一、简介
setContentView我们在Activity中经常见到，它的作用就是把我们的布局文件放在Activity中显示，下面我们根据源码分析setContentView是如何做到的
二、源码分析
1.两种setContentView
注意Activity的setContentView和AppCompatActivity的setContentView是有一些区别的，所以我们要分析两钟setContentView，下面先分析Activity的
2.Activity的setContentView
(1).从Activity的setContentView这个方法开始
public void setContentView(@LayoutRes int layoutResID) {getWindow().setContentView(layoutResID);initWindowDecorActionBar();}可以看到第一句getWindow().setContentView(layoutResID)，这个getWindow是获取当前Activity的Window，在Android中Window的实现类是phoneWindow,所以我们要看phoneWindow的setContentView
顺便提一下Activity的window的创建时机是在Activity的attach方法:
(2).继续跟踪到phoneWindow的setContentView
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(); //⭐这句关键流程} 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);//⭐这句关键流程 }mContentParent.requestApplyInsets();final Callback cb = getCallback();if (cb != null && !isDestroyed()) {cb.onContentChanged();}mContentParentExplicitlySet = true; //⭐这个flag记一下}上面代码中我标记了三处重点，我们下面继续分析这三个重点都干了什么，先分析第一个installDecor()
(2.1).分析phoneWindow的setContentView的第一个关键点installDecor()
installDecor主要是我用红框标记出来的是重点，我们先分析generateDecor(-1)这个方法:
(2.1.1).分析installDecor()方法中的generateDecor(-1)方法
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, this);if (mTheme != -1) {context.setTheme(mTheme);}}} else {context = getContext();}return new DecorView(context, featureId, this, getAttributes());//⭐重点}创建了一个DecorView并且返回之后赋值给了mDecor，我们先看一下这个DecorVIew是什么:
很明显是一个FrameLayout，这下我们知道了创建了一个FrameLayout类型的DecorView然后赋值给了mDecor变量,下面继续分析installDecor的第二个重点：generateLayout(mDecor)
(2.1.2).分析installDecor的第二个重点：generateLayout(mDecor)
protected ViewGroup generateLayout(DecorView decor) {...else {// Embedded, so no decoration is needed.layoutResource = R.layout.screen_simple; // System.out.println("Simple!");} mDecor.startChanging();mDecor.onResourcesLoaded(mLayoutInflater, layoutResource); //⭐重点下面的方法 ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);if (contentParent == null) {throw new RuntimeException("Window couldn't find content container view");}...return contentParent;} //⭐DecorView的onResourcesLoaded方法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);//⭐重点主线流程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. ⭐重点主线流程addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));}mContentRoot = (ViewGroup) root;initializeElevation();}这个方法的作用就是，通过我们设置的style或者requestWindowFuture等来选出一个系统自带的布局文件，默认的是R.layout.screen_simple，选出布局文件后，通过调用mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);方法inflate出来后，add到DecorView上，我们详细看一下R.layout.screen_simple这个布局文件:
就是这个布局文件，被inflate到DecorView上，然后通过
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);这一句获取到这个是content的FrameLayout，然后返回了这个contentParent
到这里installDecor这个方法分析完了，总结一下，installDecor方法主要就是创建DecorView，然后把选出的布局文件add到DecorView上，然后再通过findViewbyId找到类型是FrameLayout的content赋值给contentParent 返回，其实还有一步是DecorView和phoneWindow结合，这里不细说，以后有FrameWorker源码解析再说。
(2.2).继续分析phoneWindow的setContentView的第二个关键流程重点
mLayoutInflater.inflate(layoutResID, mContentParent);这句很明显，layoutResID是我们的activity_main.layout这种自己写的布局文件，把它inflate到mContentParent中，通过图片让大家有一个更清晰的感官:
(2.3).继续分析phoneWindow的setContentView的第三个关键流程重点
mContentParentExplicitlySet = true;这个flag的作用首先我们先看一段代码:
public class MainActivity extends AppCompatActivity { @Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);requestWindowFeature(Window.FEATURE_NO_TITLE);}}这段代码运行会报错:
requestFeature() must be called before adding content为什么会报这个错误呢，从代码上来找:
