详解Android布局加载流程源码

我们会发现DecorView里面包裹的内容可能会随着不同的情况而变化，但是在Decor之前的层次关系都是固定的。即Activity包裹PhoneWindow，PhoneWindow包裹DecorView。接下来我们首先看一下三者分别是如何创建的。

首先看到入口类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) { ...}

有句特别关键的代码，即

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

所以activity是Instrumentation类的newActivity方法创建的，追踪过去，源码如下

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); }

追踪源码，可知getFactory方法返回一个AppComponentFactory对象，然后调用AppComponentFactory的instantiateActivity方法，继续追踪

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

到这里就结束了，我们可以发现Activity是通过反射创建的。

我们还是回到ActivityThread的performLaunchActivity方法，在刚刚展示的那一段的下面有如下部分代码

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这个方法中，传入了一个Window对象，追踪这个attach方法，里面有一句关键代码

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

此时就创建了PhoneWindow。所以我们可以知道，在Activity创建完之后，会为当前的Activity创建一个PhoneWindow对象。

DecorView的创建就不是performLaunchActivity方法里面了，这次我们从Activity的setContentView的源码开始分析。下面的Activity的setContentView方法的内容。

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

我们发现，Activity的setContentView实际是调用了PhoneWindow的setContentView方法，跟踪源码。我们会首先进入Window抽象类，然后我们找其子类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();}...

当mContentParent为null时，会调用installDecor方法，追踪进入

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);}...

它调用了generateDecor方法，追踪进入

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()); }

会发现generateDecor方法会创建一个DecorView对象，并且作为返回值返回。再追踪DecorView

public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks

会发现DecorView其实是一个FrameLayout 。到这就介绍完DecorView是如何创建的了

我们回到PhoneWindow的installDecor方法，再刚刚看的部分的下面，有（2692行）

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

追踪进入

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; }

分析源码和源码注释可以看出generateLayout方法的核心功能是完成DecorView的布局加载，而且根据不同的主题样式会加载不同的系统默认布局。那么比如有FrameLayout布局，它如何加载到DecorView中呢？在generateLayout方法中，有这么一句

mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);

追踪进入

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(); }

关键在这里

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作为参数，通过inflate方法进行解析加载。然后作为参数，传入addView方法，将布局挂载到上面。至此完成了DecorView的默认布局加载。如果是我们自己编写的布局是如何加载呢？再回到generateLayout的

mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);

，在此之后，有这么一句

ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);

在DecorView执行onResourcesLoaded方法加载完默认布局后，会查找Id为ID_ANDROID_CONTENT的控件(默认布局中的FrameLayout控件)并作为generateLayout方法的返回值，我们回到setContentView方法中，可观察到以下代码：

mLayoutInflater.inflate(layoutResID, mContentParent);

所以，我们自己编写的布局是被加载到DecorView中Id为ID_ANDROID_CONTENT的控件上

当Activity创建后会创建出一个PhoneWindow对象，当在Activity中调用setContentView时，实际上是调用了PhoneWindow的setContentView方法，此时PhoneWindow会创建根布局DecorView，并根据主题样式，为DecorView加载对应的默认系统布局，在默认的系统布局中包含了一个Id为ID_ANDROID_CONTENT的控件，而我们自己编写的布局就是加载到这个控件中的。

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