我们通常写Activity的时候在onCreate里面都会写上setContentView()这一步，该方法会将你指定的布局文件加载并显示到屏幕中，在其中会经历一系列操作，其中也就涉及到了Activity,Window，DecorView,ViewRootImpl，今天就让我们借助这个方法来看看这些对象之间的关系。

如果你在5.0之前就开始学习android了，那你会发现一个Activity的改变，就是默认创建Activity类时所继承的类不同了，在5.0之前默认继承的是Activity类，在5.0之后默认继承的是AppCompatActivity类。

5.0之前

setContentView是这样的

public void setContentView(int layoutResID) {getWindow().setContentView(layoutResID);initWindowDecorActionBar();
}
public void setContentView(View view) {getWindow().setContentView(view);initWindowDecorActionBar();
}
public void setContentView(View view, ViewGroup.LayoutParams params) {getWindow().setContentView(view, params);initWindowDecorActionBar();
}

一共有三个构造方法

布局文件id
View对象
View对象和ViewGroup的布局参数

我们最常用的便是第一个了，首先获取一个window对象然后调用它的setContentView方法，initWindowDecorActionBar();

()会初始化actionBar。

    public Window getWindow() {return mWindow;}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, ActivityConfigCallback activityConfigCallback) {attachBaseContext(context);.............................................................mWindow = new PhoneWindow(this, window, activityConfigCallback);mWindow.setWindowControllerCallback(this);mWindow.setCallback(this);mWindow.setOnWindowDismissedCallback(this);mWindow.getLayoutInflater().setPrivateFactory(this);...............................................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());}mWindowManager = mWindow.getWindowManager();..................................................}

可以看到window对象是在attach方法中被赋值的，它本身是一个PhoneWindow对象，还为其设置了callback等对象

window设置了窗口管理器WindowManager用于管理窗口，使用context.getSystemService(Context.WINDOW_SERVICE)得到WindowManager

    public void setWindowManager(WindowManager wm, IBinder appToken, String appName,boolean hardwareAccelerated) {mAppToken = appToken;mAppName = appName;mHardwareAccelerated = hardwareAccelerated|| SystemProperties.getBoolean(PROPERTY_HARDWARE_UI, false);if (wm == null) {wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);}mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);}public WindowManagerImpl createLocalWindowManager(Window parentWindow) {return new WindowManagerImpl(mContext, parentWindow);}

调用setWindowManager传入WindowManager，在其中还会再次检查WindowManager对象是否是空，最后调用createLocalWindowManager方法创建WinodwManager的实现类

    public PhoneWindow(Context context, Window preservedWindow,ActivityConfigCallback activityConfigCallback) {this(context);.............................if (preservedWindow != null) {mDecor = (DecorView) preservedWindow.getDecorView();mElevation = preservedWindow.getElevation();..............................}...............................}

在PhoneWindow中会给mDecor赋值

    @Overridepublic void 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) {//构造DecorView对象并赋值给mDecor，并进行mContentParent的初始化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 {//将Resource对于的id等于layoutResID的xml布局文件，add到mContentParent中mLayoutInflater.inflate(layoutResID, mContentParent);}mContentParent.requestApplyInsets();final Callback cb = getCallback();if (cb != null && !isDestroyed()) {cb.onContentChanged();}mContentParentExplicitlySet = true;}

在PhoneWindow的setContentView中会先对mContentParent对象进行判空，如果为空就创建DecorView，如果有视图的话就清空视图，请求重新Layout,刷新视图

    public void removeAllViews() {removeAllViewsInLayout();requestLayout();invalidate(true);}

private void installDecor() {.......................mDecor = generateDecor(-1);mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);mDecor.setIsRootNamespace(true);.....................................mContentParent = generateLayout(mDecor);.........................}

installDecor方法很长，这里只贴了主要代码，generateDecor会生成DecorView对象并为其设置Theme，generateLayout的源码超长，主要就是根据Theme的不同加载不同的布局样式，然后再向mContentParent中加入子View

    protected DecorView 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());}

就这样吧一个decorview就建立完成了，下面我们来看看5.0之后的流程是怎样的

5.0之后

首先看AppCompatActivity中的setContentView()方法

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

getDelegate()得到代理对象

    @NonNullpublic AppCompatDelegate getDelegate() {if (this.mDelegate == null) {this.mDelegate = AppCompatDelegate.create(this, this);}return this.mDelegate;}

    public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {return new AppCompatDelegateImpl(activity, activity.getWindow(), callback);}public void setContentView(View v) {this.ensureSubDecor();ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(16908290);contentParent.removeAllViews();contentParent.addView(v);this.mOriginalWindowCallback.onContentChanged();}