public final boolean requestWindowFeature(int featureId) {return getWindow().requestFeature(featureId);} //我们已经知道，getWindow其实获取的是PhoneWindow所以调用的是PhoneWindow的requestFeature@Overridepublic boolean requestFeature(int featureId) {if (mContentParentExplicitlySet) { //⭐这就是报错的根源throw new AndroidRuntimeException("requestFeature() must be called before adding content"); }final int features = getFeatures();final int newFeatures = features | (1 << featureId);if ((newFeatures & (1 << FEATURE_CUSTOM_TITLE)) != 0 &&(newFeatures & ~CUSTOM_TITLE_COMPATIBLE_FEATURES) != 0) {// Another feature is enabled and the user is trying to enable the custom title feature// or custom title feature is enabled and the user is trying to enable another featurethrow new AndroidRuntimeException("You cannot combine custom titles with other title features");}if ((features & (1 << FEATURE_NO_TITLE)) != 0 && featureId == FEATURE_ACTION_BAR) {return false; // Ignore. No title dominates.}if ((features & (1 << FEATURE_ACTION_BAR)) != 0 && featureId == FEATURE_NO_TITLE) {// Remove the action bar feature if we have no title. No title dominates.removeFeature(FEATURE_ACTION_BAR);} if (featureId == FEATURE_INDETERMINATE_PROGRESS &&getContext().getPackageManager().hasSystemFeature(PackageManager.FEATURE_WATCH)) {throw new AndroidRuntimeException("You cannot use indeterminate progress on a watch.");}return super.requestFeature(featureId);}看到了报错的根源，其实就是mContentParentExplicitlySet这个flag，在setContentView执行完就设置成了true，所以调用requestWindowFeature(Window.FEATURE_NO_TITLE);方法必须在setContentView之前，否则就会抛出异常，最后从设计的角度分析，为什么要设计这么一个flag呢，或者说为什么非要在setContentView之前执行requestFeature，因为在setContentView中需要通过设置的这些requestWindowFeature的flag去选择一个布局文件然后add到DecorView上，如果在setContentView后面设置就起不到作用，所以有了这个设计。
3.AppCompatActivity的setContentView
(1).AppCompatActivity的setContentView源码
@Overridepublic void setContentView(@LayoutRes int layoutResID) {getDelegate().setContentView(layoutResID);}(1.1).getDelegate()
@NonNullpublic AppCompatDelegate getDelegate() {if (mDelegate == null) {mDelegate = AppCompatDelegate.create(this, this);}return mDelegate;}实际上实现类是AppCompatDelegate，getDelegate().setContentView(layoutResID);的setContentView实际上是AppCompatDelegate的setContentView方法
(2).AppCompatDelegate的setContentView方法
@Overridepublic void setContentView(int resId) {ensureSubDecor(); //⭐重点主线ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content); //⭐重点主线contentParent.removeAllViews();LayoutInflater.from(mContext).inflate(resId, contentParent);//⭐重点主线mAppCompatWindowCallback.getWrapped().onContentChanged();}(2.1).分析ensureSubDecor()方法
private void ensureSubDecor() {if (!mSubDecorInstalled) {mSubDecor = createSubDecor(); // ⭐重点主线流程 // If a title was set before we installed the decor, propagate it nowCharSequence title = getTitle();if (!TextUtils.isEmpty(title)) {if (mDecorContentParent != null) {mDecorContentParent.setWindowTitle(title);} else if (peekSupportActionBar() != null) {peekSupportActionBar().setWindowTitle(title);} else if (mTitleView != null) {mTitleView.setText(title);}} applyFixedSizeWindow(); onSubDecorInstalled(mSubDecor); mSubDecorInstalled = true;//⭐这个flag参数 // Invalidate if the panel menu hasn't been created before this.// Panel menu invalidation is deferred avoiding application onCreateOptionsMenu// being called in the middle of onCreate or similar.// A pending invalidation will typically be resolved before the posted message// would run normally in order to satisfy instance state restoration.PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);if (!mIsDestroyed && (st == null || st.menu == null)) {invalidatePanelMenu(FEATURE_SUPPORT_ACTION_BAR);}}}主要的第一句:
mSubDecor = createSubDecor();mSubDecor是一个ViewGroup类型的对象，下面我们分析createSubDecor()
(2.1.1).ensureSubDecor()方法中的createSubDecor()方法
