一、概述

Activity作為Android四大元件之一，也是我們平時開發中使用的最多的元件，相信大家已經瞭解了它的基本使用，那麼是否知道Activity的onCreate()、onStart()、onResume()方法是什麼時候呼叫的呢？接下來就從原始碼角度來分析Activity的工作原理

二、從main()方法原始碼開始分析

我們知道每一個程式都有一個入口，類似於c語言中的入口是main()方法一樣，Android中也有個main()方法，它就是ActivityThrea中的main()方法。

    public static void main(String[] args) {
        SamplingProfilerIntegration.start();

        // CloseGuard defaults to true and can be quite spammy.  We
 

        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);

        Environment.initForCurrentUser();

        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new
 
 EventLoggingReporter());

        Security.addProvider(new AndroidKeyStoreProvider());

        Process.setArgV0("<pre-initialized>");

        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        if (sMainThreadHandler == null
 
) {
            sMainThreadHandler = thread.getHandler();
        }

        AsyncTask.init();

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        Looper.loop();

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

從上面程式碼可以看出，首先初始化了Looper，這也解釋了為什麼我們在子執行緒中使用Handler不是建立Looper，因為系統已經幫我們實現了。接下來，建立ActivityThread物件，並且呼叫attach()方法。

    private void attach(boolean system) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        if (!system) {
            ViewRootImpl.addFirstDrawHandler(new Runnable() {
                public void run() {
                    ensureJitEnabled();
                }
            });
            android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
                                                    UserHandle.myUserId());
            RuntimeInit.setApplicationObject(mAppThread.asBinder());
            IActivityManager mgr = ActivityManagerNative.getDefault();
            try {
                mgr.attachApplication(mAppThread);
            } catch (RemoteException ex) {
                // Ignore
            }
        } else {
            // Don't set application object here -- if the system crashes,
            // we can't display an alert, we just want to die die die.
            android.ddm.DdmHandleAppName.setAppName("system_process",
                                                    UserHandle.myUserId());
            try {
                mInstrumentation = new Instrumentation();
                ContextImpl context = new ContextImpl();
                context.init(getSystemContext().mPackageInfo, null, this);
                Application app = Instrumentation.newApplication(Application.class, context);
                mAllApplications.add(app);
                mInitialApplication = app;
                app.onCreate();
            } catch (Exception e) {
                throw new RuntimeException(
                        "Unable to instantiate Application():" + e.toString(), e);
            }
        }

        // add dropbox logging to libcore
        DropBox.setReporter(new DropBoxReporter());

        ViewRootImpl.addConfigCallback(new ComponentCallbacks2() {
            public void onConfigurationChanged(Configuration newConfig) {
                synchronized (mPackages) {
                    // We need to apply this change to the resources
                    // immediately, because upon returning the view
                    // hierarchy will be informed about it.
                    if (applyConfigurationToResourcesLocked(newConfig, null)) {
                        // This actually changed the resources!  Tell
                        // everyone about it.
                        if (mPendingConfiguration == null ||
                                mPendingConfiguration.isOtherSeqNewer(newConfig)) {
                            mPendingConfiguration = newConfig;

                            queueOrSendMessage(H.CONFIGURATION_CHANGED, newConfig);
                        }
                    }
                }
            }
            public void onLowMemory() {
            }
            public void onTrimMemory(int level) {
            }
        });
    }

    final ApplicationThread mAppThread = new ApplicationThread();

從原始碼可以看出，因為system為false，所以首先獲取ActivityManagerService物件，然後呼叫ActivityManagerService的attachApplication()方法。至於這裡為什麼說IActivityManager是ActivityManagerService，將會在第三節中提到。繼續看ActivityManagerService的attachApplication()。

    @Override
    public final void attachApplication(IApplicationThread thread) {
        synchronized (this) {
            int callingPid = Binder.getCallingPid();
            final long origId = Binder.clearCallingIdentity();
            attachApplicationLocked(thread, callingPid);
            Binder.restoreCallingIdentity(origId);
        }
    }

