Activity -- setContentView

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

public Window getWindow() {
    return mWindow;
}

final void attach(虎谢。脑融。。) {
    ...
    mWindow = new PhoneWindow(this, window, activityConfigCallback);
    ...
}

在Activity的setContentView方法中調(diào)用了Window對應(yīng)的方法，Window本身是一個(gè)抽象類吊履，這里它的實(shí)現(xiàn)類是PhoneWindow，實(shí)際上調(diào)用的是PhoneWindow的setContentView方法调鬓。

PhoneWindow -- setContentView

ViewGroup mContentParent;

@Override
public void setContentView(int layoutResID) {
    if (mContentParent == null) {
        installDecor();   //注釋1
    } 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;
}

第一次調(diào)用的時(shí)候mContentParent為null艇炎，會執(zhí)行installDecor()方法，在該方法中會創(chuàng)建mContentParent;繼續(xù)執(zhí)行下面會把傳入的布局添加到mContentParent中：mLayoutInflater.inflate(layoutResID, mContentParent);

//注釋1
private void installDecor() {
    ...
    if (mDecor == null) {
        mDecor = generateDecor(-1);
        ...
    } else {
        mDecor.setWindow(this);
    }
    if (mContentParent == null) {
        mContentParent = generateLayout(mDecor);
        ...
    }
}

在installDecor方法中腾窝，會創(chuàng)建mDecor(DecorView)和mContentParent

protected DecorView generateDecor(int featureId) {
    Context context;
    if (mUseDecorContext) {
        Context applicationContext = getContext().getApplicationContext();
        if (applicationContext == null) {
            context = getContext();
        } else {
            context = new DecorContext(applicationContext, getContext().getResources());
            if (mTheme != -1) {
                context.setTheme(mTheme);
            }
        }
    } else {
        context = getContext();
    }
    return new DecorView(context, featureId, this, getAttributes());
}

public class DecorView extends FrameLayout

直接new DecorView冕臭，DecorView本質(zhì)是一個(gè)FrameLayout。

protected ViewGroup generateLayout(DecorView decor) {
    ......
   //根據(jù)設(shè)置的feature來選擇Activity的根部局
    int layoutResource;
    int features = getLocalFeatures();    
    if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
        layoutResource = R.layout.screen_swipe_dismiss;
        setCloseOnSwipeEnabled(true);
    } 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 {
        layoutResource = R.layout.screen_simple;   
    }

    mDecor.startChanging();
    //往DecorView加載根部局
    mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
    //找到內(nèi)容布局：不管選擇哪個(gè)根部局燕锥，內(nèi)容布局id都設(shè)置為ID_ANDROID_CONTENT這個(gè)參數(shù)值
    ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);  
    ...
    return contentParent;
}

public abstract class Window{
    public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;
}

根據(jù)feature來選擇Activity不同的布局，然后將布局添加到mDecor中悯蝉，不管是哪種布局归形，內(nèi)容的布局id都是ID_ANDROID_CONTENT，然后獲取內(nèi)容布局賦值給contentParent鼻由。

R.layout.screen_simple

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    android:orientation="vertical">
    <ViewStub android:id="@+id/action_mode_bar_stub"
              android:inflatedId="@+id/action_mode_bar"
              android:layout="@layout/action_mode_bar"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:theme="?attr/actionBarTheme" />
    <FrameLayout
         android:id="@android:id/content"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:foregroundInsidePadding="false"
         android:foregroundGravity="fill_horizontal|top"
         android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

R.layout.screen_simple添加到DecorView中暇榴，DecorView在PhoneWindow中，所以通過findViewById(ID_ANDROID_CON)獲取到的是FrameLayout返回給mContentParent 蕉世。通過setContentView設(shè)置布局就是往mContentParent里面添加內(nèi)容蔼紧。它本質(zhì)是一個(gè)FrameLayout。

布局層級圖

我們打開一個(gè)應(yīng)用狠轻，最外層是一個(gè)PhoneWindow奸例，它里層是一個(gè)DecorView，本質(zhì)就是一個(gè)FrameLayout向楼，然后它里層是一個(gè)LinearLayout查吊，是一個(gè)垂直方向的，上面是延時(shí)加載的ActionBar湖蜕，因?yàn)锳ctivity可能沒有actionBar逻卖，所以用ViewStub布局；下面是一個(gè)FrameLayout昭抒，我們通過setContentView設(shè)置的布局评也，就是往該FrameLayout中添加內(nèi)容。

Kotlin項(xiàng)目實(shí)戰(zhàn)