private ViewGroup createSubDecor() {TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);//⭐这个错误是不是曾经见到过，如果用的Theme不是AppCompatTheme的就会报错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.");}...... // Now let's make sure that the Window has installed its decor by retrieving itensureWindow(); mWindow.getDecorView();final LayoutInflater inflater = LayoutInflater.from(mContext);ViewGroup subDecor = null;if (!mWindowNoTitle) {......} else {...... subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);......}...... 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 viewwhile (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.windowContentView.setId(View.NO_ID);contentView.setId(android.R.id.content); // The decorContent may have a foreground drawable set (windowContentOverlay).// Remove this as we handle it ourselvesif (windowContentView instanceof FrameLayout) {((FrameLayout) windowContentView).setForeground(null);}} // Now set the Window's content view with the decormWindow.setContentView(subDecor);...... return subDecor;}上面显示出来的基本上都很重要，我们一句一句分析:
(2.1.1.1).ensureWindow();
private void ensureWindow() {// We lazily fetch the Window for Activities, to allow DayNight to apply in// attachBaseContextif (mWindow == null && mHost instanceof Activity) {attachToWindow(((Activity) mHost).getWindow());}if (mWindow == null) {throw new IllegalStateException("We have not been given a Window");}}首先我们要明确AppCompatActivity是继承自Activity的，所以window也是在attach方法中创建的，在AppCompatDelegateImpl中也维护了一个Window类型的变量是mWindow，就是通过这个ensureWindow方法经过检查后赋值过来的。说白了ensureWindow方法就是把AppCompatActivity中的Window对象赋值到AppCompatDelegateImpl对象中，当然对window设置的callBack啥的也换成AppCompatDelegateImpl中的。
(2.1.1.2).mWindow.getDecorView()方法
Window的实现类，所以我们要看PhoneWindow的getDecorView()方法:
@Overridepublic final @NonNull View getDecorView() {if (mDecor == null || mForceDecorInstall) {installDecor();}return mDecor;}可以看到是调用了installDecor，和Activity有了相同的部分，我们简单回忆一下installDecor干了啥，首先创建了DecorView，然后通过解析我们style的设置选出合适的系统自带布局文件，把它add到DecorView上，并且返回了一个id是com.android.internal.R.id.content的FrameLayout（将来我们要把我们的activity_mai的layout文件add到这个content上面）。
(2.1.1.3).后面要分析的这一堆代码就是Activity和AppCompatActivity的setContentView的主要区别
.... subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);....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 viewwhile (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.windowContentView.setId(View.NO_ID);contentView.setId(android.R.id.content); ......} // Now set the Window's content view with the decormWindow.setContentView(subDecor);...... return subDecor;}通过我们设置的style选出一个布局文件，这一步好像在installDecor中已经做过了，这样好像重复了，为什么有这样的一个重复” />
abc_screen_content_include的布局文件:
把这个布局文件inflate成view赋值给subDecor，subDecor是一个ViewGroup：
下面重点来了:
final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(R.id.action_bar_activity_content);这一句是获取subDecor中的id是 action_bar_activity_content的ContentFrameLayout类型的View赋值给contentView
final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);windowContentView是获取installDecor中选择的那个布局的id是android.R.id.content的那个FrameLayout
while (windowContentView.getChildCount() > 0) {final View child = windowContentView.getChildAt(0);windowContentView.removeViewAt(0);contentView.addView(child);}如果windowContentView中有子View，那就全部转移到contentView上
windowContentView.setId(View.NO_ID);contentView.setId(android.R.id.content);然后把windowContentView的id设置成NO_ID，把contentView的id设置成android.R.id.content，这就是想把AppCompatDelegateImple中选的这个系统自带布局文件的content替换掉之前的installDecor方法中布局文件的content，以后我们的activity_main的layout布局文件就加载在替换之后的content上
mWindow.setContentView(subDecor);这句调用的是phoneWindow的setContentView方法
@Overridepublic void setContentView(View view) {setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));} @Overridepublic void setContentView(View view, ViewGroup.LayoutParams params) {// 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)) {view.setLayoutParams(params);final Scene newScene = new Scene(mContentParent, view);transitionTo(newScene);} else {mContentParent.addView(view, params); //⭐把contentView添加到mContentParent上}mContentParent.requestApplyInsets();final Callback cb = getCallback();if (cb != null && !isDestroyed()) {cb.onContentChanged();}mContentParentExplicitlySet = true;}我们看一下mContentParent是什么：
mContentParent = generateLayout(mDecor);是我们取消ID的那个之前的content，也就是说把AppCompatDelegateImpl的选择的系统自带布局文件(subDecor)，添加到之前的content中，最后返回subDecor注意这个subDecor是系统自带布局inflate出来的，接下来我们通过一张图加深理解：
