我们的Activity通常继承自Activity或者AppCompatActivity，这两个setContentView流程是不同的

一、继承自Activity

1、Activity.setContentView

Activity中setContentVIew调用了getWindow().setContentView(view, params);
getWindow返回的是mWindow，mWindow是PhoneWindow对象，在attach中初始化

当创建Activity、Dialog、PopupWindow、Toast时都会创建PhoneWindow对象

2、PhoneWindow.setContentView

该方法主要目的是创建DecorView 拿到Content

2.1 installDecor

在installDecor方法中创建DecorView并拿到Content：


generateDecor创建DecorView

generateLayout较长，用于生成和配置窗口（Activity 的视图容器）的布局和样式。

上图红框为两行较为重要的代码
onResourcesLoaded是将上面的R.layout.simple等这种layout添加到DecorView中
第二个红框是拿到ViewGroup对象，最后返回的就是这个对象

在这之上，该方法会加载一些layout，以最简单的screen_simple举例

注意：DecorView其实就是个FrameLayout，是整个窗口的根视图，它包括应用程序内容的视图以及窗口可能有的系统级视图，如状态栏和导航栏。

其中ID为content的FrameLayout就是mContentParent

二、继承自AppCompatActivity

1、AppCompatActivity.setContentView

与继承自Activity的不同，这里是调用的AppCompatDelegate的setContentView
AppCompatDelegate是一个接口，具体实现在AppCompatDelegateImpl：

1.1、ensureSubDecor

调用ensureSubDecor，下面的与继承Activity的类似
ensureSubDecor又调用createSubDecor

createSubDecor又调用ensureWindow，ensureWindow是对window对象检查，给mWindow变量赋值，之后执行getDecorView

此时mWindow是一个PhoneWindow对象，调用PhoneWindow的getDecorView

getDecorView中又调用了installDecor，这个方法在继承Activity中出现过，此后流程不再赘述

crateDecor之后还会去获取subDecor，看最简单的abc_screen_simple

abc_screen_content_include.xml

此时subDecor有布局了
之后会通过findViewById去获取上面的ContentFrameLayout

之后执行final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);

该处获取的是Activity的，也就是R.layout.screen_simple的content

之后将这个原始的content id置为NO_ID 将subDecerView的id置为content 进行一个替换

三、LayoutInflater.from(mContext).inflate(resId, contentParent);

不管是继承自Activity还是AppCompatActivity的setContentVIew，之后都会调用这么一句代码，该代码就是创建R.layout.main_activity的View

调用res.getLayout进行xml解析
调用inflate进行渲染

    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {synchronized (mConstructorArgs) {Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");final Context inflaterContext = mContext;final AttributeSet attrs = Xml.asAttributeSet(parser);Context lastContext = (Context) mConstructorArgs[0];mConstructorArgs[0] = inflaterContext;View result = root;try {advanceToRootNode(parser); //寻找根节点final String name = parser.getName(); //节点名称if (DEBUG) {System.out.println("**************************");System.out.println("Creating root view: "+ name);System.out.println("**************************");}//处理merge标签if (TAG_MERGE.equals(name)) {if (root == null || !attachToRoot) {throw new InflateException("<merge /> can be used only with a valid "+ "ViewGroup root and attachToRoot=true");}rInflate(parser, root, inflaterContext, attrs, false);} else {// Temp is the root view that was found in the xml//创建根视图final View temp = createViewFromTag(root, name, inflaterContext, attrs);ViewGroup.LayoutParams params = null;if (root != null) {if (DEBUG) {System.out.println("Creating params from root: " +root);}// Create layout params that match root, if supplied//处理布局参数params = root.generateLayoutParams(attrs);if (!attachToRoot) {// Set the layout params for temp if we are not// attaching. (If we are, we use addView, below)temp.setLayoutParams(params);}}if (DEBUG) {System.out.println("-----> start inflating children");}// Inflate all children under temp against its context.//解析并创建子视图，该方法是一个递归方法rInflateChildren(parser, temp, attrs, true);if (DEBUG) {System.out.println("-----> done inflating children");}// We are supposed to attach all the views we found (int temp)// to root. Do that now.if (root != null && attachToRoot) {root.addView(temp, params);}// Decide whether to return the root that was passed in or the// top view found in xml.if (root == null || !attachToRoot) {result = temp;}}} catch (XmlPullParserException e) {final InflateException ie = new InflateException(e.getMessage(), e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;} catch (Exception e) {final InflateException ie = new InflateException(getParserStateDescription(inflaterContext, attrs)+ ": " + e.getMessage(), e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;} finally {// Don't retain static reference on context.mConstructorArgs[0] = lastContext;mConstructorArgs[1] = null;Trace.traceEnd(Trace.TRACE_TAG_VIEW);}return result;}}

