在我的上一篇文章Android开发Activity的setContentView源码分析中介绍了Activity的setContentView源码，但是我们在开发过程中用的基本都是AppCompatActivity，那么让我们看看AppCompatActivity的setContentView方法跟Activity的setContentView方法有什么不同。
首先来回忆以下Activity的setContentView方法中有两个很重要的东西：mDecor和mContentParent。前者是整个界面的根部局，后者是加载我们自定义布局的容器。其实AppCompatActivity的布局的层级跟Activity基本是一样的，只是在mContentParent中又多了一层布局而已。
话不多说，点开AppCompatActivity的setContentView方法：
@Override
public void setContentView(@LayoutRes int layoutResID) {
getDelegate().setContentView(layoutResID);
}
@NonNull
public AppCompatDelegate getDelegate() {
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
}
public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
return new AppCompatDelegateImpl(activity, activity.getWindow(), callback);
}
可以看到，里面是调用了一个委托对象的setContentView方法，这个对象就是AppCompatDelegateImpl，那么我们找到AppCompatDelegateImpl的setContentView方法：
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
明白了Activity布局加载的过程我们可以猜到，方法中第二行获取到的contentParent的功能就相当于Activity中mContentParent，我们自定义的布局就是加载到这个contentParent中的，接下来的代码LayoutInflater.from(mContext).inflate(resId, contentParent)马上就证实了我们的判断。
那么第一行的ensureSubDecor()是干啥的呢？在开篇的时候说过，AppCompatActivity的布局的层级跟Activity基本是一样的，只是在mContentParent中又多了一层布局。这里提前给出结论：mContentParent中又多的那一层布局就是mSubDecor，而contentParent又是在mSubDecor下面的一个子布局。
点进ensureSubDecor方法：
private void ensureSubDecor() {
if (!mSubDecorInstalled) {
mSubDecor = createSubDecor();
……
}
可以看到通过createSubDecor方法创建mSubDecor。
private ViewGroup createSubDecor() {
TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);
if (!a.hasValue(R.styleable.AppCompatTheme_windowActionBar)) {
a.recycle();
throw new IllegalStateException(
"You need to use a Theme.AppCompat theme (or descendant) with this activity.");
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowNoTitle, false)) {
requestWindowFeature(Window.FEATURE_NO_TITLE);
} else if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBar, false)) {
// Don't allow an action bar if there is no title.
requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR);
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBarOverlay, false)) {
requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
}
if (a.getBoolean(R.styleable.AppCompatTheme_windowActionModeOverlay, false)) {
requestWindowFeature(FEATURE_ACTION_MODE_OVERLAY);
}
mIsFloating = a.getBoolean(R.styleable.AppCompatTheme_android_windowIsFloating, false);
a.recycle();
// Now let's make sure that the Window has installed its decor by retrieving it
ensureWindow();
//注释1
mWindow.getDecorView();
final LayoutInflater inflater = LayoutInflater.from(mContext);
ViewGroup subDecor = null;
//根据设置给subDecor加载不同的布局
if (!mWindowNoTitle) {
if (mIsFloating) {
// If we're floating, inflate the dialog title decor
subDecor = (ViewGroup) inflater.inflate(
R.layout.abc_dialog_title_material, null);
// Floating windows can never have an action bar, reset the flags
mHasActionBar = mOverlayActionBar = false;
} else if (mHasActionBar) {
/**
* This needs some explanation. As we can not use the android:theme attribute
* pre-L, we emulate it by manually creating a LayoutInflater using a
* ContextThemeWrapper pointing to actionBarTheme.
*/
TypedValue outValue = new TypedValue();
mContext.getTheme().resolveAttribute(R.attr.actionBarTheme, outValue, true);
Context themedContext;
if (outValue.resourceId != 0) {
themedContext = new ContextThemeWrapper(mContext, outValue.resourceId);
} else {
themedContext = mContext;
}
// Now inflate the view using the themed context and set it as the content view
subDecor = (ViewGroup) LayoutInflater.from(themedContext)
.inflate(R.layout.abc_screen_toolbar, null);
mDecorContentParent = (DecorContentParent) subDecor
.findViewById(R.id.decor_content_parent);
mDecorContentParent.setWindowCallback(getWindowCallback());
/**
* Propagate features to DecorContentParent
*/
if (mOverlayActionBar) {
mDecorContentParent.initFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
}
if (mFeatureProgress) {
mDecorContentParent.initFeature(Window.FEATURE_PROGRESS);
}
if (mFeatureIndeterminateProgress) {
mDecorContentParent.initFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
}
}
} else {
if (mOverlayActionMode) {
subDecor = (ViewGroup) inflater.inflate(
R.layout.abc_screen_simple_overlay_action_mode, null);
} else {
subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);
}
if (Build.VERSION.SDK_INT >= 21) {
// If we're running on L or above, we can rely on ViewCompat's