(2.1.2).ensureSubDecor()方法中的一个boolean值mSubDecorInstalled
这个mSubDecorInstalled和Activity中的mContentParentExplicitlySet一样，作用也是防止在setContentView之后调用
supportRequestWindowFeature(Window.FEATURE_NO_TITLE);，我们发现和Activity的不一样了，我们思考一下，为什么调用supportRequestWindowFeature而不是调用requestWindowFeature？
Activity中处理这些style是在PhoneWIndow的installDecor中，而AppCompatActivity在处理一些style时是在AppCompatDelegateImpl中，requestWindowFeature是影响的installDecor，supportRequestWindowFeature是影响的AppCompatDelegateImpl，拿window的Title来举例，看上面放过的两张图，Activity的title是要在installDecor方法中决定的显示与隐藏的，而AppCompatActivity的title是放在AppCompatDelegateImpl中决定显示与隐藏的，我们用AppCompatActivity肯定是要在AppCompatDelegateImpl进行一些操作，而不是对installDecor中进行操作。
下面看一下supportRequestWindowFeature调用逻辑:
public boolean supportRequestWindowFeature(int featureId) {return getDelegate().requestWindowFeature(featureId);} @Overridepublic boolean requestWindowFeature(int featureId) {featureId = sanitizeWindowFeatureId(featureId); if (mWindowNoTitle && featureId == FEATURE_SUPPORT_ACTION_BAR) {return false; // Ignore. No title dominates.}if (mHasActionBar && featureId == Window.FEATURE_NO_TITLE) {// Remove the action bar feature if we have no title. No title dominates.mHasActionBar = false;} switch (featureId) {case FEATURE_SUPPORT_ACTION_BAR:throwFeatureRequestIfSubDecorInstalled();mHasActionBar = true;return true;case FEATURE_SUPPORT_ACTION_BAR_OVERLAY:throwFeatureRequestIfSubDecorInstalled();mOverlayActionBar = true;return true;case FEATURE_ACTION_MODE_OVERLAY:throwFeatureRequestIfSubDecorInstalled();mOverlayActionMode = true;return true;case Window.FEATURE_PROGRESS:throwFeatureRequestIfSubDecorInstalled();mFeatureProgress = true;return true;case Window.FEATURE_INDETERMINATE_PROGRESS:throwFeatureRequestIfSubDecorInstalled();mFeatureIndeterminateProgress = true;return true;case Window.FEATURE_NO_TITLE:throwFeatureRequestIfSubDecorInstalled();mWindowNoTitle = true;return true;} return mWindow.requestFeature(featureId);} //⭐mSubDecorInstalled 这参数眼熟吧private void throwFeatureRequestIfSubDecorInstalled() {if (mSubDecorInstalled) {throw new AndroidRuntimeException("Window feature must be requested before adding content");}}(2.2).AppCompatDelegateImpl中的setContentView(View v)剩下的一起说
ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content); //⭐重点主线contentParent.removeAllViews();LayoutInflater.from(mContext).inflate(resId, contentParent);//⭐重点主线获取contentview，注意这个contentView和Activity的contentView的区别，看上面那张图，把所有的view清除掉，然后把我们的activity_main的layout这种我们自己的布局加载上去
3.分析LayoutInflater.from(mContext).inflate(resId, contentParent);方法
(1).很经典的一道面试题，inflate三个参数都有什么作用
或者说这三种写法有什么区别：
我们先看源码，看完之后总结:
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {final Resources res = getContext().getResources();if (DEBUG) {Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("+ Integer.toHexString(resource) + ")");} View view = tryInflatePrecompiled(resource, res, root, attachToRoot);if (view != null) {return view;}XmlResourceParser parser = res.getLayout(resource); //⭐把布局文件用xml解析器解析 try {return inflate(parser, root, attachToRoot); //调用inflate重载的方法} finally {parser.close();}}继续往下看inflate的重载方法，这个方法中就有这道面试题的答案:
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; //⭐重点 try {advanceToRootNode(parser); //⭐确保下面的代码是首先解析的根布局标签final String name = parser.getName(); if (DEBUG) {System.out.println("**************************");System.out.println("Creating root view: "+ name);System.out.println("**************************");} if (TAG_MERGE.equals(name)) {if (root == null || !attachToRoot) {throw new InflateException(" can be used only with a valid "+ "ViewGroup root and attachToRoot=true");} rInflate(parser, root, inflaterContext, attrs, false);} else { //⭐重点 else单独分析// Temp is the root view that was found in the xmlfinal 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 suppliedparams = 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.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;}}上面的代码太多我把关于这道题的主要代码逻辑拿出来:
