1 從我們自己新建的一個Activity的setContentView 就開始

public class ActivityTest extends AppCompatActivity{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_test);

    }

}
                ↓
                ↓ setContentView(R.layout.activity_test);
                ↓
public class AppCompatActivity extends ... {

     public void setContentView(@LayoutRes int layoutResID) {
        this.getDelegate().setContentView(layoutResID);
     }


                ↓
                ↓ getDelegate().setContentView(layoutResID);先找getDelegate()
                ↓ getDelegate()也在AppCompatActivity 中
                ↓
 @NonNull
    public AppCompatDelegate getDelegate() {
        if (mDelegate == null) {
            mDelegate = AppCompatDelegate.create(this, this);
        }
        return mDelegate;
    }
}
                ↓
                ↓  getDelegate() = AppCompatDelegate.create(this, this);
                ↓
public abstract class AppCompatDelegate {
  public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
        return new AppCompatDelegateImpl(activity, activity.getWindow(), callback);
    }   
}

所以最后呼叫的是

2 AppCompatDelegateImpl 中的 setContentView 方法：

class AppCompatDelegateImpl extends ...{

    public void setContentView(int resId) {
        this.ensureSubDecor();
        ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(16908290);
        contentParent.removeAllViews();
        LayoutInflater.from(this.mContext).inflate(resId, contentParent);
        this.mOriginalWindowCallback.onContentChanged();
    }

}

接著去看 ensureSubDecor 方法：

2.1 ensureSubDecor

private void ensureSubDecor() {
        if (!this.mSubDecorInstalled) {
            this.mSubDecor = this.createSubDecor();//核心代碼

           //省略其它代碼，，，
        }

    }

2.2 createSubDecor

 private ViewGroup createSubDecor() {
        ?這里是獲取Activity的theme，并根據主題風格為subDecor確定加載哪個布局 
        TypedArray a = this.mContext.obtainStyledAttributes(styleable.AppCompatTheme);
        if (!a.hasValue(styleable.AppCompatTheme_windowActionBar)) {
            a.recycle();
            throw new IllegalStateException("You need to use a Theme.AppCompat theme (or descendant) with this activity.");
        } else {
             

            this.mIsFloating = a.getBoolean(styleable.AppCompatTheme_android_windowIsFloating, false);
            a.recycle();
            this.mWindow.getDecorView(); ---> ???step1 創建DecirView
            LayoutInflater inflater = LayoutInflater.from(this.mContext);
            ViewGroup subDecor = null; ???Step2 開始
            if (!this.mWindowNoTitle) {
                if (this.mIsFloating) {
                    subDecor = (ViewGroup)inflater.inflate(layout.abc_dialog_title_material, (ViewGroup)null);
                    
                } else if (this.mHasActionBar) {
                    
                    subDecor = (ViewGroup)LayoutInflater.from((Context)themedContext).inflate(layout.abc_screen_toolbar, (ViewGroup)null); 
                    
                }
            } else {
                if (this.mOverlayActionMode) {
                    subDecor = (ViewGroup)inflater.inflate(layout.abc_screen_simple_overlay_action_mode, (ViewGroup)null);
                } else {
                    subDecor = (ViewGroup)inflater.inflate(layout.abc_screen_simple, (ViewGroup)null);
                }

                //其它代碼省略...
                
                
            }
            ? 通過上面subDecor = inflate(布局檔案)可以看出,subDecor 的布局檔案是下面四種布局檔案之一：
                  1  abc_dialog_title_material
                  2  abc_screen_toolbar
                  3  abc_screen_simple_overlay_action_mode
                  4  abc_screen_simple
            if (subDecor == null) {
                ? 如果subDecor為空就拋出例外，這個例外看起來是不是很熟悉 
                throw new IllegalArgumentException("AppCompat does not support the current theme features: { windowActionBar: " + this.mHasActionBar + ", windowActionBarOverlay: " + this.mOverlayActionBar + ", android:windowIsFloating: " + this.mIsFloating + ", windowActionModeOverlay: " + this.mOverlayActionMode + ", windowNoTitle: " + this.mWindowNoTitle + " }");
            } else {
                if (this.mDecorContentParent == null) {
                    this.mTitleView = (TextView)subDecor.findViewById(id.title);
                }

                ViewUtils.makeOptionalFitsSystemWindows(subDecor);
                 //? 上面說了 subDecor 是四個布局檔案中的一個創建，
                 // ?每個布局檔案都有一action_bar_activity_content   
                ContentFrameLayout contentView = (ContentFrameLayout)subDecor.findViewById(id.action_bar_activity_content);
                ViewGroup windowContentView = (ViewGroup)this.mWindow.findViewById(16908290);
                if (windowContentView != null) {
                    while(windowContentView.getChildCount() > 0) {
                        View child = windowContentView.getChildAt(0);
                        windowContentView.removeViewAt(0);
                        contentView.addView(child);
                    }

                    windowContentView.setId(-1);
                    contentView.setId(16908290);--->???Step2 結束 contentView的id被設定成16908290
                    if (windowContentView instanceof FrameLayout) {
                        ((FrameLayout)windowContentView).setForeground((Drawable)null);
                    }
                }

                this.mWindow.setContentView(subDecor);---> ???Step3 把subDecor加載到DecorView布局檔案中的容器中
                contentView.setAttachListener(new OnAttachListener() {
                    public void onAttachedFromWindow() {
                    }

                    public void onDetachedFromWindow() {
                        AppCompatDelegateImpl.this.dismissPopups();
                    }
                });
                return subDecor; ? 把subDecor回傳賦值給mSubDecor
            }
        }
    }

