protected void onCreate(final Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

setContentView()方法會將我們的視圖設(shè)置到哪兒去了？

public void setContentView(@LayoutRes int layoutResID) {
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}

Activity會調(diào)用當(dāng)前Activity的Window的setContentView()方法，而Window類是一個(gè)抽象類，唯一實(shí)現(xiàn)類PhoneWindow。

PhoneWindwo對象在Activity中的attach()方法中創(chuàng)建胁住。

 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,
        NonConfigurationInstances lastNonConfigurationInstances,
        Configuration config, String referrer, IVoiceInteractor voiceInteractor,
        Window window) {
    attachBaseContext(context);
    
    mFragments.attachHost(null /*parent*/);
    
    mWindow = new PhoneWindow(this, window);
    mWindow.setWindowControllerCallback(this);
    mWindow.setCallback(this);
    mWindow.setOnWindowDismissedCallback(this);
    mWindow.getLayoutInflater().setPrivateFactory(this);
    //......
}

那我們接著看PhoneWindow的setContentView()方法趁猴。

public void setContentView(int layoutResID) {
    
    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();
    }
}

當(dāng)我們第一此setContentView()時(shí)，mContentParent為空彪见，會調(diào)用installDecor()方法做些初始化工作躲叼。然后再將我們setContentView里的View加載到mContentParent上面去。

mLayoutInflater.inflate(layoutResID, mContentParent);

來看看installDecor()做了什么偉大的事情:

private DecorView mDecor;

private ViewGroup mContentParent;

private ViewGroup mContentRoot;

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();
        //....
    }
}

mDecor是DecorView類對象企巢，而DecorView繼承自FrameLayout枫慷，generateDecor()方法就是初始化創(chuàng)建一個(gè)空的FrameLayout。來看generateLayout()方法是怎樣初始化我們需要的mContentParent的：

protected ViewGroup generateLayout(DecorView decor) {   
    TypedArray a = getWindowStyle();
    if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {
        requestFeature(FEATURE_NO_TITLE);
    } else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {
        requestFeature(FEATURE_ACTION_BAR);
    }
    
    if (a.getBoolean(R.styleable.Window_windowActionBarOverlay, false)) {
        requestFeature(FEATURE_ACTION_BAR_OVERLAY);
    }
    //....
    if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
        layoutResource = R.layout.screen_swipe_dismiss;
    } else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
        if (mIsFloating) {
            TypedValue res = new TypedValue();
            getContext().getTheme().resolveAttribute(
                    R.attr.dialogTitleIconsDecorLayout, res, true);
            layoutResource = res.resourceId;
        } else {
            layoutResource = R.layout.screen_title_icons;
        }
        removeFeature(FEATURE_ACTION_BAR);
    } else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
            && (features & (1 << FEATURE_ACTION_BAR)) == 0) {
        layoutResource = R.layout.screen_progress;
     
    } else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {}
    //.....
    int layoutResource;
    //.....
    View in = mLayoutInflater.inflate(layoutResource, null);
    decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    
    mContentRoot = (ViewGroup) in;
    
    ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
    //.....
    return contentParent;
 }

generateLayout()方法會根據(jù)我們Acivity主題樣式浪规，選擇加載不同的系統(tǒng)布局資源或听，并將該視圖添加到DecorView中去。

View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
mContentRoot = (ViewGroup) in;

無論選擇哪一個(gè)布局資源里面都一個(gè)id名為content的FrameLayout笋婿。再從該布局中找到我們需要的contentParent誉裆。

(ViewGroup)findViewById(ID_ANDROID_CONTENT);

最后返回contentParent，將setContentView()中的視圖加載到mContentParent上去缸濒。

20180509223239.jpg