Activity启动过程

Activity启动过程（API27）.png

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

Activity setContentView .png

Activity setContentView 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>