總結一下createSubDecor方法，一共做了三件事：

1 this.mWindow.getDecorView(); 創建Decorview,并為它加載一個布局檔案，找到這個布局檔案中 R.id.content 的容器，賦值給 mContentParent，

這樣我們就準備好了一個DecorView和其布局中id為R.id.content 的容器，

2 給ViewGroup subDecor根據主題、style選擇合適布局檔案并加載到subDecor中：

ContentFrameLayout contentView = (ContentFrameLayout)subDecor.findViewById(id.action_bar_activity_content);

ViewGroup windowContentView = (ViewGroup)this.mWindow.findViewById(R.id.content);? 這里就是上一步里面那個布局檔案的R.id.content 容器

windowContentView.setId(View.NO_ID); ? 把windowContentView的id設定為View.NO_ID 即 -1

contentView.setId(android.R.id.content); ? 把contentView 的id設定為R.id.content

這樣我們準備好了subDecor和其布局中 id為action_bar_activity_content的容器，并把這個容器的id改成 R.id.content

3 this.mWindow.setContentView(subDecor); 將第2步的subDecor添加到 第1步準備好的DecorView的容器mContentParent中，

下面分析這三步驟

Step1 this.mWindow.getDecorView(); this.mWindow 這個是啥呢？看下面

 AppCompatDelegateImpl(Context context, Window window, AppCompatCallback callback) {
        ......

       mWindow 的初始化是在AppCompatDelegateImpl建構式里
                ↓
       this.mWindow = window;

        .....
       
    }
        想要知道mWindow是啥就要找到AppCompatDelegateImpl(context,window,callback)這個
        建構式初始化的時候傳入的window是啥
        還記得最開始我們從setContentView點進來的代碼么
        
                ↓
                ↓  
                ↓
*********************************************************************** 
public class ActivityTest extends AppCompatActivity{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_test);

    }

}
                ↓
                ↓ setContentView(R.layout.activity_test);
                ↓
public class AppCompatActivity extends ... {

     public void setContentView(@LayoutRes int layoutResID) {
        this.getDelegate().setContentView(layoutResID);
     }


                ↓
                ↓ getDelegate().setContentView(layoutResID);先找getDelegate()
                ↓ getDelegate()也在AppCompatActivity 中
                ↓
 @NonNull
    public AppCompatDelegate getDelegate() {
        if (mDelegate == null) {
            mDelegate = AppCompatDelegate.create(this, this);
        }
        return mDelegate;
    }
}
                ↓
                ↓  getDelegate() = AppCompatDelegate.create(this, this);
                ↓
public abstract class AppCompatDelegate {
  public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
        在這里初始化的，activity就是AppCompatActivity ，window就是activity.getWindow()
        return new AppCompatDelegateImpl(activity, activity.getWindow(), callback);
    }   
}
***************************************************************************************

window就是AppCompatActivity.getWindow(),但是AppCompatActivity中沒有getWindow()方法，
getWindow()是在其父類Activity中實作
public class Activity extends ... ... {

       private Window mWindow;
        
      final void attach(Context context, ......) {
        attachBaseContext(context);

           

        mWindow = new PhoneWindow(this, window, activityConfigCallback);
        
        ......
       }

      public @Nullable Window getWindow() {
            return mWindow;
       }
}

最后找到window就是PhoneWindow物件，

那就到里PhoneWindow面看一下 getDecorView()是如何實作的

PhoneWindow.java: 
@Override
    public final View getDecorView() {
        if (mDecor == null || mForceDecorInstall) {
            installDecor();  --->  ?呼叫installDecor()
        }
        return mDecor;
    }

 private void installDecor() {
        mForceDecorInstall = false;
        if (mDecor == null) {
            mDecor = generateDecor(-1); ---> ? generateDecor就是創建DecorView物件
           
            ....省略其它代碼....
        } else {
            mDecor.setWindow(this);
        }
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor); ---> ? 給DecorView物件加載布局檔案
                                                              并準備好R.id.content容器               
            ....省略其它代碼.....
            }
}

先來看 mDecor = generateDecor(-1); generateDecor方法很簡單就是創建了一個DecorView物件并回傳賦值給 mDecor

 protected DecorView generateDecor(int featureId) {
      
        ...其它代碼省略...        

        return new DecorView(context, featureId, this, getAttributes());
    }

接著看 mContentParent = generateLayout(mDecor);

1：mContentParent是一個ViewGroup；

2 ：generateLayout(mDecor);將上面剛剛創建的DecorView作為引數傳了進去

protected ViewGroup generateLayout(DecorView decor) {
      ? 獲取windowStyle,給DecorView加載哪種布局就是根據這個來判斷的！
      TypedArray a = getWindowStyle();        

        ...其它代碼省略...

