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
&＃63; 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 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 <
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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