调用了AppCompatDelegateImpl的setContentView方法，首先会调用ensureSubDecor，在该方法内部会调用createSubDecor来创建SubDecor，并为它设置标题栏。下面的代码很长，重点的部分我在注释中写出来了

private void ensureSubDecor() {if (!this.mSubDecorInstalled) {this.mSubDecor = this.createSubDecor();CharSequence title = this.getTitle();if (!TextUtils.isEmpty(title)) {if (this.mDecorContentParent != null) {this.mDecorContentParent.setWindowTitle(title);} else if (this.peekSupportActionBar() != null) {this.peekSupportActionBar().setWindowTitle(title);} else if (this.mTitleView != null) {this.mTitleView.setText(title);}}。。。。。。。。。。。。。。。。。。。。。。。}}private ViewGroup createSubDecor() {TypedArray a = this.mContext.obtainStyledAttributes(styleable.AppCompatTheme);if (!a.hasValue(styleable.AppCompatTheme_windowActionBar)) {a.recycle();//当使用了AppCompatActivity但是没有设置一个Theme.AppCompat的主题，则会报这个Exceptionthrow new IllegalStateException("You need to use a Theme.AppCompat theme (or descendant) with this activity.");} else {//设置一些Window属性if (a.getBoolean(styleable.AppCompatTheme_windowNoTitle, false)) {this.requestWindowFeature(1);} else if (a.getBoolean(styleable.AppCompatTheme_windowActionBar, false)) {this.requestWindowFeature(108);}if (a.getBoolean(styleable.AppCompatTheme_windowActionBarOverlay, false)) {this.requestWindowFeature(109);}if (a.getBoolean(styleable.AppCompatTheme_windowActionModeOverlay, false)) {this.requestWindowFeature(10);}this.mIsFloating = a.getBoolean(styleable.AppCompatTheme_android_windowIsFloating, false);a.recycle();//在这里创建了DecorViewthis.mWindow.getDecorView();LayoutInflater inflater = LayoutInflater.from(this.mContext);ViewGroup subDecor = null;//根据标志来决定inflate哪个layout 下面好大一片if (!this.mWindowNoTitle) {if (this.mIsFloating) {subDecor = (ViewGroup)inflater.inflate(layout.abc_dialog_title_material, (ViewGroup)null);this.mHasActionBar = this.mOverlayActionBar = false;} else if (this.mHasActionBar) {TypedValue outValue = new TypedValue();this.mContext.getTheme().resolveAttribute(attr.actionBarTheme, outValue, true);Object themedContext;if (outValue.resourceId != 0) {themedContext = new ContextThemeWrapper(this.mContext, outValue.resourceId);} else {themedContext = this.mContext;}subDecor = (ViewGroup)LayoutInflater.from((Context)themedContext).inflate(layout.abc_screen_toolbar, (ViewGroup)null);this.mDecorContentParent = (DecorContentParent)subDecor.findViewById(id.decor_content_parent);this.mDecorContentParent.setWindowCallback(this.getWindowCallback());if (this.mOverlayActionBar) {this.mDecorContentParent.initFeature(109);}if (this.mFeatureProgress) {this.mDecorContentParent.initFeature(2);}if (this.mFeatureIndeterminateProgress) {this.mDecorContentParent.initFeature(5);}}} 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);}if (VERSION.SDK_INT >= 21) {ViewCompat.setOnApplyWindowInsetsListener(subDecor, new OnApplyWindowInsetsListener() {public WindowInsetsCompat onApplyWindowInsets(View v, WindowInsetsCompat insets) {int top = insets.getSystemWindowInsetTop();int newTop = AppCompatDelegateImpl.this.updateStatusGuard(top);if (top != newTop) {insets = insets.replaceSystemWindowInsets(insets.getSystemWindowInsetLeft(), newTop, insets.getSystemWindowInsetRight(), insets.getSystemWindowInsetBottom());}return ViewCompat.onApplyWindowInsets(v, insets);}});} else {((FitWindowsViewGroup)subDecor).setOnFitSystemWindowsListener(new OnFitSystemWindowsListener() {public void onFitSystemWindows(Rect insets) {insets.top = AppCompatDelegateImpl.this.updateStatusGuard(insets.top);}});}}if (subDecor == null) {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);ContentFrameLayout contentView = (ContentFrameLayout)subDecor.findViewById(id.action_bar_activity_content);// 获取PhoneWindow中的content布局对象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);// 设置contentView的id 这里的数字在R文件中可以查出来是R.id.contentcontentView.setId(16908290);if (windowContentView instanceof FrameLayout) {((FrameLayout)windowContentView).setForeground((Drawable)null);}}this.mWindow.setContentView(subDecor);contentView.setAttachListener(new OnAttachListener() {public void onAttachedFromWindow() {}public void onDetachedFromWindow() {AppCompatDelegateImpl.this.dismissPopups();}});return subDecor;}}}