// setOnApplyWindowInsetsListener
ViewCompat.setOnApplyWindowInsetsListener(subDecor,
new OnApplyWindowInsetsListener() {
@Override
public WindowInsetsCompat onApplyWindowInsets(View v,
WindowInsetsCompat insets) {
final int top = insets.getSystemWindowInsetTop();
final int newTop = updateStatusGuard(top);
if (top != newTop) {
insets = insets.replaceSystemWindowInsets(
insets.getSystemWindowInsetLeft(),
newTop,
insets.getSystemWindowInsetRight(),
insets.getSystemWindowInsetBottom());
}
// Now apply the insets on our view
return ViewCompat.onApplyWindowInsets(v, insets);
}
});
} else {
// Else, we need to use our own FitWindowsViewGroup handling
((FitWindowsViewGroup) subDecor).setOnFitSystemWindowsListener(
new FitWindowsViewGroup.OnFitSystemWindowsListener() {
@Override
public void onFitSystemWindows(Rect insets) {
insets.top = updateStatusGuard(insets.top);
}
});
}
}
if (subDecor == null) {
throw new IllegalArgumentException(
"AppCompat does not support the current theme features: { "
+ "windowActionBar: " + mHasActionBar
+ ", windowActionBarOverlay: "+ mOverlayActionBar
+ ", android:windowIsFloating: " + mIsFloating
+ ", windowActionModeOverlay: " + mOverlayActionMode
+ ", windowNoTitle: " + mWindowNoTitle
+ " }");
}
if (mDecorContentParent == null) {
mTitleView = (TextView) subDecor.findViewById(R.id.title);
}
// Make the decor optionally fit system windows, like the window's decor
ViewUtils.makeOptionalFitsSystemWindows(subDecor);
//注释2
final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
R.id.action_bar_activity_content);
final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
if (windowContentView != null) {
// There might be Views already added to the Window's content view so we need to
// migrate them to our content view
while (windowContentView.getChildCount() > 0) {
final View child = windowContentView.getChildAt(0);
windowContentView.removeViewAt(0);
contentView.addView(child);
}
// Change our content FrameLayout to use the android.R.id.content id.
// Useful for fragments.
//清除windowContentView的id
windowContentView.setId(View.NO_ID);
//将contentView的id设置成android.R.id.content
contentView.setId(android.R.id.content);
// The decorContent may have a foreground drawable set (windowContentOverlay).
// Remove this as we handle it ourselves
if (windowContentView instanceof FrameLayout) {
((FrameLayout) windowContentView).setForeground(null);
}
}
// Now set the Window's content view with the decor
//注释3
mWindow.setContentView(subDecor);
contentView.setAttachListener(new ContentFrameLayout.OnAttachListener() {
@Override
public void onAttachedFromWindow() {}
@Override
public void onDetachedFromWindow() {
dismissPopups();
}
});
return subDecor;
}
在注释1处调用了mWindow的getDecorView方法，这里的mWindow就是PhoneWindow了，在PhoneWinow中找到getDecorView方法：
@Override
public final View getDecorView() {
if (mDecor == null || mForceDecorInstall) {
installDecor();
}
return mDecor;
}
可以看到里面调用了installDecor方法，在前面的文章里提过，做的事情就是初始化mDecor和mContentParent。接下来是根据设置给subDecor加载不同的布局。再接着，在注释2处，通过subDecor的findViewById(R.id.action_bar_activity_content)方法获取到了id为R.id.action_bar_activity_content的contentView，然后再通过mWindow的findViewById(android.R.id.content)方法获取到了windowContentView（对应着Activity中的mContentParent），接着通过windowContentView.setId(View.NO_ID)方法将windowContentView的id清除，之后再用contentView.setId(android.R.id.content)将contentView的id设为android.R.id.content。这样一来，contentView 就成为了Activity中的mContentParent，我们编写的的布局加载到contentView中。
最后在注释3处，调用了PhoneWindow的setContentView(subDecor)方法将创建的subDecor放到mContentParent中，但是此时的mContentParent已经不是加载我们自己编写布局的那个容器了，加载我们编写的布局的容器已经变成了subDecor中的contentView。
让我们再回到AppCompatDelegateImpl的setContentView方法：
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
经过ensureSubDecor方法后，下面获取到的contentParent已经替换成了刚刚提到的contentView了，接下来通过LayoutInflater.from(mContext).inflate(resId, contentParent)将自己编写的布局加载到这个contentParent中来。
来看看整个流程：
AppCompatActivity布局层级如下：
最后总结一下Activity和AppCompatActivity的setContView方法区别：
Activity的setContView直接将我们的布局渲染到mContentParent容器里面
AppCompactActivity的setContView会根据不同的主题特性在mContentParent容器里面添加一个不同主题的subDecor容器，在subDecor容器里面有一个id为action_bar_activity_content的ContentFrameLayout容器（后来被替换成了R.id.content），并将我们的布局渲染到ContentFrameLayout里面(ContentFrameLayout也继承自FrameLayout)。
以上就是我对AppCompatActivity的setContentView加载布局的理解，欢迎大家在评论区留言指正。
文章参考自：
（1）AppCompatActivity和Activity的setContentView方法的区别
（2）从源码角度分析AppCompactActivity#setContentView
（3）Android 之 setContentView 源码阅读
（4）深入解析Android中的setContentView加载布局原理