PS：本文为转载文章，原文可读性会更好，文末有原文链接PS：这篇文章基于AndroidApi26来分析源码。活动是四大组成部分之一。当然，其内部工作流程体系也做了很多封装。这种封装使得启动Activity变得极其简单。在显式调用的情况下，假设我使用MainActivity来启动Main2Activity，那么可以使用下面的代码来完成；为了更好的理解Activity启动时两个Activity的生命周期的方法调用过程，我们先列出MainActivity和Main2Activity的代码；（1）MainActivity代码；（2）Main2Activity代码；运行app，打印log如下；看，跳转时两个activity的声明周期方法的调用顺序是：MainActivity.onPause->Main2Activity.onCreate->Main2Activity.onStart->Main2Activity.onResume->MainActivity.onStop。由于上面提到的5个生命周期的过程涉及到很多源码，这里我们先来寻找一下MainActivity的onPause方法的流向。当MainActivity启动Main2Activity时，它内部是如何进行的呢？我们看一下Activity的startActivity(Intentintent)方法的实现；startActivity(Intentintent)方法调用Activity的startActivity(Intentintent,@NullableBundleoptions)方法；由于options参数为空，执行的是注2代码块，我们看一下Activity的startActivityForResult(@RequiresPermissionIntentintent,intrequestCode)方法；可空捆绑选项）方法；看，为什么会执行Note3中的代码？这里首先要判断mParent==null是否为真，mParent是当前Activity的父Activity，而我们的Activity是MainActivity，没有父Activity，所以这里mParent==null的结果为真，其实，mParent常用于ActivityGroup，ActivityGroup从API14开始弃用。我们看一下注释4中的代码，mInstrumentation是一个Instrumentation类型的对象。我们看Instrumentation的execStartActivity(Contextwho,IBindercontextThread,IBindertoken,Activitytarget,Intentintent,intrequestCode,Bundleoptions)方法；我们先来看Note6。代码，看方法名，好像是检查Activity启动的结果，好吧，我们看一下Instrumentation的checkStartActivityResult(intres,Objectintent)方法；各种检查的结果很多，我省略了很多；见注释7，如果我们没有在AndroidManifest.xml文件中注册Activity，那么UnabletofindexplicitactivityclasshaveyoudeclaredthisactivityinyourAndroidManifest.xml?将抛出异常。好了，我们再回头看一下注释5中的代码，ActivityManager.getService()其实是获取ActivityManagerService的，所以我们再往下看ActivityManagerService的startActivity(IApplicationThreadcaller,StringcallingPackage,Intentintent,StringresolvedType,IBinderresultTo,StringresultWho,intrequestCode,intstartFlags,ProfilerInfoprofilerInfo,BundlebOptions)方法；ActivityManagerService的startActivity(IApplicationThreadcaller,StringcallingPackage,Intentintent,StringresolvedType,IBinderresultTo,StringresultWho,intrequestCode,intOfoundlestartFlags,Profiler)方法又调用了ActivityManagerService的startActivityAsUser方法；mActivityStarter是一个ActivityStarter实例对User的启动对象(ActivityManagersService的startApplicationAsUser，ActivityManagersServicecaller,StringcallingPackage,Intentintent,StringresolvedType,IBinderresultTo,StringresultWho,intrequestCode,intstartFlags,ProfilerInfoprofilerInfo,BundlebOptions,intuserId)方法调用ActivityStarter的startActivityMayWait(IApplicationThreadcaller，intcallingUid，StringcallingPackage，Intentintent，StringresolvedType，IVoiceInteractionSe-ssionvoiceSession，IVoiceInteractorvoiceInteractor，IBinderresultTo，StringresultWho，intrequestCode，intstartFlags，ProfilerInfoprofilerInfo，WaitResultoutResult，ConfigurationglobalConfig，BundlebOptions，booleanignoreTargetSecurity，intuserId,IActivityContaineriContainer,TaskRecordinTask,Stringreason)方法；finalintstartActivityMayWait(IApplicationThreadcaller,intcallingUid,StringcallingPackage,Intentintent,StringresolvedType,IVoiceInteractionSessionvoiceSession,IVoiceInteractorvoiceInteractor,IBinderresultTo,StringresultWho,intrequestCode,intstartFlags,ProfilerInfoprofilerInfo,WaitResultoutResult,配置globalConfig,BundlebOptions,booleanignoreTargetSecurity,intuserId,IActivityContaineriContainer,TaskRecordinTask,Stringreason){......synchronized(mService){......//9、intres=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);......