->ActivityThread.performPauseActivity
  ->ActivityThread.performPauseActivityIfNeeded
  ->Instrumentation.callActivityOnPause
  ->Activity.performPause
  ->Activity.onPause
 ->PauseActivityItem.postExecute
  ->ActivityTaskManagerService.activityPaused
  ->ActivityStack.activityPausedLocked [ActivityStack对象是在ActivityRecord]
  ->ActivityStack.completePauseLocked
  ->RootActivityContainer.resumeFocusedStacksTopActivities
  ->ActivityStack.resumeTopActivityUncheckedLocked
  ->ActivityStack.resumeTopActivityInnerLocked
->ActivityStack.resumeTopActivityInnerLocked [暂停流程结束又回到了这个方法]
->ActivityStackSupervisor.startSpecificActivityLocked [分两种情况，没有应用进程和有]
->ActivityStackSupervisor.realStartActivityLocked [有应用程序进程]
->ActivityTaskManagerService.mH.sendMessage [没有应用程序进程]
这里说明下：sendMessage其实是发送了一个Function，即ActivityManagerInternal::startProcess。下面就是分析startProcess
 ->ActivityManagerInternal.startProcess [AMS.LocalService.startProcess]
 ->ActivityManagerService.LocalService.startProcess
 ->ActivityManagerService.startProcessLocked
 ->ProcessList.startProcessLocked
 ->ProcessList.startProcessLocked [三个参数]
 ->ProcessList.startProcessLocked [五个参数]
 ->ProcessList.startProcessLocked [十二个参数]
 ->ProcessList.startProcess
 ->Process.start [非zygote进程]
 ->ZygoteProcess.start
 ->ZygoteProcess.startViaZygote
  ->ZygoteProcess.openZygoteSocketIfNeeded [这是由下一个方法调用]
  ->ZygoteProcess.attemptConnectionToPrimaryZygote [尝试连接Zygote]
  ->ZygoteState.connect
 ->ZygoteProcess.zygoteSendArgsAndGetResult
 ->ZygoteProcess.attemptZygoteSendArgsAndGetResult
 ->zygoteState.mZygoteOutputWriter.write(msgStr) [localsocket通信]
->执行到这里，就已经发送了Socket给Zygote进程，下面分析Zygote进程启动
 ->app_main.cpp的main方法
 ->AndroidRuntime.start
 ->ZygoteInit.main [app启动Zygote进程]
 ->new ZygoteServer()
 ->zygoteServer.runSelectLoop(abiList) [循环监听AMS发来的消息]
 ->ZygoteServer.acceptCommandPeer [pollIndex == 0]
  ->ZygoteSocket.accept()
 ->ZygoteServer.createNewConnection
 ->new ZygoteConnection
 ->ZygoteConnection.processOneCommand [pollIndex < usapPoolEventFDIndex]
 ->Zygote.forkAndSpecialize [这里就是fork流程]
  ->Zygote.nativeForkAndSpecialize
  ->com_android_internal_os_Zygote.cpp#nativeForkAndSpecialize
  ->com_android_internal_os_Zygote.cpp#ForkAndSpecializeCommon
  ->fork 不再往底层分析 processOneCommand
 ->ZygoteConnection.handleChildProc [parsedArgs.mInvokeWith=null]
 ->ZygoteInit.zygoteInit [isZygote=false]
  ->RuntimeInit.redirectLogStreams()
  ->RuntimeInit.commonInit()
  ->ZygoteInit.nativeZygoteInit() [在AndroidRuntime里面注册]
   ->com_android_internal_os_ZygoteInit_nativeZygoteInit
   ->gCurRuntime->onZygoteInit #具体实现在AppRuntime即app_main里面
   ->ProcessState::self->startThreadPool #开启binder线程池
  ->RuntimeInit.applicationInit [回到ZygoteInit]
  ->RuntimeInit.findStaticMain
  ->new MethodAndArgsCaller [最终返回给runSelectLoop]
  ->MethodAndArgsCaller.run [在ZygoteInit.main取到runnable执行]
  ->Method.invoke [在MethodAndArgsCaller.run中]
  ->ActivityThread.main [传到MethodAndArgsCaller中]
  ->new ActivityThread
  ->ActivityThread.attach
  ->ActivityManagerService.attachApplication [传入IApplicationThread这个Binder]
  ->ActivityManagerService.attachApplicationLocked [处理attachApplicationLocked]
   ->ApplicationThread.bindApplication [绑定AMS这个binder]
   ->ActivityThread.sendMessage [发送BIND_APPLICATION]
   ->ActivityThread.handleBindApplication
    ->ActivityThread.getPackageInfoNoCheck [获取LoadedApk]
     ->ContextImpl.createAppContext
    ->反射创建Instrumentation
    ->LoadedApk.makeApplication
     ->Instrumentation.newApplication
      ->Application.attach
    ->Instrumentation.callApplicationOnCreate
  ->ActivityManagerService.attachApplicationLocked [回到attachApplicationLocked]
   ->AtmInternal.attachApplication [ActivityTaskManagerService]
  ->ActivityStackSupervisor.realStartActivityLocked [回到了启动Activity的流程]
  ->通过ClientTransaction启动Activity

二、参考

Android10.0应用进程创建过程以及Zygote的fork流程-[Android取经之路]
在Android 10.0中Activity的启动流程