無論怎么說,?Activity都是Android最核心的組件, 主要負責向用戶展示應用信息. Activity的生命周期由Android系統控制, 啟動與繪制都是自動完成. 對于開發人員而言, 僅僅是一句startActivity, 就完成了全部, 但是在平凡的表象下隱藏著驚人的秘密. 讓我們撥開迷霧, 一起探索Activity的啟動框架! Know more the world, know more ourselves.

Activity

本文源碼來源Android SDK 25(即7.1), 邏輯與低版本不同. 在AMS與ASS之間又添加ActivityStarter, 負責管理Activity的啟動.

Activity的啟動框架圖

Activity的啟動框架

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Activity與Instrumentation

最簡單的Activity啟動方式, 如下:

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

startActivity方法存在于Activity類中, Activity類的調用流程:

Activity#startActivity(Intent)

Activity#startActivityForResult(Intent, int, Bundle)

Activity#startActivityForResult核心是mInstrumentation.execStartActivity

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,@Nullable Bundle options) {if (mParent == null) {// ...Instrumentation.ActivityResult ar =mInstrumentation.execStartActivity(this, mMainThread.getApplicationThread(), mToken, this,intent, requestCode, options); // 核心方法// ...} else {// ...} }

Instrumentation類負責監控系統與應用之間的交互.

execStartActivity核心是ActivityManagerNative類的getDefault#startActivity, 獲取IActivityManager單例, 本質是ServiceManager.getService("activity"), 即ActivityManagerService. 通過Binder, 遠程調用ActivityManagerService類的startActivity方法.

public ActivityResult execStartActivity(Context who, IBinder contextThread, IBinder token, Activity target,Intent intent, int requestCode, Bundle options) {// ...try {// ...int result = ActivityManagerNative.getDefault().startActivity(whoThread, who.getBasePackageName(), intent,intent.resolveTypeIfNeeded(who.getContentResolver()),token, target != null ? target.mEmbeddedID : null,requestCode, 0, null, options); // 核心方法// ...} // ...return null; }

Activity啟動通過遠程調用, 由當前應用交給ActivityManagerService(AMS)處理.

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ActivityManagerService

ActivityManagerService類是負責啟動Activity的核心服務, 簡稱AMS. 啟動邏輯包含在ActivityStarter, ActivityStackSupervisor和ActivityStack和三個類中, ActivityStarter負責啟動, ActivityStackSupervisor負責管理Stack和TaskRecord, ActivityStack負責管理棧內的Activity.

Stack和TaskRecord示例:

Stack #1:Running activities (most recent first):TaskRecord{3caa65e3 #2711 A=me.chunyu.spike.wcl_activity_launchmode_demo U=0 sz=2}Run #1: ActivityRecord{36b06e99 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.TestAActivity t2711}Run #0: ActivityRecord{27396226 u0 me.chunyu.spike.wcl_activity_launchmode_demo/.MainActivity t2711}Stack #0:Running activities (most recent first):TaskRecord{27d796c9 #2695 A=com.miui.home U=0 sz=1}Run #0: ActivityRecord{2e5712cb u0 com.miui.home/.launcher.Launcher t2695}

ActivityManagerService類的調用過程:

ActivityManagerService#startActivity

ActivityManagerService#startActivityAsUser

ActivityManagerService調用ActivityStarter的startActivityMayWait方法, 執行啟動.

@Override 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) {// ...return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,profilerInfo, null, null, bOptions, false, userId, null, null); }

ActivityStarter類負責處理Activity的Intent和Flags的邏輯, 還有管理Stack和TaskRecord.

ActivityStarter類的調用流程:

ActivityStarter#startActivityMayWait

ActivityStarter#startActivityLocked

ActivityStarter#startActivityUnchecked

startActivityMayWait: 根據Intent獲取Activity的啟動信息(ResolveInfo和ActivityInfo), 獲取調用者的Pid和Uid.
startActivityLocked: 創建ActivityRecord, 含有Activity的核心信息.
startActivityUnchecked: 根據啟動的Flag信息, 設置TaskRecord, 完成后執行ActivityStackSupervisor類的resumeFocusedStackTopActivityLocked方法, 繼續啟動.

private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask) {// ...// 所有啟動準備完成后, dontStart是true.final boolean dontStart = top != null && mStartActivity.resultTo == null&& top.realActivity.equals(mStartActivity.realActivity)&& top.userId == mStartActivity.userId&& top.app != null && top.app.thread != null&& ((mLaunchFlags & FLAG_ACTIVITY_SINGLE_TOP) != 0|| mLaunchSingleTop || mLaunchSingleTask);if (dontStart) {ActivityStack.logStartActivity(AM_NEW_INTENT, top, top.task);// doResume是true, mDoResume也是true if (mDoResume) {mSupervisor.resumeFocusedStackTopActivityLocked();}// ...}// ... }

ActivityStackSupervisor類與ActivityStack類配合使用. ActivityStackSupervisor負責管理Task和Stack, 而ActivityStack負責管理在Stack和Task中的Activity. 因此, 對于Stack和Task的操作, AMS使用ActivityStackSupervisor進行管理; 對于Activity的操作, AMS使用ActivityStack進行管理. 兩者相互調用, 最終完成啟動Activity.

ActivityStackSupervisor類與ActivityStack類的調用流程:

ActivityStackSupervisor#resumeFocusedStackTopActivityLocked

ActivityStack#resumeTopActivityUncheckedLocked

ActivityStack#resumeTopActivityInnerLocked

ActivityStackSupervisor#startSpecificActivityLocked

核心在于ActivityStack類的resumeTopActivityInnerLocked方法, 根據ActivityRecord和ActivityOptions完成Activity的切換, 移至棧頂, 最后調用ActivityStackSupervisor類的startSpecificActivityLocked方法, 執行啟動.

ActivityStackSupervisor類的startSpecificActivityLocked方法調用realStartActivityLocked方法, 執行真正的啟動Activity.

void startSpecificActivityLocked(ActivityRecord r,boolean andResume, boolean checkConfig) {// 獲取啟動Activity的進程, 即ApplicationProcessRecord app = mService.getProcessRecordLocked(r.processName,r.info.applicationInfo.uid, true);r.task.stack.setLaunchTime(r);if (app != null && app.thread != null) {try {// ...// 真正啟動Activity的過程realStartActivityLocked(r, app, andResume, checkConfig);return;}// ...}// ... }

在ActivityStackSupervisor的realStartActivityLocked方法中, 含有啟動的核心方法scheduleLaunchActivity, 即調用IApplicationThread的scheduleLaunchActivity方法.

app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),new Configuration(task.mOverrideConfig), r.compat, r.launchedFromPackage,task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);

IApplicationThread的實現是ApplicationThread, 而ApplicationThread是ActivityThread的內部類, 即使用ApplicationThread類的scheduleLaunchActivity方法處理Activity啟動.

最終由ActivityThread完成Activity的創建與繪制.

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復雜的邏輯...

That's all! Enjoy it!

原文地址：?http://www.jianshu.com/p/269e3dc06b09

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