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

        省略

        // See if the top visible activity is waiting to run in this process...
            if (normalMode) {
                try {
                    if (mStackSupervisor.attachApplicationLocked(app)) {
                        didSomething = true;
                    }
                } catch (Exception e) {
                    Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
                    badApp = true;
                }
            }
        省略
    }

關於attachApplicationLocked方法的程式碼很多，真正啟動Activity的就是上面幾行程式碼。繼續跟進去

    boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
        final String processName = app.processName;
        boolean didSomething = false;
        for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
            ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks;
            for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) {
                final ActivityStack stack = stacks.get(stackNdx);
                if (!isFrontStack(stack)) {
                    continue;
                }
                ActivityRecord hr = stack.topRunningActivityLocked(null);
                if (hr != null) {
                    if (hr.app == null && app.uid == hr.info.applicationInfo.uid
                            && processName.equals(hr.processName)) {
                        try {
                            if (realStartActivityLocked(hr, app, true, true)) {
                                didSomething = true;
                            }
                        } catch (RemoteException e) {
                            Slog.w(TAG, "Exception in new application when starting activity "
                                  + hr.intent.getComponent().flattenToShortString(), e);
                            throw e;
                        }
                    }
                }
            }
        }
        if (!didSomething) {
            ensureActivitiesVisibleLocked(null, 0);
        }
        return didSomething;
    }

遍歷Activity棧，找到棧頂的Activity，從下面一個程式碼可以看出是startActivity了

    realStartActivityLocked(hr, app, true, true)

繼續跟進去

    省略
    final boolean realStartActivityLocked(ActivityRecord r,
            ProcessRecord app, boolean andResume, boolean checkConfig)
            throws RemoteException {

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

這裡還只是看核心程式碼，從上文可以看出，呼叫app.thread的scheduleLaunchActivity方法，而這裡的app.thread又是ApplicationThread，而ApplicationThread又是什麼呢？其實它是ActivityThread的內部類，當呼叫ApplicationThread的scheduleLaunchActivity的時候，會通過Handler傳送一個訊息，這個Handler就是在ActivityThread中的H這個類，在H中處理又交給ActivityThread的方法來處理。OK，看原始碼。

    public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
                ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
                Bundle state, List<ResultInfo> pendingResults,
                List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
                String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {
            ActivityClientRecord r = new ActivityClientRecord();

            r.token = token;
            r.ident = ident;
            r.intent = intent;
            r.activityInfo = info;
            r.compatInfo = compatInfo;
            r.state = state;

            r.pendingResults = pendingResults;
            r.pendingIntents = pendingNewIntents;

            r.startsNotResumed = notResumed;
            r.isForward = isForward;

            r.profileFile = profileName;
            r.profileFd = profileFd;
            r.autoStopProfiler = autoStopProfiler;

            updatePendingConfiguration(curConfig);

            queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
        }

    private void queueOrSendMessage(int what, Object obj, int arg1, int arg2) {
        synchronized (this) {
            if (DEBUG_MESSAGES) Slog.v(
                TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
                + ": " + arg1 + " / " + obj);
            Message msg = Message.obtain();
            msg.what = what;
            msg.obj = obj;
            msg.arg1 = arg1;
            msg.arg2 = arg2;
            mH.sendMessage(msg);
        }
    }

    private class H extends Handler {
        public static final int LAUNCH_ACTIVITY         = 100;
        省略
        String codeToString(int code) {
            if (DEBUG_MESSAGES) {
                switch (code) {
                    case LAUNCH_ACTIVITY: return "LAUNCH_ACTIVITY";
                    省略
                }
            }
            return Integer.toString(code);
        }
        public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
            switch (msg.what) {
                case LAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
                    ActivityClientRecord r = (ActivityClientRecord)msg.obj;

                    r.packageInfo = getPackageInfoNoCheck(
                            r.activityInfo.applicationInfo, r.compatInfo);
                    handleLaunchActivity(r, null);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                省略
            }
        }

    private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();

        if (r.profileFd != null) {
            mProfiler.setProfiler(r.profileFile, r.profileFd);
            mProfiler.startProfiling();
            mProfiler.autoStopProfiler = r.autoStopProfiler;
        }

