直接撸代码

扛不住直接翻到后面，但是不看代码，只看结论，对于学习是没有什么帮助的

先初始化PhoneWindow:
Activity 在创建出来之后，会调用attach方法，在这里面会创建PhoneWindow，并初始化WindowManager，WindowManagerService就是Binder的本地代理对象，用于向系统Window中添加我们视图；

 final void Activity.attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor,
            Window window, ActivityConfigCallback activityConfigCallback) {
        attachBaseContext(context);
        mWindow = new PhoneWindow(this, window, activityConfigCallback);
        ...
        mWindow.setWindowManager(
                (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        ...
}

调用PhoneWindow的setContentView，初始化后，再进行设置ActionBar；

public void Activity.setContentView(View view) {
        getWindow().setContentView(view);
        initWindowDecorActionBar();
}

private void initWindowDecorActionBar() {
        Window window = getWindow();
        // Initializing the window decor can change window feature flags.
        // Make sure that we have the correct set before performing the test below.
        window.getDecorView();
        if (isChild() || !window.hasFeature(Window.FEATURE_ACTION_BAR) || mActionBar != null) {
            return;
        }
        mActionBar = new WindowDecorActionBar(this);
        mActionBar.set ...
}

接下来就进入到PhoneWindow中;
这里面就能看到整体的创建流程；可以得出几个结论：

1. Activity的根View就是DecorView；
   在mContentParent没有初始化过，那就要先进行初始化DecorView；进入到installDecor()；
2. 当View设置了变换动画，需要先清空View，然后让这些View去执行变换动画；比如Android5.0的转场动画；
   也就是在场景Scene变化时，是通过Transitions变换来实现转场动画的；
   更多场景和变换动画可参考http://www.jcodecraeer.com/a/anzhuokaifa/androidkaifa/2015/0113/2310.html
   image.png
3. mContentParent就是com.android.internal.R.id.content，我们称为RootView
4. 最后如果是ResLayoutId就通过LayoutInflater来解析填充，并挂载到mContentParent上；

public void PhoneWindow.setContentView(int layoutResID) {
    // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
    // decor, when theme attributes and the like are crystalized. Do not check the feature
    // before this happens.
    if (mContentParent == null) {
        installDecor();
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        mContentParent.removeAllViews();
    }
    if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID, getContext());
        transitionTo(newScene);
    } else {
        mLayoutInflater.inflate(layoutResID, mContentParent);
    }
    mContentParent.requestApplyInsets();
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        cb.onContentChanged();
    }
    mContentParentExplicitlySet = true;
}

再看下DecorView和mContentParent的生成：

//初始化DecorView和mContentParent
 private void PhoneWindow.installDecor() {
        mForceDecorInstall = false;
        if (mDecor == null) {
            mDecor = generateDecor(-1);
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        } else {
            mDecor.setWindow(this);
        }
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);
            // Set up decor part of UI to ignore fitsSystemWindows if appropriate.
            mDecor.makeOptionalFitsSystemWindows();
            final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById( R.id.decor_content_parent);
        }
}
//创建DecorView
protected DecorView PhoneWindow.generateDecor(int featureId) {
        // System process doesn't have application context and in that case we need to directly use
        // the context we have. Otherwise we want the application context, so we don't cling to the
        // activity.
        Context context;
        if (mUseDecorContext) {
            Context applicationContext = getContext().getApplicationContext();
            if (applicationContext == null) {
                context = getContext();
            } else {
                context = new DecorContext(applicationContext, getContext());
                if (mTheme != -1) {
                    context.setTheme(mTheme);
                }
            }
        } else {
            context = getContext();
        }
        return new DecorView(context, featureId, this, getAttributes());
}
//创建mContentParent，里面代码省略了一大堆
protected ViewGroup PhoneWindowgenerateLayout(DecorView decor){
        // Apply data from current theme.

        TypedArray a=getWindowStyle();
        ...
        if(a.getBoolean(R.styleable.Window_windowFullscreen,false)){
        setFlags(FLAG_FULLSCREEN,FLAG_FULLSCREEN&(~getForcedWindowFlags()));
        }
        ...设置各种style

        if(a.hasValue(R.styleable.Window_windowFixedWidthMajor)){
        if(mFixedWidthMajor==null)mFixedWidthMajor=new TypedValue();
        a.getValue(R.styleable.Window_windowFixedWidthMajor,
        mFixedWidthMajor);
        }
        ...设置Window属性
        WindowManager.LayoutParams params=getAttributes();

        if(params.windowAnimations==0){
        params.windowAnimations=a.getResourceId(
        R.styleable.Window_windowAnimationStyle,0);
        }
        ...创建DecorView的属性
        // Inflate the window decor.
        int layoutResource;
        int features=getLocalFeatures();
        // System.out.println("Features: 0x" + Integer.toHexString(features));
        if((features&(1<<FEATURE_ACTION_MODE_OVERLAY))!=0){
        layoutResource=R.layout.screen_simple_overlay_action_mode;
        }else{
        // Embedded, so no decoration is needed.
        layoutResource=R.layout.screen_simple;
        // System.out.println("Simple!");
        }

        mDecor.startChanging();
        mDecor.onResourcesLoaded(mLayoutInflater,layoutResource);
        // 根据不同的属性，选择不同的layout，比Theme带ActionBar的选用ActionBar的layout；
        //但是里面都个id是ID_ANDROID_CONTENT，也就是com.android.internal.R.id.content
        ViewGroup contentParent=(ViewGroup)findViewById(ID_ANDROID_CONTENT);
        if(contentParent==null){
        throw new RuntimeException("Window couldn't find content container view");
        }
        ...
        mDecor.finishChanging();
}

到此全部创建完成。

整体流程是：

1. attach时创建PhoneWindow，用于挂载到系统上；
2. 在PhoneWindow中调用generateDecor(-1)创建DecorView，设置为根View，在DecorView实际是个FrameLayout；并在这时候设置各种系统属性；
3. 在DecorView中创建mContentParent，实际是通过不同的主题，直接选择不同Layout，然后填充成mRootView；并将mRootView添加到DecorView中；最后通过findViewById(ID_ANDROID_CONTENT)，取出mContentParent；
4. 设置场景和变换动画；
5. 如果我们传入的是ResLayoutId，就将mContentParent做为parent使用LayoutInflater填充；否则直接addView挂载到mContentParent上；