Activity启动过程
Activity的Window的创建过程
Activity启动最终会执行到ActivityThread#performLaunchActivity，调用Activity#attach
ActivityThread
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
activity.attach(...);
}
Activity#attach完成mWindow的初始化
final void attach(Context context, ActivityThread aThread,...) {
mWindow = new PhoneWindow(this, window, activityConfigCallback);
}
Activity setContentView 的过程
ActivitysetContentView 会调用PhoneWindow的 setContentView，将 layout 填充到DecorView的 id 为android.id.content 的 FramLayout 中。DecorView 初始化时，会根据主题样式加载不同的布局，共同点就是都含有 id.content 的 FramLayout
Activity启动时会在onCreate中回调setContentView，传入用户自定义布局进行渲染。
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
...
}
getWindow()会返回mWindow，而mWindow是在Activity#attach()中被初始化的，具体实现是PhoneWindow
PhoneWindow#setContentView()
@Override
public void setContentView(int layoutResID) {
...
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent 在installDecor() 被初始化
private void installDecor() {
...
mContentParent = generateLayout(mDecor);
}
再来看看generateLayout(DecorView decor)
/**
* 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;
protected ViewGroup generateLayout(DecorView decor) {
...
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
...
return contentParent;
}
Window
public <T extends View> T findViewById(@IdRes int id) {
return getDecorView().findViewById(id);
}
PhoneWindow
@Override
public final View getDecorView() {
if (mDecor == null || mForceDecorInstall) {
installDecor();
}
return mDecor;
}
installDecor()
private void installDecor() {
...
mDecor = generateDecor(-1);
}
generateDecor()
protected DecorView generateDecor(int featureId) {
...
return new DecorView(context, featureId, this, getAttributes());
}
DecorView 类声明
public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks
构造方法，如下，并没有调用任何inflate方法
DecorView(Context context, int featureId, PhoneWindow window,
WindowManager.LayoutParams params) {
...
}
搜索inflate，找到如下代码
void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
final View root = inflater.inflate(layoutResource, null);
...
addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
}
在PhoneWindow搜索mDecor的引用，可以找到mDecor.onResourcesLoaded
protected ViewGroup generateLayout(DecorView decor) {
...
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
}
再看看布局参数layoutResource的来历，还是在generateLayout()中，可以看到，根据feature和mIsFloating，layoutResource会被赋值成不同的layout文件
int layoutResource;
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
...
} else {
layoutResource = R.layout.screen_simple;
}
其中最简单的是 R.layout.screen_simple，在这里我们可以id: android:id/content ，也就是上文中提到的ID_ANDROID_CONTENT = com.android.internal.R.id.content。到这里可以看明白，Activity通过setContentView()设置的布局文件最终经过一系列的传递，传入了DecorView的FrameLayout之中。
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
android:orientation="vertical">
<ViewStub android:id="@+id/action_mode_bar_stub"
android:inflatedId="@+id/action_mode_bar"
android:layout="@layout/action_mode_bar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="?attr/actionBarTheme" />
<FrameLayout
android:id="@android:id/content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:foregroundInsidePadding="false"
android:foregroundGravity="fill_horizontal|top"
android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>
