Activity的启动流程分析

Activity启动分为很多种情况，这里说的是打开新的应用程序第一个Activity的流程。 

1. AcitivityManager产生新进程，新进程从android.app.ActivityThread.main开始运行。这里就是一般意义上的程序入口点，类似于C的main函数。

ActivityManagerService.java 

 private final void startProcessLocked(ProcessRecord app, String hostingType, String hostingNameStr) {
       // Process里面通知Zygote服务，Zygote真正产生新的进程（准确说是复制一个）
       int pid = Process.start("android.app.ActivityThread",
                    mSimpleProcessManagement ? app.processName : null, uid, uid,
  }

 2.ActivityThread的main函数中将Looper准备起来，prepareMainLooper标志着这是应用程序主线程的Looper对象。

ActivityThread.java  

public static final void main(String[] args) {
    Looper.prepareMainLooper();
    ActivityThread thread = new ActivityThread();
    thread.attach(false);
    // 这里闭合消息循环 
    Looper.loop();
}

 3. 接下来调用attach，参数为false，表明这不是系统进程，是给普通应用程序用使用的进程。

ActivityThread.java   

private final void attach(boolean system) {
             ViewRoot.addFirstDrawHandler(new Runnable() {
                public void run() {
                    ensureJitEnabled();
                }
            });
            RuntimeInit.setApplicationObject(mAppThread.asBinder());
            IActivityManager mgr = ActivityManagerNative.getDefault();
            mgr.attachApplication(mAppThread);
    }

    mAppThread是ApplicationThread对象，是提供给ActivityManagerService控制ActivityThread的回调接口。

private final class ApplicationThread extends ApplicationThreadNative {

  public final void schedulePauseActivity(IBinder token, boolean finished,boolean userLeaving, int  configChanges){

            queueOrSendMessage(finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,
                    token, (userLeaving ? 1 : 0),  configChanges);
        }

  public final void scheduleStopActivity(IBinder token, boolean showWindow, int configChanges) {
           queueOrSendMessage(showWindow ? H.STOP_ACTIVITY_SHOW : H.STOP_ACTIVITY_HIDE,
                token, 0, configChanges);
        }

  .........

}

    ActivityManagerService要Pause当前Activity，就会调用schedulePauseActivity想本地的消息循环中加入一个H.PAUSE_ACTIVITY的消息，然后立即返回以避免ActivityManagerService的阻塞。

 4.现在又回到了ActivityManagerService中

ActivityManagerService.java   

 // ActivityManagerService中XXXLocked函数才是真正干活的地方，XXX只是个套

    public final void attachApplication(IApplicationThread thread) {
        int callingPid = Binder.getCallingPid();
        attachApplicationLocked(thread, callingPid);
    }

 

    private final boolean attachApplicationLocked(IApplicationThread thread, int pid) {

        // 这里取出对应该Pid的ProcessRecord对象，如果取不出来，你就等着out吧

        app = mPidsSelfLocked.get(pid)

        // 为ProcessRecord对象补充信息

        app.thread = thread;
        app.curAdj = app.setAdj = -100;
        app.curSchedGroup = Process.THREAD_GROUP_DEFAULT;
        app.setSchedGroup = Process.THREAD_GROUP_BG_NONINTERACTIVE;
        app.forcingToForeground = null;
        app.foregroundServices = false;
        app.debugging = false;
        // 清除timeout监测 
        mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);

 

        // 回调之前的ActivityThread，让它记住自己在ActivityManagerService中的相关信息，传这么大坨东西，真给力

        thread.bindApplication(processName, app.instrumentationInfo != null
                    ? app.instrumentationInfo : app.info, providers,
                    app.instrumentationClass, app.instrumentationProfileFile,
                    app.instrumentationArguments, app.instrumentationWatcher, testMode, 
                    isRestrictedBackupMode || !normalMode,
                    mConfiguration, getCommonServicesLocked());

       // topRunningActivityLocked的意思没看太明白

       HistoryRecord hr = topRunningActivityLocked(null);

       // 启动Activity

       realStartActivityLocked(hr, app, true, true);

    }

 

    private final boolean realStartActivityLocked(HistoryRecord r,
            ProcessRecord app, boolean andResume, boolean checkConfig){

        // 这里又跑到ActivityThread中去了

       app.thread.scheduleLaunchActivity(new Intent(r.intent), r, System.identityHashCode(r),
                    r.info, r.icicle, results, newIntents, !andResume, isNextTransitionForward());

    }

 

5.ActivityThread开始调度用户的Activity启动了

ActivityThread.java

private final void handleLaunchActivity(ActivityRecord r, Intent customIntent) {
       Activity a = performLaunchActivity(r, customIntent);

       ......

}

private final void performLaunchActivity(ActivityRecord r, Intent customIntent) {

       java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
       // 产生Activity 
       activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);

       // 将新生的Activity与当前应用关联

       activity.attach(appContext, this, getInstrumentation(), r.token,
                       r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstance,
                        r.lastNonConfigurationChildInstances, config);

      .....

}

 6.Acivity与当前App关联，直到这里，平时应用所见的Activity才真正被构建

Actiivty.java 

final void attach(Context context, ActivityThread aThread, Instrumentation instr, IBinder token, int ident,
    Application application, Intent intent, ActivityInfo info,  CharSequence title, Activity parent, String id,
    Object lastNonConfigurationInstance, HashMap<String,Object> lastNonConfigurationChildInstances,
            Configuration config) {

       // 记录传入ContexImpl实例，这就是平时所见的Activity Context 
       attachBaseContext(context);
       // 创建与Activity关联的Window，可以简单理解为显示窗口 
       mWindow = PolicyManager.makeNewWindow(this);

       mUiThread = Thread.currentThread();

       mMainThread = aThread;
       mInstrumentation = instr;
       mToken = token;
       mIdent = ident;
       mApplication = application;
       mIntent = intent;
       mComponent = intent.getComponent();
       mActivityInfo = info;
       mTitle = title;
       mParent = parent;
       mEmbeddedID = id;
      // 构建WindowManager的代理LocalWindowManager 
       mWindow.setWindowManager(null, mToken, mComponent.flattenToString());
       if (mParent != null) {
            // 难道说整个Activity栈中的Activity都有同一个Container 
            mWindow.setContainer(mParent.getWindow());
       }
       mWindowManager = mWindow.getWindowManager();

    }

 7.ActivityThread

ActivityThread.java

private final void performLaunchActivity(ActivityRecord r, Intent customIntent) {

       ......

       // 回调Activity::onCreate

       mInstrumentation.callActivityOnCreate(activity, r.state);

        // 记录新产生的Activity 
       mActivities.put(r.token, r);

}

private final void handleLaunchActivity(ActivityRecord r, Intent customIntent) {

       ......

       handleResumeActivity(r.token, false, r.isForward);

}

 final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) {

       ActivityRecord r = performResumeActivity(token, clearHide);

 

}

public final ActivityRecord performResumeActivity(IBinder token, boolean clearHide) {

        if (r.pendingIntents != null) {
              deliverNewIntents(r, r.pendingIntents);
              r.pendingIntents = null;
       }
       if (r.pendingResults != null) {
              deliverResults(r, r.pendingResults);
              r.pendingResults = null;
       }
       //回调Activity::onResume 
       r.activity.performResume();

}

      到这里，Activity从产生到初始化的过程就全部结束了。之后就是等待用户界面的消息，根据消息进行相应的处理。整个过程中，在ActivityManagerServer、ActivityThread的互相协作下构建出Activity，并在相应的时机回调Activity的相应接口，完成Activity的初始化。