一、概述
setContenView()是用来给activity设置布局使用的，但是我们不能只会使用而不深入源码，只有掌握了源码，才能看的更高走的更远。其实分析setContentView的源码就是分析如何将我们设置的布局资源挂载到android.R.id.content上进而形成一个完整的view树的过程。
整个view树的结构如下图所示，android.R.layout.screen_simple是其中一种系统布局，具体使用哪个布局需要看我们设置的主题（如透明主题，无标题主题等）
setContentView源码-1
二、源码分析
1.调用AppCompatActivity的setContentView()
@Override
public void setContentView(@LayoutRes int layoutResID) {
getDelegate().setContentView(layoutResID);
}
2.分析下getDelegate()是什么鬼，这里我感觉是代理，封装了具体实现
@NonNull
public AppCompatDelegate getDelegate() {
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
}
3.看下AppCompatDelegate.create(this, this)做了哪些操作？
public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
return create(activity, activity.getWindow(), callback);
}
private static AppCompatDelegate create(Context context, Window window, AppCompatCallback callback) {
if (VERSION.SDK_INT >= 24) {
return new AppCompatDelegateImplN(context, window, callback);
} else {
return (AppCompatDelegate)(VERSION.SDK_INT >= 23 ? new AppCompatDelegateImplV23(context, window, callback) : new AppCompatDelegateImplV14(context, window, callback));
}
}
create会进行版本判断,>=24会使用AppCompatDelegateImplN，=23会使用AppCompatDelegateImplV23，<23会使用AppCompatDelegateImplV14。
其实我们查看这些imp实现代码，发现都是会调用父类AppCompatDelegateImplV9中的setContentView9()方法,我们就拿v9来做分析。
4.查看AppCompatDelegateImplV9中的setContentView()方法。
public void setContentView(int resId) {
this.ensureSubDecor();//步骤1
ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(16908290);//步骤2
contentParent.removeAllViews();
LayoutInflater.from(this.mContext).inflate(resId, contentParent);//步骤3
this.mOriginalWindowCallback.onContentChanged();
}
4.1 我将上述代码分为三个步骤，先来看下步骤1this.ensureSubDecor()，看名字应该是确保DecorView初始化的。
private void ensureSubDecor() {
if (!this.mSubDecorInstalled) {
this.mSubDecor = this.createSubDecor();
CharSequence title = this.getTitle();
if (!TextUtils.isEmpty(title)) {
this.onTitleChanged(title);
}
this.applyFixedSizeWindow();
this.onSubDecorInstalled(this.mSubDecor);
this.mSubDecorInstalled = true;
AppCompatDelegateImplV9.PanelFeatureState st = this.getPanelState(0, false);
if (!this.isDestroyed() && (st == null || st.menu == null)) {
this.invalidatePanelMenu(108);
}
}
}
我们主要关注this.mSubDecor = this.createSubDecor()这句代码。
private ViewGroup createSubDecor() {
//...代码较多已省略
this.mWindow.getDecorView();
subDecor = (ViewGroup)inflater.inflate(layout.abc_screen_simple, (ViewGroup)null);
return subDecor;
}
往下走，看下PhoneWindow中的getDecorView()方法
@Override
public final View getDecorView() {
if (mDecor == null) {
installDecor();
}
return mDecor;
}
看下PhoneWindow中的installDecor()只是初始化DecorView，然后执行generateLayout()
private void installDecor() {
if (mDecor == null) {
//获取DecorView，然后他是一个FrameLayout
mDecor = generateDecor();
mDecor.setIsRootNamespace(true);
}
if (mContentParent == null) {
//初始化mContentParent 对象
mContentParent = generateLayout(mDecor);
}
}
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;
.......
return new DecorView(context, featureId, this, getAttributes());
}
generateLayout我的理解是在decorView上根据不同的系统样式设置不同的布局
protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.
//首先通过WindowStyle中设置的各种属性，对Window进行requestFeature或者setFlags
TypedArray a = getWindowStyle();
mIsFloating = a.getBoolean(R.styleable.Window_windowIsFloating, false);
//中间省略一些代码
.................
// 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;
} 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;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
// System.out.println("Title Icons!");
} else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
&& (features & (1 << FEATURE_ACTION_BAR)) == 0) {
// Special case for a window with only a progress bar (and title).
// XXX Need to have a no-title version of embedded windows.
layoutResource = R.layout.screen_progress;
// System.out.println("Progress!");
} else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
// Special case for a window with a custom title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogCustomTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_custom_title;
}
// XXX Remove this once action bar supports these features.
removeFeature(FEATURE_ACTION_BAR);
} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
// If no other features and not embedded, only need a title.
// If the window is floating, we need a dialog layout
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;
}
// System.out.println("Title!");
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = R.layout.screen_simple_overlay_action_mode;
} else {
// Embedded, so no decoration is needed.
layoutResource = R.layout.screen_simple;
// System.out.println("Simple!");
}
//前方高能请注意！！！！！！！！！！！！！
mDecor.startChanging();
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
..............
mDecor.finishChanging();
return contentParent;
}
根据不同的主题设置完布局之后，看下mDecor.onResourcesLoaded(mLayoutInflater, layoutResource)方法，其实这个地方我们可以猜想一下：就是把传进来的layoutResource通过mLayoutInflater添加到decorView中。
void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
mStackId = getStackId();
if (mBackdropFrameRenderer != null) {
loadBackgroundDrawablesIfNeeded();
mBackdropFrameRenderer.onResourcesLoaded(
this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable,
mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState),
getCurrentColor(mNavigationColorViewState));
}
//这个我猜想是快照，应该不需要考虑，mDecorCaptionView应该会为null
mDecorCaptionView = createDecorCaptionView(inflater);
//看重点,这句就是inflate出我们的布局
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 {
//最后addView应该是会执行这个
addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
}
mContentRoot = (ViewGroup) root;
initializeElevation();
}
到目前为止4中的第一个步骤已经通过4.1讲解完成。其实4.1的整个过程只是初始化DecorView，并且初始化DecorView中的那个LinearLayout。
4.2
4.1中讲的是下边代码中步骤1的内容，其实就是初始化DecorView及其中的LinearLayout，接下来是步骤2和步骤3`
public void setContentView(int resId) {
this.ensureSubDecor();//步骤1
ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(16908290);//步骤2
contentParent.removeAllViews();
LayoutInflater.from(this.mContext).inflate(resId, contentParent);//步骤3
this.mOriginalWindowCallback.onContentChanged();
}
ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(16908290);//步骤2
,mSubDecor看名字我们应该也能知道是decorView中的那个LinearLayout。
所以contentParent拿到的是id为android.R.id.content的frameLayout，及下图中的ContentView。
setContentView源码-1
4.3 LayoutInflater.from(this.mContext).inflate(resId, contentParent);//步骤3
步骤3是setContentView源码的最后一步，把我们设置的resId设置到contentParent
5.总结
整个流程其实比较简单：
1.初始化decorView
2.根据不同主题给decorView设置不同的LinearLayout
3.将我们的resId通过layoutInflate.inflate()的方式添加到LinearLayout中的android.R.id.content中。
6.反思
1中的getDelegate().setContentView（）这种代理，或者说隐藏代码细节的方式值得我们学习。