1、createViewFromTag

红框中会判断name中是否有点，有执行onCreateView没有执行createView
有点一般的就是一些第三方组件或者自定义组件，比如androidx.constraintlayout.widget.ConstraintLayout

Layoutinflater是一个抽象类，onCreateView的具体实现在其实现类PhoneLayoutInflater
在onCreateView中，最终还是调用了createView

此处的prefix是一些前缀，用于在调用createView时进行补全
最后返回了super.onCreateView,父类的onCreateView中补全了android.view

然后看createView

    @Nullablepublic final View createView(@NonNull Context viewContext, @NonNull String name,@Nullable String prefix, @Nullable AttributeSet attrs)throws ClassNotFoundException, InflateException {Objects.requireNonNull(viewContext);Objects.requireNonNull(name);Constructor<? extends View> constructor = sConstructorMap.get(name);if (constructor != null && !verifyClassLoader(constructor)) {constructor = null;sConstructorMap.remove(name);}Class<? extends View> clazz = null;try {Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);if (constructor == null) {// Class not found in the cache, see if it's real, and try to add it//由于前面补全了前缀，所以这个可以通过包名进行反射，然后创建View对象clazz = Class.forName(prefix != null ? (prefix + name) : name, false,mContext.getClassLoader()).asSubclass(View.class);if (mFilter != null && clazz != null) {boolean allowed = mFilter.onLoadClass(clazz);if (!allowed) {failNotAllowed(name, prefix, viewContext, attrs);}}//获取构造对象constructor = clazz.getConstructor(mConstructorSignature);constructor.setAccessible(true);sConstructorMap.put(name, constructor);} else {// If we have a filter, apply it to cached constructorif (mFilter != null) {// Have we seen this name before?Boolean allowedState = mFilterMap.get(name);if (allowedState == null) {// New class -- remember whether it is allowedclazz = Class.forName(prefix != null ? (prefix + name) : name, false,mContext.getClassLoader()).asSubclass(View.class);boolean allowed = clazz != null && mFilter.onLoadClass(clazz);mFilterMap.put(name, allowed);if (!allowed) {failNotAllowed(name, prefix, viewContext, attrs);}} else if (allowedState.equals(Boolean.FALSE)) {failNotAllowed(name, prefix, viewContext, attrs);}}}Object lastContext = mConstructorArgs[0];mConstructorArgs[0] = viewContext;Object[] args = mConstructorArgs;args[1] = attrs;try {//执行构造，创建View对象final View view = constructor.newInstance(args);if (view instanceof ViewStub) {// Use the same context when inflating ViewStub later.final ViewStub viewStub = (ViewStub) view;viewStub.setLayoutInflater(cloneInContext((Context) args[0]));}return view;} finally {mConstructorArgs[0] = lastContext;}} catch (NoSuchMethodException e) {final InflateException ie = new InflateException(getParserStateDescription(viewContext, attrs)+ ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;} catch (ClassCastException e) {// If loaded class is not a View subclassfinal InflateException ie = new InflateException(getParserStateDescription(viewContext, attrs)+ ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;} catch (ClassNotFoundException e) {// If loadClass fails, we should propagate the exception.throw e;} catch (Exception e) {final InflateException ie = new InflateException(getParserStateDescription(viewContext, attrs) + ": Error inflating class "+ (clazz == null ? "<unknown>" : clazz.getName()), e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;} finally {Trace.traceEnd(Trace.TRACE_TAG_VIEW);}}

到这里rootView创建好了，之后要创建子View

2、rInflateChildren

rInflateChildren调用的是rinflate

    void rInflate(XmlPullParser parser, View parent, Context context,AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {final int depth = parser.getDepth();int type;boolean pendingRequestFocus = false;//循环解析XMLwhile (((type = parser.next()) != XmlPullParser.END_TAG ||parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {//跳过非开始标签if (type != XmlPullParser.START_TAG) {continue;}//获取标签名称final String name = parser.getName();//处理<requestFocus>标签if (TAG_REQUEST_FOCUS.equals(name)) {pendingRequestFocus = true;consumeChildElements(parser);} else if (TAG_TAG.equals(name)) {//处理<tag>标签parseViewTag(parser, parent, attrs);} else if (TAG_INCLUDE.equals(name)) {//处理<include>标签if (parser.getDepth() == 0) {throw new InflateException("<include /> cannot be the root element");}parseInclude(parser, context, parent, attrs);} else if (TAG_MERGE.equals(name)) {//处理<merge>标签throw new InflateException("<merge /> must be the root element");} else {//对于其他标签，使用createViewFromTag创建视图，生成布局参数，递归调用rInflateChildrenfinal View view = createViewFromTag(parent, name, context, attrs);final ViewGroup viewGroup = (ViewGroup) parent;final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);rInflateChildren(parser, view, attrs, true);viewGroup.addView(view, params);}}if (pendingRequestFocus) {parent.restoreDefaultFocus();}if (finishInflate) {parent.onFinishInflate();}}