        // Make sure we are running with the most recent config.
        handleConfigurationChanged(null, null);

        if (localLOGV) Slog.v(
            TAG, "Handling launch of " + r);
        Activity a = performLaunchActivity(r, customIntent);

        if (a != null) {
            r.createdConfig = new Configuration(mConfiguration);
            Bundle oldState = r.state;
            handleResumeActivity(r.token, false, r.isForward,
                    !r.activity.mFinished && !r.startsNotResumed);

            if (!r.activity.mFinished && r.startsNotResumed) {
                // The activity manager actually wants this one to start out
                // paused, because it needs to be visible but isn't in the
                // foreground.  We accomplish this by going through the
                // normal startup (because activities expect to go through
                // onResume() the first time they run, before their window
                // is displayed), and then pausing it.  However, in this case
                // we do -not- need to do the full pause cycle (of freezing
                // and such) because the activity manager assumes it can just
                // retain the current state it has.
                try {
                    r.activity.mCalled = false;
                    mInstrumentation.callActivityOnPause(r.activity);
                    // We need to keep around the original state, in case
                    // we need to be created again.  But we only do this
                    // for pre-Honeycomb apps, which always save their state
                    // when pausing, so we can not have them save their state
                    // when restarting from a paused state.  For HC and later,
                    // we want to (and can) let the state be saved as the normal
                    // part of stopping the activity.
                    if (r.isPreHoneycomb()) {
                        r.state = oldState;
                    }
                    if (!r.activity.mCalled) {
                        throw new SuperNotCalledException(
                            "Activity " + r.intent.getComponent().toShortString() +
                            " did not call through to super.onPause()");
                    }

                } catch (SuperNotCalledException e) {
                    throw e;

                } catch (Exception e) {
                    if (!mInstrumentation.onException(r.activity, e)) {
                        throw new RuntimeException(
                                "Unable to pause activity "
                                + r.intent.getComponent().toShortString()
                                + ": " + e.toString(), e);
                    }
                }
                r.paused = true;
            }
        } else {
            // If there was an error, for any reason, tell the activity
            // manager to stop us.
            try {
                ActivityManagerNative.getDefault()
                    .finishActivity(r.token, Activity.RESULT_CANCELED, null);
            } catch (RemoteException ex) {
                // Ignore
            }
        }
    }

從handleLaunchActivity()方法中，首先呼叫performLaunchActivity()方法建立Activity。

     private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

        ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

        ComponentName component = r.intent.getComponent();
        if (component == null) {
            component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
            r.intent.setComponent(component);
        }

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }

        Activity activity = null;
        try {
            java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            StrictMode.incrementExpectedActivityCount(activity.getClass());
            r.intent.setExtrasClassLoader(cl);
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to instantiate activity " + component
                    + ": " + e.toString(), e);
            }
        }

        try {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);

            if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
            if (localLOGV) Slog.v(
                    TAG, r + ": app=" + app
                    + ", appName=" + app.getPackageName()
                    + ", pkg=" + r.packageInfo.getPackageName()
                    + ", comp=" + r.intent.getComponent().toShortString()
                    + ", dir=" + r.packageInfo.getAppDir());

            if (activity != null) {
                Context appContext = createBaseContextForActivity(r, activity);
                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
                Configuration config = new Configuration(mCompatConfiguration);
                if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                        + r.activityInfo.name + " with config " + config);
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config);

                if (customIntent != null) {
                    activity.mIntent = customIntent;
                }
                r.lastNonConfigurationInstances = null;
                activity.mStartedActivity = false;
                int theme = r.activityInfo.getThemeResource();
                if (theme != 0) {
                    activity.setTheme(theme);
                }

                activity.mCalled = false;
                mInstrumentation.callActivityOnCreate(activity, r.state);
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                        " did not call through to super.onCreate()");
                }
                r.activity = activity;
                r.stopped = true;
                if (!r.activity.mFinished) {
                    activity.performStart();
                    r.stopped = false;
                }
                if (!r.activity.mFinished) {
                    if (r.state != null) {
                        mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
                    }
                }
                if (!r.activity.mFinished) {
                    activity.mCalled = false;
                    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) {
            throw e;

