我們會發現DecorView里面包裹的內容可能會隨著不同的情況而變化，但是在Decor之前的層次關系都是固定的。即Activity包裹PhoneWindow，PhoneWindow包裹DecorView。接下來我們首先看一下三者分別是如何創建的。

首先看到入口類ActivityThread的performLaunchActivity方法:

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { ...ContextImpl appContext = createBaseContextForActivity(r);Activity activity = null;try { java.lang.ClassLoader cl = appContext.getClassLoader(); activity = mInstrumentation.newActivity( cl, component.getClassName(), r.intent); StrictMode.incrementExpectedActivityCount(activity.getClass()); r.intent.setExtrasClassLoader(cl); r.intent.prepareToEnterProcess(); if (r.state != null) {r.state.setClassLoader(cl); }} catch (Exception e) { ...}

有句特別關鍵的代碼，即

activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);

所以activity是Instrumentation類的newActivity方法創建的，追蹤過去，源碼如下

public Activity newActivity(ClassLoader cl, String className, Intent intent) throws InstantiationException, IllegalAccessException, ClassNotFoundException {String pkg = intent != null && intent.getComponent() != null? intent.getComponent().getPackageName() : null;return getFactory(pkg).instantiateActivity(cl, className, intent); }

追蹤源碼，可知getFactory方法返回一個AppComponentFactory對象，然后調用AppComponentFactory的instantiateActivity方法，繼續追蹤

public @NonNull Activity instantiateActivity(@NonNull ClassLoader cl, @NonNull String className, @Nullable Intent intent) throws InstantiationException, IllegalAccessException, ClassNotFoundException {return (Activity) cl.loadClass(className).newInstance(); }

到這里就結束了，我們可以發現Activity是通過反射創建的。

我們還是回到ActivityThread的performLaunchActivity方法，在剛剛展示的那一段的下面有如下部分代碼

Window window = null;if (r.mPendingRemoveWindow != null && r.mPreserveWindow) { window = r.mPendingRemoveWindow; r.mPendingRemoveWindow = null; r.mPendingRemoveWindowManager = null;}// Activity resources must be initialized with the same loaders as the// application context.appContext.getResources().addLoaders(app.getResources().getLoaders().toArray(new ResourcesLoader[0]));appContext.setOuterContext(activity);activity.attach(appContext, this, getInstrumentation(), r.token,r.ident, app, r.intent, r.activityInfo, title, r.parent,r.embeddedID, r.lastNonConfigurationInstances, config,r.referrer, r.voiceInteractor, window, r.configCallback,r.assistToken);

在activity.attach這個方法中，傳入了一個Window對象，追蹤這個attach方法，里面有一句關鍵代碼

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

此時就創建了PhoneWindow。所以我們可以知道，在Activity創建完之后，會為當前的Activity創建一個PhoneWindow對象。

DecorView的創建就不是performLaunchActivity方法里面了，這次我們從Activity的setContentView的源碼開始分析。下面的Activity的setContentView方法的內容。

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

我們發現，Activity的setContentView實際是調用了PhoneWindow的setContentView方法，跟蹤源碼。我們會首先進入Window抽象類，然后我們找其子類PhoneWindow，在里面找到setContentView方法

public void setContentView(int layoutResID) {if (mContentParent == null) { installDecor();} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) { mContentParent.removeAllViews();}...

當mContentParent為null時，會調用installDecor方法，追蹤進入

private void 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);}...

它調用了generateDecor方法，追蹤進入

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, this);if (mTheme != -1) { context.setTheme(mTheme);} }} else { context = getContext();}return new DecorView(context, featureId, this, getAttributes()); }

會發現generateDecor方法會創建一個DecorView對象，并且作為返回值返回。再追蹤DecorView

public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks

會發現DecorView其實是一個FrameLayout 。到這就介紹完DecorView是如何創建的了

我們回到PhoneWindow的installDecor方法，再剛剛看的部分的下面，有（2692行）

if (mContentParent == null) {mContentParent = generateLayout(mDecor);

