Android Framework之Activity啓動流程(三)

各位看官好，本文是Android Framework之Activity啓動流程的第三篇，本篇將分析Activity生命週期的回調，新世界的大門就在眼前,走起。
第一篇：Android Framework之Activity啓動流程(一)
第二篇：Android Framework之Activity啓動流程(二)

執行完ApplicationThread# handleBindApplication ()之後，這時候新進程已經啓動。
回到AMS# attachApplicationLocked(),代碼邏輯來到了上篇文章的註釋5處，調用了ActivityStackSupervisor# attachApplicationLocked,在裏面又執行了realStartActivityLocked(),這樣就回到了第一篇末尾所說到的第二種情況，接下來分析第二種情況。
我們看下ApplicationThread# scheduleLaunchActivity()

public final void scheduleLaunchActivity(/*參數省略*/){
    //……省略代碼
    sendMessage(H.LAUNCH_ACTIVITY, r);  
}

sendMessage內部使用了Hanlder，發送了一個H.LAUNCH_ACTIVITY類型的消息，然後調用handleLaunchActivity()->performLaunchActivity()方法。

看看這兩個方法到底幹了什麼事。
ActivityThread# handleLaunchActivity()

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
    // 當前進程正活躍，避免GC
    unscheduleGcIdler();
    //確保使用的是最近的配置
    handleConfigurationChanged(null, null);
    //在創建Activity之前初始化WindowManagerService
    if (!ThreadedRenderer.sRendererDisabled) {
        GraphicsEnvironment.earlyInitEGL()
    }
    WindowManagerGlobal.initialize();
    //======== 註釋9 ========
    //執行performLaunchActivity(),並返回Activity對象
    Activity a = performLaunchActivity(r, customIntent);

    if (a != null) {
        r.createdConfig = new Configuration(mConfiguration);
        reportSizeConfigurations(r);
        Bundle oldState = r.state;
        //======== 註釋10=========
        //啓動成功後，恢復Activity
        handleResumeActivity(r.token, false, r.isForward,!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
        //……省略代碼
    }
}

參數列表中的ActivityClientRecord可以看做是一個Activity的實例，同時還包括了其他一些屬性，主要是對Activity狀態進行記錄。
handleLaunchActivity()主要有兩個操作：
1.註釋9:performLaunchActivity()
該方法主要是調用Activity的onCreate(),onStart(),onRestoreInstance(),onPostCreate()生命週期
2.註釋10:handleResumeActivity()
該方法會調用Activity的onResume()生命週期

先來看看ActivityThread# performLaunchActivity()

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ActivityInfo aInfo = r.activityInfo;
    if (r.packageInfo == null) {
        //從PackageManagerService獲取應用包信息
        r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                Context.CONTEXT_INCLUDE_CODE);
    }
    ComponentName component = r.intent.getComponent();
    if (component == null) {
        //獲取組件信息
        component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
        r.intent.setComponent(component);
    }
    //如果提前設置好了目標Activity，則重新設置組件信息
    if (r.activityInfo.targetActivity != null) {
        component = new ComponentName(r.activityInfo.packageName,
                r.activityInfo.targetActivity);
    }

    ContextImpl appContext = createBaseContextForActivity(r);
    Activity activity = null;
    try {
        //利用ClassLoader去加載Activity
        java.lang.ClassLoader cl = appContext.getClassLoader();
        //利用Instrumentation創建Activity實例
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        StrictMode.incrementExpectedActivityCount(activity.getClass());
        r.intent.setExtrasClassLoader(cl);
        r.intent.prepareToEnterProcess();
        if (r.state != null) {
            r.state.setClassLoader(cl);
        }
    } catch (Exception e) {
        //……省略代碼
    }
    try{
        Application app = r.packageInfo.makeApplication(false, mInstrumentation);
        if (activity != null) {
            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);
        }
        ……
        //回調Activity的onCreate()方法
        //這裏回調的重載函數由ActivityInfo的persistableMode參數決定
        if (r.isPersistable()) {
            mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
        } else {
            mInstrumentation.callActivityOnCreate(activity, r.state);
        }
        r.activity = activity;
        r.stopped = true;
        if (!r.activity.mFinished) {
            //回調Activity的onStart()方法，同時會改變FragmentManager的狀態信息
            // mInstrumentation.callActivityOnStart(this);
            activity.performStart();
            r.stopped = false;
        }

