一入源码深似海，半天再也出来
从超级熟悉的入口开始
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
setContentView()做了什么？直接点进去
/**
* Set the activity content from a layout resource. The resource will be
* inflated, adding all top-level views to the activity.
*
* @param layoutResID Resource ID to be inflated.
*
* @see #setContentView(android.view.View)
* @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
*/
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
发现调用了getWindow()的setContentView(), 进去看下getWindow()
/**
* Retrieve the current {@link android.view.Window} for the activity.
* This can be used to directly access parts of the Window API that
* are not available through Activity/Screen.
*
* @return Window The current window, or null if the activity is not
* visual.
*/
public Window getWindow() {
return mWindow;
}
直接返回了mWindow，点mWindow
private Window mWindow
发现是个Window对象，点Window看看是什么玩意
* Abstract base class for a top-level window look and behavior policy. An
* instance of this class should be used as the top-level view added to the
* window manager. It provides standard UI policies such as a background, title
* area, default key processing, etc.
*
* <p>The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a
* Window.
*/
public abstract class Window {
...
}
代码还没看，直接在注释里发现The only existing implementation of this abstract class is android.view.PhoneWindow。
那直接打开PhoneWindow类，看其中的setContentView方法：
@Override
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
...
}
发现有个叫Decor的玩意正在初始化，直接点进去：
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);
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);
...
}
...
}
进来又发现两个玩意在初始化：
mDecor = generateDecor(-1);
mContentParent = generateLayout(mDecor);
先点进去generateDecor看看:
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, getContext());
if (mTheme != -1) {
context.setTheme(mTheme);
}
}
} else {
context = getContext();
}
return new DecorView(context, featureId, this, getAttributes());
}
发现return了一个DecorView，点进去看看是个什么东西
public class DecorView extends FrameLayout
发现就是一个FrameLayout的子类，也就是一个容器，
然后回到generateLayout(mDecor)，这个方法把初始化好的一个DecorView传了进去，点进去：
protected ViewGroup generateLayout(DecorView decor) {
//这里贼长... 不贴了，想看的自己去看看，就是设置样式主题什么的
...
...
// Inflate the window decor.
int layoutResource;
int features = getLocalFeatures();
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
layoutResource = R.layout.screen_swipe_dismiss;
setCloseOnSwipeEnabled(true);
} else if (...){
...
//再省略一段，这里是根据不同的features给layoutResource赋值。
}
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
...
return contentParent;
}
这里有两个关键的点：
- 发现DecorView调用了onResourcesLoaded(mLayoutInflater, layoutResource)这么个方法，接收了mLayoutInflater和layoutResource
- 初始化一个ViewGroup： ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);并且返回了这个ViewGroup
先进去onResourcesLoaded看看做了什么：
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();
}
阅读DecorView的这个函数，发现把layoutResource解析并调用addView添加到了DecorView，也就是上面的实例，mDecor中。
然后回去看看findViewById(ID_ANDROID_CONTENT)， 点击这个ID_ANDROID_CONTENT看看是什么：
/**
* 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;
这里说到这个资源id是主容器中的id，而且一定存在的。
总结一下generateLayout做了什么
- 根据不同主题或者特性设置了不同的layoutResource（这些layoutResource都是系统布局，查看这些布局会发现里面容器里包含一个ActionBar的title，一个id为com.android.internal.R.id.content的FrameLayout容器）
- 把layoutResource解析到mDecor(DecorView)中
- 得到id固定为com.android.internal.R.id.content的ViewGroup并返回
看完installDecor()中主要的函数generateDecor() 和generateLayout() 到这里分析完了。然后再回到上面调用installDecor()的地方，上面贴过了，就是在PhoneWindow的setContentView中。再贴一次：
@Override
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
...
}
看这里的这一句 mLayoutInflater.inflate(layoutResID, mContentParent)，mContentParent就是刚才通过generateLayout得到的id为com.android.internal.R.id.content的容器，layoutResID是在Activity里设置的资源id，到此设置完成，在总结一下：
- Activity中调用setContentView(layoutResID)其实是调用了Window的setContentView(layoutResID),而Window的唯一实现类是PhoneWindow。所以调用的是PhoneWindow的setContentView(layoutResID)
- PhoneWindow的setContentView()里调用了两个方法：installDecor()和mLayoutInflater.inflate(layoutResID, mContentParent)。installDecor是为了得到容器mContentParent， 然后mLayoutInflater.inflate把资源解析上去
- installDecor()为了得到容器mContentParent主要调用了两个方法：mDecor = generateDecor(-1)和 mContentParent = generateLayout(mDecor)。 generateDecor是为了实例化一个DecorView，generateLayout接收该DecorView的实例，然后根据不同主题、样式设置不同的layoutResource，并返回一个固定id为com.android.internal.R.id.content的FrameLayout（这些不同的系统layoutResource中都会有这么一个FrameLayout），返回的这个FrameLayout。就是上一步中我们想要得到的mContentParent，也就是接收在Activity中通过setContentView 设置的layoutResID的容器。
最后一张图：
view层级.jpeg