        // Inflate the window decor.

        ? 這里精簡保留的，layoutResource = R.layout.布局檔案 
        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) {
              
                layoutResource = res.resourceId;
            } else {
                layoutResource = R.layout.screen_title_icons;
            }
            
        } 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) {
           
            if (mIsFloating) {
               
                layoutResource = res.resourceId;
            } else {
                layoutResource = R.layout.screen_custom_title;
            }
           
        } else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
             
            if (mIsFloating) {
              
                layoutResource = res.resourceId;
            } else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
                layoutResource = a.getResourceId(
                        R.styleable.Window_windowActionBarFullscreenDecorLayout,
                        R.layout.screen_action_bar);
            } else {
                layoutResource = R.layout.screen_title;
            }
            
        } else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
            layoutResource = R.layout.screen_simple_overlay_action_mode;
        } else {
             
            layoutResource = R.layout.screen_simple;
            
        }

        mDecor.startChanging();

       ? layoutResource就是上面根據WindowStyle決定的布局檔案之中的一個，
            然后加載到mDecor即DecorView中
                    ↓
        mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
       
       ? 而上面布局檔案中它們都有一個id是content的
       ? 而且這個findViewById是在PhoneWindow直接使用？它是Activity?文章最后分析
                      ↓
        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
        if (contentParent == null) {
            throw new RuntimeException("Window couldn't find content container view");
        }

        
        return contentParent; ← ?回傳DecorView的布局檔案中id為content的控制元件
    }

到這里 step1 this.mWindow.getDecorView() 就完成了，PhoneWindow新建了一個DecorView并為其加載好布局檔案，并將布局檔案中R.id.content容器準備好，

Step2 具體分析已在上面代碼中注釋

step3 this.mWindow.setContentView(subDecor)

接著我們就來看看3 this.mWindow.setContentView(subDecor);同樣是this.mWindow我們就到PhoneWindow中找setContentView方法

@Override
    public void setContentView(View view) {
        setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    }

    @Override
    public void setContentView(View view, ViewGroup.LayoutParams params) {
        // 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)) {
            view.setLayoutParams(params);
            final Scene newScene = new Scene(mContentParent, view);
            transitionTo(newScene);
        } else {
           ?還記得 step1 的時候準備好的mContentParent，現在就是把subDecor加載到其中
            ↓  
            mContentParent.addView(view, params);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

mContentParent.addView(view, params);

|--- mContentParent 就是上面DecorView中加載布局檔案的 content 控制元件

|--- view 就是我們傳遞進來的subDecor

到這里Step3 就結束了，

再來看最開始的setContentView

 public void setContentView(int resId) {
        this.ensureSubDecor(); ? 我們上面這么久的呼叫就是走完這一行代碼，想不到吧，

        ?找到id為R.id.content的控制元件，即subDecor布局中的控制元件
        ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(R.id.content);

        contentParent.removeAllViews();

        ?把我們的布局檔案加載到R.id.content的控制元件中即subDecor布局中
        LayoutInflater.from(this.mContext).inflate(resId, contentParent);

        this.mOriginalWindowCallback.onContentChanged();
    }

我們自己的布局檔案被加載到了subDecor布局中id為 action_bar_activity_content的 ContentFrameLayout中，

而subDecor被加載到了DecorView布局中id為content的FrameLayout中；

DecorView是由PhoneWindow 創建

PhoneWindow 在Activity attach方法中初始化，

這樣整個加載程序就串起來了，

是時候放上這樣一張圖了

DecorView加載的xml檔案為下面中的一個：

screen_swipe_dismiss

screen_title_icons

screen_progress

screen_custom_title

screen_action_bar

screen_title

screen_simple_overlay_action_mode

screen_simple

而他們都有一個id為content的FrameLayout控制元件用來加載subDcor.

subDcor加載的xml檔案為下面中的一個：

abc_dialog_title_material
abc_screen_toolbar
abc_screen_simple_overlay_action_mode
abc_screen_simple

而他們都有一個id為action_bar_activity_content 的 ContentFrameLayout 控制元件用來加載我們自定義的布局檔案，就是我們創建Activity時候創建的xml檔案，

上面還有一個問題PhoneWindow為啥可以直接findViewById? 點進去就會發現是在其父類Window中實作的

public abstract class Window {
     @Nullable
    public <T extends View> T findViewById(@IdRes int id) {
        return getDecorView().findViewById(id);
    }

    public abstract View getDecorView();

}

getDecorView的具體實作又回到了PhoneWindow 中；

    @Override
    public final View getDecorView() {
        if (mDecor == null || mForceDecorInstall) {
            installDecor();
        }
        return mDecor;
    }

這個方法看起來是不是很熟悉，就是我們初始化DecorView物件的方法，回傳的就是我們初始化并加載了布局的DecorView.

所以PhoneWindow進行findViewById其實就是對其持有的DecorView進行操作，