        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to start activity " + component
                    + ": " + e.toString(), e);
            }
        }

        return activity;
    }

從上面可以看出，先建立Activity、再建立Application，接著依次執行Activity的onCreate()、onStart()方法主要是呼叫Instrumentation類中的方法完成生命週期方法。再回到handleLaunchActivity()，再獲取了Activity物件之後，又呼叫了handleResumeActivity()方法.
再看這個方法之前，我們首先要知道當執行onResume()方法完成了什麼？相信大家都知道，就是Activity從不可見到可見狀態。

    final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward,
            boolean reallyResume) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();

        ActivityClientRecord r = performResumeActivity(token, clearHide);

        if (r != null) {
            final Activity a = r.activity;

            if (localLOGV) Slog.v(
                TAG, "Resume " + r + " started activity: " +
                a.mStartedActivity + ", hideForNow: " + r.hideForNow
                + ", finished: " + a.mFinished);

            final int forwardBit = isForward ?
                    WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;

            // If the window hasn't yet been added to the window manager,
            // and this guy didn't finish itself or start another activity,
            // then go ahead and add the window.
            boolean willBeVisible = !a.mStartedActivity;
            if (!willBeVisible) {
                try {
                    willBeVisible = ActivityManagerNative.getDefault().willActivityBeVisible(
                            a.getActivityToken());
                } catch (RemoteException e) {
                }
            }
            if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow();
                View decor = r.window.getDecorView();
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
                if (a.mVisibleFromClient) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                }

            // If the window has already been added, but during resume
            // we started another activity, then don't yet make the
            // window visible.
            } else if (!willBeVisible) {
                if (localLOGV) Slog.v(
                    TAG, "Launch " + r + " mStartedActivity set");
                r.hideForNow = true;
            }

從上面可以看出首先呼叫performResumeActivity()，接著就是完成一些使Activity可見的操作，比如獲取DecorView，並條件到windowManager上等，這裡就不詳細說了。從這裡我們就大致瞭解Activity啟動過程。

三、從startActivity()原始碼開始分析

首先從Activity的startActivity()方法開始看,幾種過載都是呼叫startActivityForResult()方法

    @Override
    public void startActivity(Intent intent, Bundle options) {
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    }

startActivityForResult()如下：

    public void startActivityForResult(Intent intent, int requestCode, Bundle options) {
        if (mParent == null) {
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }
            if (requestCode >= 0) {
                // If this start is requesting a result, we can avoid making
                // the activity visible until the result is received.  Setting
                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
                // activity hidden during this time, to avoid flickering.
                // This can only be done when a result is requested because
                // that guarantees we will get information back when the
                // activity is finished, no matter what happens to it.
                mStartedActivity = true;
            }
        } else {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                // Note we want to go through this method for compatibility with
                // existing applications that may have overridden it.
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

將Activity的啟動交給了Instrumentation

     public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    if (am.match(who, null, intent)) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return requestCode >= 0 ? am.getResult() : null;
                        }
                        break;
                    }
                }
            }
        }
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess();
            int result = ActivityManagerNative.getDefault()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
        }
        return null;
    }

然後Activity的啟動又交給了ActivityManagerNative.getDefault()

    public abstract class ActivityManagerNative extends Binder implements IActivityManager

ActivityManagerNative是一個抽象類，它繼承Binder、實現IActivityManager，因此是一個Binder，接著看getDefault()方法返回的是什麼？

      static public IActivityManager getDefault() {
        return gDefault.get();
    }

再看

    public final class ActivityManagerService  extends ActivityManagerNative

由此可以看出AMS是具體實現。

然後就是AMS.startActivity()了,之後就是ActivityStack和ActivitySupervisor之間方法的切換，最後在ActivitySupervisor的realStartActivityLocked()方法中真正啟動，接下來的和第二節的類似了，就不重複分析了。

四、分析setContentView()

相信setContentView()這個方法大家熟悉不過了，就是給Activity設定佈局，那麼它的底層到底是怎麼實現的呢？

    public void setContentView(int layoutResID) {
        getWindow().setContentView(layoutResID);
        initActionBar();
    }

