从setContentView()源码看起

2.1 Activity::setContentView()
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
public Window getWindow() {
return mWindow;
}
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2.2 mWindow的初始化是在Activity的attach方法中执行的。了解过Activity启动流程，我们知道：attach方法是在Activity启动流程中，ActivityThread::performLaunchActivity()方法中被调用的，是在Activity被创建后，完成初始化操作的。
//android.app.Activity#attach()
final void attach(Context context, ActivityThread aThread, ... Window window ...) {
...
//这里进行mWindow的初始化，可以看到Activity中的Window实现类是PhoneWindow，
//目前为止，PhoneWindow也是Window的唯一实现类
mWindow = new PhoneWindow(this, window, activityConfigCallback);
//这里是给Window设置了WindowManager,WindowManager是通过IPC获取的系统服务，
//WindowManager只是一个接口类型，具体实现是WindowManagerImpl类，
//当然WindowManagerImpl又将实际的逻辑实现交给了WindowManagerGlobal类
mWindow.setWindowManager( (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
mToken, mComponent.flattenToString(),
(info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
//Activity持有的WindowManager也是从Window中拿过来的
mWindowManager = mWindow.getWindowManager();
....
}
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2.3 继续看PhoneWindow的setContentView()实现代码
@Override
public void setContentView(int layoutResID) {
//mContentParent其实就是android.R.id.content布局对应的实际展示的内容
if (mContentParent == null) {
//这个方法时构造一个顶层的DecorView对象，其实是直接通过new DecorView产生的实例对象，并赋值给mDecor变量
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
//第二次调用setContentView()方法时走这里，会先remove掉所有的子View再通过inflate进行加载布局
mContentParent.removeAllViews();
}

if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
//这个变量，会在requestFeature()方法调用时判断时机是否正确，
//如果实在setContentView之后调用的，会抛出"requestFeature() must be called before adding content"的异常
mContentParentExplicitlySet = true;
}
//installDecorView()：构造Activity视图框架的根视图，并通过LayoutInflater加载mContentParent，
//我们一般操作的setContentView其实就是将布局展示到了mContentParent中
private void installDecor() {
...
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);
}
if (mContentParent == null) {
//DecorView已经构造好了，可以从DecorView中通过findViewById的方式实例化mContentParent对象了
mContentParent = generateLayout(mDecor);
}
...
}
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2.4 generateLayout(mDecor)也是获取mContentParent对象的关键方法
protected ViewGroup generateLayout(DecorView decor) {
...
// Inflate the window decor.
int layoutResource;
int features = getLocalFeatures();
// System.out.println("Features: 0x" + Integer.toHexString(features));
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
layoutResource = R.layout.screen_swipe_dismiss;
setCloseOnSwipeEnabled(true);
} else 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;
}
removeFeature(FEATURE_ACTION_BAR);
} 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) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogCustomTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_custom_title;
}
removeFeature(FEATURE_ACTION_BAR);
} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
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;
}
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = R.layout.screen_simple_overlay_action_mode;
} else {
layoutResource = R.layout.screen_simple;
}
....
//mDecorView虽然已经初始化了，但是他的布局还未加载，通过上面对features变量值的一堆if-else判断，
//获取到对应的feature值的布局文件，再通过inflater对象加载布局
mDecor.startChanging();
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
//这里获取的就是mContentParent对象，findViewById是View的方法，这里是间接调用了DecorView的findViewById方法，
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
...
return contentParent;
}
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2.5 查找ID_ANDROID_CONTENT变量，可以看到：其值是android.R.id.content
/**
* The ID that the main layout in the XML layout file should have.
*/
public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;
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2.6 另外： DecorView加载布局文件资源的方法是：onResourcesLoaded()
void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
...
//这里inflate方法的root直接传null，的确这已经是跟布局了，肯定是没有父View可以传了
final View root = inflater.inflate(layoutResource, null);
...
}