追蹤進入

protected ViewGroup generateLayout(DecorView decor) {// Apply data from current theme.TypedArray a = getWindowStyle();if (false) { System.out.println('From style:'); String s = 'Attrs:'; for (int i = 0; i < R.styleable.Window.length; i++) {s = s + ' ' + Integer.toHexString(R.styleable.Window[i]) + '='+ a.getString(i); } System.out.println(s);}mIsFloating = a.getBoolean(R.styleable.Window_windowIsFloating, false);int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)& (~getForcedWindowFlags());if (mIsFloating) { setLayout(WRAP_CONTENT, WRAP_CONTENT); setFlags(0, flagsToUpdate);} else { setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate); getAttributes().setFitInsetsSides(0); getAttributes().setFitInsetsTypes(0);}if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) { requestFeature(FEATURE_NO_TITLE);} else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) { // Don’t allow an action bar if there is no title. requestFeature(FEATURE_ACTION_BAR);}if (a.getBoolean(R.styleable.Window_windowActionBarOverlay, false)) { requestFeature(FEATURE_ACTION_BAR_OVERLAY);}if (a.getBoolean(R.styleable.Window_windowActionModeOverlay, false)) { requestFeature(FEATURE_ACTION_MODE_OVERLAY);}if (a.getBoolean(R.styleable.Window_windowFullscreen, false)) { setFlags(FLAG_FULLSCREEN, FLAG_FULLSCREEN & (~getForcedWindowFlags()));}if (a.getBoolean(R.styleable.Window_windowTranslucentStatus,false)) { setFlags(FLAG_TRANSLUCENT_STATUS, FLAG_TRANSLUCENT_STATUS & (~getForcedWindowFlags()));}if (a.getBoolean(R.styleable.Window_windowTranslucentNavigation,false)) { setFlags(FLAG_TRANSLUCENT_NAVIGATION, FLAG_TRANSLUCENT_NAVIGATION & (~getForcedWindowFlags()));}if (a.getBoolean(R.styleable.Window_windowShowWallpaper, false)) { setFlags(FLAG_SHOW_WALLPAPER, FLAG_SHOW_WALLPAPER&(~getForcedWindowFlags()));}if (a.getBoolean(R.styleable.Window_windowEnableSplitTouch,getContext().getApplicationInfo().targetSdkVersion>= android.os.Build.VERSION_CODES.HONEYCOMB)) { setFlags(FLAG_SPLIT_TOUCH, FLAG_SPLIT_TOUCH&(~getForcedWindowFlags()));}a.getValue(R.styleable.Window_windowMinWidthMajor, mMinWidthMajor);a.getValue(R.styleable.Window_windowMinWidthMinor, mMinWidthMinor);if (DEBUG) Log.d(TAG, 'Min width minor: ' + mMinWidthMinor.coerceToString()+ ', major: ' + mMinWidthMajor.coerceToString());if (a.hasValue(R.styleable.Window_windowFixedWidthMajor)) { if (mFixedWidthMajor == null) mFixedWidthMajor = new TypedValue(); a.getValue(R.styleable.Window_windowFixedWidthMajor, mFixedWidthMajor);}if (a.hasValue(R.styleable.Window_windowFixedWidthMinor)) { if (mFixedWidthMinor == null) mFixedWidthMinor = new TypedValue(); a.getValue(R.styleable.Window_windowFixedWidthMinor, mFixedWidthMinor);}if (a.hasValue(R.styleable.Window_windowFixedHeightMajor)) { if (mFixedHeightMajor == null) mFixedHeightMajor = new TypedValue(); a.getValue(R.styleable.Window_windowFixedHeightMajor, mFixedHeightMajor);}if (a.hasValue(R.styleable.Window_windowFixedHeightMinor)) { if (mFixedHeightMinor == null) mFixedHeightMinor = new TypedValue(); a.getValue(R.styleable.Window_windowFixedHeightMinor, mFixedHeightMinor);}if (a.getBoolean(R.styleable.Window_windowContentTransitions, false)) { requestFeature(FEATURE_CONTENT_TRANSITIONS);}if (a.getBoolean(R.styleable.Window_windowActivityTransitions, false)) { requestFeature(FEATURE_ACTIVITY_TRANSITIONS);}mIsTranslucent = a.getBoolean(R.styleable.Window_windowIsTranslucent, false);final Context context = getContext();final int targetSdk = context.getApplicationInfo().targetSdkVersion;final boolean targetPreL = targetSdk < android.os.Build.VERSION_CODES.LOLLIPOP;final boolean targetPreQ = targetSdk < Build.VERSION_CODES.Q;if (!mForcedStatusBarColor) { mStatusBarColor = a.getColor(R.styleable.Window_statusBarColor, 0xFF000000);}if (!mForcedNavigationBarColor) { mNavigationBarColor = a.getColor(R.styleable.Window_navigationBarColor, 0xFF000000); mNavigationBarDividerColor = a.getColor(R.styleable.Window_navigationBarDividerColor, 0x00000000);}if (!targetPreQ) { mEnsureStatusBarContrastWhenTransparent = a.getBoolean( R.styleable.Window_enforceStatusBarContrast, false); mEnsureNavigationBarContrastWhenTransparent = a.getBoolean( R.styleable.Window_enforceNavigationBarContrast, true);}WindowManager.LayoutParams params = getAttributes();// Non-floating windows on high end devices must put up decor beneath the system bars and// therefore must know about visibility changes of those.if (!mIsFloating) { if (!targetPreL && a.getBoolean( R.styleable.Window_windowDrawsSystemBarBackgrounds, false)) {setFlags(FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS,FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS & ~getForcedWindowFlags()); } if (mDecor.mForceWindowDrawsBarBackgrounds) {params.privateFlags |= PRIVATE_FLAG_FORCE_DRAW_BAR_BACKGROUNDS; }}if (a.getBoolean(R.styleable.Window_windowLightStatusBar, false)) { decor.setSystemUiVisibility( decor.getSystemUiVisibility() | View.SYSTEM_UI_FLAG_LIGHT_STATUS_BAR);}if (a.getBoolean(R.styleable.Window_windowLightNavigationBar, false)) { decor.setSystemUiVisibility( decor.getSystemUiVisibility() | View.SYSTEM_UI_FLAG_LIGHT_NAVIGATION_BAR);}if (a.hasValue(R.styleable.Window_windowLayoutInDisplayCutoutMode)) { int mode = a.getInt(R.styleable.Window_windowLayoutInDisplayCutoutMode, -1); if (mode < LAYOUT_IN_DISPLAY_CUTOUT_MODE_DEFAULT || mode > LAYOUT_IN_DISPLAY_CUTOUT_MODE_ALWAYS) {throw new UnsupportedOperationException('Unknown windowLayoutInDisplayCutoutMode: '+ a.getString(R.styleable.Window_windowLayoutInDisplayCutoutMode)); } params.layoutInDisplayCutoutMode = mode;}if (mAlwaysReadCloseOnTouchAttr || getContext().getApplicationInfo().targetSdkVersion>= android.os.Build.VERSION_CODES.HONEYCOMB) { if (a.getBoolean( R.styleable.Window_windowCloseOnTouchOutside, false)) {setCloseOnTouchOutsideIfNotSet(true); }}if (!hasSoftInputMode()) { params.softInputMode = a.getInt( R.styleable.Window_windowSoftInputMode, params.softInputMode);}if (a.getBoolean(R.styleable.Window_backgroundDimEnabled,mIsFloating)) { /* All dialogs should have the window dimmed */ if ((getForcedWindowFlags()&WindowManager.LayoutParams.FLAG_DIM_BEHIND) == 0) {params.flags |= WindowManager.LayoutParams.FLAG_DIM_BEHIND; } if (!haveDimAmount()) {params.dimAmount = a.getFloat(android.R.styleable.Window_backgroundDimAmount, 0.5f); }}if (params.windowAnimations == 0) { params.windowAnimations = a.getResourceId( R.styleable.Window_windowAnimationStyle, 0);}// The rest are only done if this window is not embedded; otherwise,// the values are inherited from our container.if (getContainer() == null) { if (mBackgroundDrawable == null) {if (mFrameResource == 0) { mFrameResource = a.getResourceId(R.styleable.Window_windowFrame, 0);}if (a.hasValue(R.styleable.Window_windowBackground)) { mBackgroundDrawable = a.getDrawable(R.styleable.Window_windowBackground);} } if (a.hasValue(R.styleable.Window_windowBackgroundFallback)) {mBackgroundFallbackDrawable =a.getDrawable(R.styleable.Window_windowBackgroundFallback); } if (mLoadElevation) {mElevation = a.getDimension(R.styleable.Window_windowElevation, 0); } mClipToOutline = a.getBoolean(R.styleable.Window_windowClipToOutline, false); mTextColor = a.getColor(R.styleable.Window_textColor, Color.TRANSPARENT);}// Inflate the window decor.int layoutResource;int features = getLocalFeatures();// System.out.println('Features: 0x' + Integer.toHexString(features));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; } // XXX Remove this once action bar supports these features. removeFeature(FEATURE_ACTION_BAR); // System.out.println('Title Icons!');} else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0&& (features & (1 << FEATURE_ACTION_BAR)) == 0) { // Special case for a window with only a progress bar (and title). // XXX Need to have a no-title version of embedded windows. layoutResource = R.layout.screen_progress; // System.out.println('Progress!');} else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) { // Special case for a window with a custom title. // If the window is floating, we need a dialog layout if (mIsFloating) {TypedValue res = new TypedValue();getContext().getTheme().resolveAttribute(R.attr.dialogCustomTitleDecorLayout, res, true);layoutResource = res.resourceId; } else {layoutResource = R.layout.screen_custom_title; } // XXX Remove this once action bar supports these features. removeFeature(FEATURE_ACTION_BAR);} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) { // If no other features and not embedded, only need a title. // If the window is floating, we need a dialog layout if (mIsFloating) {TypedValue res = new TypedValue();getContext().getTheme().resolveAttribute(R.attr.dialogTitleDecorLayout, res, true);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; } // System.out.println('Title!');} else 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);ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);if (contentParent == null) { throw new RuntimeException('Window couldn’t find content container view');}if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) { ProgressBar progress = getCircularProgressBar(false); if (progress != null) {progress.setIndeterminate(true); }}// Remaining setup -- of background and title -- that only applies// to top-level windows.if (getContainer() == null) { mDecor.setWindowBackground(mBackgroundDrawable); final Drawable frame; if (mFrameResource != 0) {frame = getContext().getDrawable(mFrameResource); } else {frame = null; } mDecor.setWindowFrame(frame); mDecor.setElevation(mElevation); mDecor.setClipToOutline(mClipToOutline); if (mTitle != null) {setTitle(mTitle); } if (mTitleColor == 0) {mTitleColor = mTextColor; } setTitleColor(mTitleColor);}mDecor.finishChanging();return contentParent; }