返回资源；}}看注释9的代码，又调了ActivityStarter的startActivityLocked(IApplicationThreadcaller,Intentintent,IntentephemeralIntent,StringresolvedType,ActivityInfoaInfo,ResolveInforInfo,IVoiceInteractionSessionvoiceSession,IVoi-ceInteractorvoiceInteractor，IBinderresultTo，StringresultWho，intrequestCode，intcallingPid，intcallingUid，StringcallingPackage，intrealCallingPid，intrealCallingUid，intstartFlags，ActivityOptionsoptions，booleanignoreTargetSecurity，booleancomponentSpecified，Activity-Record[]outActivity，ActivityStackSupervisor.ActivityContainer包含-er,TaskRecordinTask,Stringreason)方法；看注解10的代码，又调了ActivityStarter的startActivity(IApplicationThreadcaller,Intentintent,IntentephemeralIntent,StringresolvedType,ActivityInfoaInfo,ResolveInforInfo,IVoiceInteractionSessionvoiceSession,IVoiceInteractorvoiceIn-teractor,IBinderresultTo，StringresultWho，intrequestCode，intcallingPid，intcallingUid，StringcallingPackage，intrealCallingPid，intrealCallingUid，intstartFlags，ActivityOptionsoptions，booleanignoreTargetSecurity，booleancomponentSpecified，ActivityRecord[]outActivity，ActivityStackSupervisor.ActivityContainercontainer，Ta-skRecordinTask)方法；看注释11，调了ActivityStarter的startActivity(finalActivityRecordr,ActivityRecordsourceRecord,IVoiceInteractionSessionvoiceSession,IVoiceInteractorvoiceInteractor,intstartFlags,booleandoResume,ActivityOptionsoptions,TaskRecordinTask,ActivityRecord[]outActivity)方1法；看法；，调到了ActivityStarter的startActivityUnchecked(finalActivityRecordr,ActivityRecordsourceRecord,IVoiceInteractionSessionvoiceSession,IVoiceInteractorvoiceInteractor,intstartFlags,booleandoResume,ActivityOptionsoptions,TaskRecordinTask,ActivityRec-ord[]outActivity)方法；//注意：这个方法应该只从{@linkstartActivity}调用。privateintstartActivityUnchecked(finalActivityRecordr,ActivityRecordsourceRecord,IVoiceInteractionSessionvoiceSession,IVoiceInteractorvoiceInteractor,intstartFlags,booleandoResume,ActivityOptions选项,TaskRecordinTask,ActivityRecord[]outActivity){......if(mDoResume){finalActivityRecordtopTaskActivity=mStartActivity.getTask().topRunningActivityLocked();if(!mTargetStack.isFocusable()||(topTaskActivity!=null&&topTaskActivity.mTaskOverlay&&mStartActivity!=topTaskActivity)){......}else{......//13、mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack、mStartActivity、mOptions）；}}else{...}......返回START_SUCCESS；}看注释13中的代码，因为我的Main2Activity的启动模式是标准的，所以就转到这里，mSupervisor是ActivityStackSupervisor类型的一个实例，我们看一下resumeFocusedStackTopActivityLock-ed(ActivityStackSupervisor的ActivityStacktargetStack,ActivityRecordtarget,活动选项离子目标选项）方法；见注释14，调用ActivityStack的resumeTopActivityUncheckedLo-cked(ActivityRecordprev,ActivityOptionsoptions)方法；见注释15，调用ActivityStack的resumeTopActivityInner-Locked(ActivityRecordprev,ActivityOptionsoptions)方法；调用ActivityStack的选项6实现了ActivityStack的startPausingLocked(booleanuserLeaving,booleanuiSleeping,ActivityRecordresuming,booleanpauseImmediately)方法；见注释17，prev.app.thread是ActivityThread的内部类ApplicatitonThread，调用了schedulePauseActivity(user,IBinderanishtoken,intconfigChanges,booleandontReport)方法；见注18，因为finished为false，所以第一个参数都是H.PAUSE_ACTIVITY，调用ActivityThread的内部类H；见注释19，调用handlePauseActivity(IBindertoken,booleanfinished,booleanuserLeaving,intconfigChanges,booleandontReport,intseq)方法；见注释20，调用ActivityThread的performPauseActivity(IBindertoken,booleanfinished,booleansaveState,Stringreason)方法；见注释21，调用ActivityThreadperformPauseActivity(Activit-yClientRecordr,booleanfinished,booleansaveState,Stringreason)方法；参见注释22，调用ActivityThread的performPauseActivity-IfNeeded(ActivityClientRecordr,Stringreason)方法；见注释23，mInstrumentation是Instrumentation类型的对象，所以调用了Instrumentation的callActivityOnPause(Activityactivity)方法；见注释24，调用了Activity的performPause方法；看注释25，Activity的onPause方法已经找到了，我们的目标已经完成，那么本文到此结束写完