getDecorView里面就和之前写的一样了

小结：

Activity的具体实现都会转交给它里面的Window来实现，调用了 PhoneWindow的setContentView首先会查看DecorView存不存在，不存在的话则创建一个DecorView出来，但是此时的DecorView还只是空白的，需要通过 generateLayout方法加载具体的布局到DecorView上面，具体的布局文件的话可能和系统版本以及主题有关系，随后会通过 LayoutInflater的inflate通过pull解析xml的方式将我们自己的布局文件添加到DecorView里面的 mContentParent中，注意只是添加到mContentParent中，接着回调Activity的onContentChanged方法，告诉Activity DecorView已经初始化好了，即Activity的布局文件已经成功的添加到了DecorView的mContentParent里面啦，但是这个时候DecorView还是没有被WindowManager添加到Window上面的，因为View不能单独存在，所以我们需要将DecorView添加到Window里面。

Activity.attach()的执行

首先启动一个Activity会调用handleLaunchActivity

private void handleLaunchActivity (ActivityClientRecord r, Intent customIntent){//会在里面调用Activity.attach,并调用onCreate,和onstartActivity a = performLaunchActivity(r, customIntent);.....................................if (a != null) {//执行Activity.onResume()handleResumeActivity(r.token, false, r.isForward,!r.activity.mFinished && !r.startsNotResumed);if (!r.activity.mFinished && r.startsNotResumed) {try {r.activity.mCalled = false;//执行Activity.onPause()mInstrumentation.callActivityOnPause(r.activity);}}}}

final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward, boolean reallyResume) {//调用ctivity.onResume()已经调用了，但是现在界面还是不可见的ActivityClientRecord r = performResumeActivity(token, clearHide);if (r != null) {final Activity a = r.activity;if (r.window == null && !a.mFinished && willBeVisible) {r.window = r.activity.getWindow();View decor = r.window.getDecorView();//decor对用户不可见decor.setVisibility(View.INVISIBLE);ViewManager wm = a.getWindowManager();WindowManager.LayoutParams l = r.window.getAttributes();a.mDecor = decor;l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;if (a.mVisibleFromClient) {a.mWindowAdded = true;//将decorview添加进WindowManagerwm.addView(decor, l);}if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {//使DecorView可见if (r.activity.mVisibleFromClient) {r.activity.makeVisible();}}}}
}

addView()

@Override
public void addView(View view, ViewGroup.LayoutParams params) {mGlobal.addView(view, params, mDisplay, mParentWindow);
}

WindowManagerGlobal是WindowManager的实现类

public void addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) {final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;...............................synchronized (mLock) {ViewRootImpl root;//实例化一个ViewRootImpl对象root = new ViewRootImpl(view.getContext(), display);view.setLayoutParams(wparams);mViews.add(view);mRoots.add(root);mParams.add(wparams);}...........................try {//将DecorView交给ViewRootImplroot.setView(view, wparams, panelParentView);} catch (RuntimeException e) {}
}

WindowManagerGlobal中有三个列表，一个是保存View的mViews列表，一个是保存ViewRootImpl的mRoots列表，一个是保存WindowManager.LayoutParams的mParams列表，在调用setView后将decorView交给ViewRootImpl，ViewRootImpl中调用performTraversals方法，然后便开始测量布局绘画了，界面才得以显示出来。

总的来说addView干了4件事：

首先进行了一些输入参数的检查，不合法的话抛出相应异常信息
接着创建了一个 ViewRootImpl对象，在创建ViewRootImpl的时候会创建一个Session对象出来，而Session对象的创建是通过调用的 WindowManagerService的openSession方法了
随后会将我们当前的ViewRootImpl对象放到mRoots列表中，将当前的View对象放到mViews中，将当前的LayoutParams对象放到mParams中
最后调用ViewRootImpl 的setView方法，这个方法里面真正的进行View添加操作是通过创建ViewRootImpl的时候创建的Session对象完成的，而在 Session中添加操作实际上是通过WindowManagerService完成的，后期的添加操作是有涉及到IPC通信的

总结

每一个Activity有一个Window，每一个Window都有一个DecorView，而每一个View在被添加的时候都会创建一个ViewRootImpl，但是Window所在的ViewRootImpl应该是属于DecorView这个View的，其实ViewRootImpl我们可以认为是View的操作者一样，因为事件分发机制，View视图的绘制都是从他开始的每一个Activity有一个Window，每一个Window都有一个DecorView，而每一个View在被添加的时候都会创建一个ViewRootImpl，但是Window所在的ViewRootImpl应该是属于DecorView这个View的，其实ViewRootImpl我们可以认为是View的操作者一样，因为事件分发机制，View视图的绘制都是从他开始的

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Activity、Window、DecorView、ViewRootImpl之间的关系

5.0之前

5.0之后

小结：

Activity.attach()的执行

总结

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