        //回調Activity的onRestoreInstanceState()方法
        //這裏的回調方法同樣由ActivityInfo的persistableMode參數決定
        if (!r.activity.mFinished) {
            if (r.isPersistable()) {
                if (r.state != null || r.persistentState != null) {
                    mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                            r.persistentState);
                }
            } else if (r.state != null) {
                mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
            }
        }
        //回調Activity的OnPostCreate()方法
        if (!r.activity.mFinished) {
            activity.mCalled = false;
            if (r.isPersistable()) {
                mInstrumentation.callActivityOnPostCreate(activity, r.state,
                        r.persistentState);
            } else {
                mInstrumentation.callActivityOnPostCreate(activity, r.state);
            }
            if (!activity.mCalled) {
                throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                                " did not call through to super.onPostCreate()");
            }
        }
        r.paused = true;
        mActivities.put(r.token, r);
    } catch(SuperNotCalledException e){
        ……
    }
    return activity;
}

ActivityThread#handleResumeActivity()

final void handleResumeActivity(IBinder token,
                                boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
    ActivityClientRecord r = mActivities.get(token);
    //======== 註釋11=========
    //回調Activity的onResume()方法
    r = performResumeActivity(token, clearHide, reason);
    if (r != null) {
        final Activity a = r.activity;
        final int forwardBit = isForward ? WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;
        //顯示window
        if (r.window == null && !a.mFinished && willBeVisible) {
            ……
            ViewManager wm = a.getWindowManager();
            ……
            //將decorView添加到WindowManager中
            wm.addView(decor, l);
        }
        ……
        //更新佈局
        wm.updateViewLayout(decor, l);
        ……
        if (reallyResume) {
            try {
                //通知AMS已經Resume了
                ActivityManager.getService().activityResumed(token);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        } else {
            try {
                //如果在onResume之前拋出異常了,則通知AMS結束該Activity
                ActivityManager.getService()
                        .finishActivity(token, Activity.RESULT_CANCELED, null,
                                Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            }
        }
    }
}

該方法主要是回調Activity的onResume()方法(實際上是調用Instrumentation.callActivityOnResume(this)來執行)，並將DecorView添加到WindowManager中，這裏的WindowManager是a.getWindowManager()得到的，其實現是WindowManagerImpl，這步操作在onResume()方法之後執行。

至此，Activity的啓動流程分析完畢。
看完之後感覺如何，是否恍然大悟？

總結

1、 startActivity()通過Instrumentation向AMS進程發起startActivity()請求
2、 AMS收到啓動請求後由ActivityStarter處理Flags和Intent信息,然後再由ActivityStackSupervisor和ActivityStack處理Task和Stack流程
3、 在ActivityStackSupervisor中判斷app進程是否存在，若不存在則請求AMS進程創建新進程，app進程存在的情況請直接跳到第7步
4、 AMS進程通過Socket方式請求Zygote fork新進程
5、 在新進程裏創建ActivityThread(主線程)並開啓Looper循環，同時將ApplicationThread綁定到AMS
6、 AMS回調ApplicationThread的bindApplication()方法將自身與新進程綁定,並在ActivityThread的handleBindApplication()方法中創建應用的Application
7、 Application創建成功後，AMS調用ActivityStackSupervisor# realStartActivityLocked向ActivityThread發送創建Activity請求
8、 ActivityThread利用ClassLoader去加載Activity,並回調其生命週期方法。