這裡呼叫window的setContentView(),我們知道這裡的window就是PhoneWindow，看原始碼

    @Override
    public void setContentView(int layoutResID) {
        if (mContentParent == null) {
            installDecor();
        } else {
            mContentParent.removeAllViews();
        }
        mLayoutInflater.inflate(layoutResID, mContentParent);
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

    private void installDecor() {
        if (mDecor == null) {
            mDecor = generateDecor();
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        }
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);

            // Set up decor part of UI to ignore fitsSystemWindows if appropriate.
            mDecor.makeOptionalFitsSystemWindows();

            mTitleView = (TextView)findViewById(com.android.internal.R.id.title);
            if (mTitleView != null) {
                mTitleView.setLayoutDirection(mDecor.getLayoutDirection());
                if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
                    View titleContainer = findViewById(com.android.internal.R.id.title_container);
                    if (titleContainer != null) {
                        titleContainer.setVisibility(View.GONE);
                    } else {
                        mTitleView.setVisibility(View.GONE);
                    }
                    if (mContentParent instanceof FrameLayout) {
                        ((FrameLayout)mContentParent).setForeground(null);
                    }
                } else {
                    mTitleView.setText(mTitle);
                }
            } else {
                mActionBar = (ActionBarView) findViewById(com.android.internal.R.id.action_bar);
                if (mActionBar != null) {
                    mActionBar.setWindowCallback(getCallback());
                    if (mActionBar.getTitle() == null) {
                        mActionBar.setWindowTitle(mTitle);
                    }
                    final int localFeatures = getLocalFeatures();
                    if ((localFeatures & (1 << FEATURE_PROGRESS)) != 0) {
                        mActionBar.initProgress();
                    }
                    if ((localFeatures & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
                        mActionBar.initIndeterminateProgress();
                    }

                    boolean splitActionBar = false;
                    final boolean splitWhenNarrow =
                            (mUiOptions & ActivityInfo.UIOPTION_SPLIT_ACTION_BAR_WHEN_NARROW) != 0;
                    if (splitWhenNarrow) {
                        splitActionBar = getContext().getResources().getBoolean(
                                com.android.internal.R.bool.split_action_bar_is_narrow);
                    } else {
                        splitActionBar = getWindowStyle().getBoolean(
                                com.android.internal.R.styleable.Window_windowSplitActionBar, false);
                    }
                    final ActionBarContainer splitView = (ActionBarContainer) findViewById(
                            com.android.internal.R.id.split_action_bar);
                    if (splitView != null) {
                        mActionBar.setSplitView(splitView);
                        mActionBar.setSplitActionBar(splitActionBar);
                        mActionBar.setSplitWhenNarrow(splitWhenNarrow);

                        final ActionBarContextView cab = (ActionBarContextView) findViewById(
                                com.android.internal.R.id.action_context_bar);
                        cab.setSplitView(splitView);
                        cab.setSplitActionBar(splitActionBar);
                        cab.setSplitWhenNarrow(splitWhenNarrow);
                    } else if (splitActionBar) {
                        Log.e(TAG, "Requested split action bar with " +
                                "incompatible window decor! Ignoring request.");
                    }

                    // Post the panel invalidate for later; avoid application onCreateOptionsMenu
                    // being called in the middle of onCreate or similar.
                    mDecor.post(new Runnable() {
                        public void run() {
                            // Invalidate if the panel menu hasn't been created before this.
                            PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
                            if (!isDestroyed() && (st == null || st.menu == null)) {
                                invalidatePanelMenu(FEATURE_ACTION_BAR);
                            }
                        }
                    });
                }
            }
        }
    }

從原始碼可以看出，首先獲取DecorView物件，然後將其傳到generateLayout(mDecor)獲取mContentParent，那麼這個mContentParent是什麼呢？其實就是我們設定的xml佈局，mContentParent + titleView就是我們說的DecorView.至於generateLayout(mDecor)是如何完成的，這裡就不詳解了。再次回到PhoneWindow的setContentView()，接著執行mLayoutInflater.inflate(layoutResID, mContentParent)，再獲取了內容之後，就開始填充了。

OK，這篇部落格就到這裡了。