分析源碼和源碼注釋可以看出generateLayout方法的核心功能是完成DecorView的布局加載，而且根據不同的主題樣式會加載不同的系統默認布局。那么比如有FrameLayout布局，它如何加載到DecorView中呢？在generateLayout方法中，有這么一句

mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);

追蹤進入

void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {if (mBackdropFrameRenderer != null) { loadBackgroundDrawablesIfNeeded(); mBackdropFrameRenderer.onResourcesLoaded( this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable, mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState), getCurrentColor(mNavigationColorViewState));}mDecorCaptionView = createDecorCaptionView(inflater);final View root = inflater.inflate(layoutResource, null);if (mDecorCaptionView != null) { if (mDecorCaptionView.getParent() == null) {addView(mDecorCaptionView,new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT)); } mDecorCaptionView.addView(root, new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));} else { // Put it below the color views. addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));}mContentRoot = (ViewGroup) root;initializeElevation(); }

關鍵在這里

mDecorCaptionView = createDecorCaptionView(inflater);final View root = inflater.inflate(layoutResource, null);if (mDecorCaptionView != null) { ... mDecorCaptionView.addView(root, new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));}

可以看到layoutResource作為參數，通過inflate方法進行解析加載。然后作為參數，傳入addView方法，將布局掛載到上面。至此完成了DecorView的默認布局加載。如果是我們自己編寫的布局是如何加載呢？再回到generateLayout的

mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);

，在此之后，有這么一句

ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);

在DecorView執行onResourcesLoaded方法加載完默認布局后，會查找Id為ID_ANDROID_CONTENT的控件(默認布局中的FrameLayout控件)并作為generateLayout方法的返回值，我們回到setContentView方法中，可觀察到以下代碼：

mLayoutInflater.inflate(layoutResID, mContentParent);

所以，我們自己編寫的布局是被加載到DecorView中Id為ID_ANDROID_CONTENT的控件上

當Activity創建后會創建出一個PhoneWindow對象，當在Activity中調用setContentView時，實際上是調用了PhoneWindow的setContentView方法，此時PhoneWindow會創建根布局DecorView，并根據主題樣式，為DecorView加載對應的默認系統布局，在默認的系統布局中包含了一個Id為ID_ANDROID_CONTENT的控件，而我們自己編寫的布局就是加載到這個控件中的。

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