(1.1).advanceToRootNode(）这个方法就是确保第一个解析的是布局的根标签
private void advanceToRootNode(XmlPullParser parser)throws InflateException, IOException, XmlPullParserException {// Look for the root node.int type;while ((type = parser.next()) != XmlPullParser.START_TAG &&type != XmlPullParser.END_DOCUMENT) {// Empty} if (type != XmlPullParser.START_TAG) {throw new InflateException(parser.getPositionDescription()+ ": No start tag found!");}}(1.2).else的逻辑单独分析
else {// Temp is the root view that was found in the xmlfinal 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 suppliedparams = 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.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;}}下面逐句分析：
(1.2.1)
final View temp = createViewFromTag(root, name, inflaterContext, attrs);这一句就是通过name，来反射创建view，下面会详细分析如何创建的view，这个temp就是布局文件的根view
(1.2.2)
if (root != null) {if (DEBUG) {System.out.println("Creating params from root: " +root);}// Create layout params that match root, if suppliedparams = 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);}}这就是如果inflate方法的第二个参数root不是null，那就执行这段代码：
params = root.generateLayoutParams(attrs);这段代码的意思是：我们把新视图的根布局参数传递给root。让root进行转换成适合它自身布局的布局参数（因为不同的布局有不同的特性，例如LinearLayout和FrameLayout，我们要把布局文件的根temp这个View放到root中，就要让temp原来的布局参数转换成适合root这个ViewGroup布局的参数），如果inflate第三个参数attachToRoot是false就把布局文件的根view设置成转换的params
(1.2.3)
rInflateChildren(parser, temp, attrs, true);之后就继续解析布局文件，通过反射创建View，具体如何解析和创建之后详细分析
(1.2.4)
if (root != null && attachToRoot) {root.addView(temp, params);}如果root不是null并且attachToRoot是true，把布局文件根生成的temp这个view添加到root上面，注意这个params还是上面转换的那个。
(1.2.5)
if (root == null || !attachToRoot) {result = temp;}如果root是null或者attachToRoot是false，方法最后的返回值就是这个布局文件生成的View
(1.2.6)
到这里我们就知道了：
1.当root不为null时，attachTORoot是true的时候就直接把我们布局生成的View添加到root(这个root是inflate方法参数的第二个参数)上面，并且方法最后的返回结果是root，如果attachTORoot是false，直接返回我们布局文件生成的View注意这个生成View的layoutParams已经set了，所以可以说：
LayoutInflater.from(this).inflate(R.layout.activity_main,root,true);等价于
View view = LayoutInflater.from(this).inflate(R.layout.activity_main,root,false);root.addView(view);2.当root为null时，直接返回我们布局文件生成的view，注意这个生成的View没有layoutParams
,而且一旦root为null，后面的attachToRoot这个参数就失效了。
(2).分析inflate方法如何解析和创建View的
(2.1).在inflate布局文件的根标签时要注意merge标签
if (TAG_MERGE.equals(name)) {if (root == null || !attachToRoot) {throw new InflateException(" can be used only with a valid "+ "ViewGroup root and attachToRoot=true");}rInflate(parser, root, inflaterContext, attrs, false);}当一个布局文件的根标签是merge时，如果root是null或者attachToRoot是false就报错，说明布局文件跟标签是merge时不能直接生成一个View，必须依附于其他View上。
(2.2).解析不是根标签的布局
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; while (((type = parser.next()) != XmlPullParser.END_TAG ||parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) { if (type != XmlPullParser.START_TAG) {continue;} final String name = parser.getName(); if (TAG_REQUEST_FOCUS.equals(name)) {pendingRequestFocus = true;consumeChildElements(parser);} else if (TAG_TAG.equals(name)) {parseViewTag(parser, parent, attrs);} else if (TAG_INCLUDE.equals(name)) {if (parser.getDepth() == 0) {throw new InflateException(" cannot be the root element");}parseInclude(parser, context, parent, attrs);} else if (TAG_MERGE.equals(name)) {throw new InflateException(" must be the root element");} else {final 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();}}(2.2.1).解析不是根布局时，注意对merge、include标签的处理
对merge标签时直接报错，因为merge标签只能用于根标签，对include标签的处理，判断如果include是跟标签则报错，因为include标签不能用作根标签。
(2.2.2).其他的view标签会走创建view，然后通过父布局生成对应的布局参数LayoutParams，然后添加在副布局上
else {final 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);}这一段中，createViewFromTag这个方法最重要，后面都是生成LayoutParams和添加到父布局上的逻辑，最后我们分析createViewFromTag这个方法
(2.2.3).createViewFromTag创建view,注意这个Activity和AppCompatActivity有差别
a.先分析Activity的createViewFromTag创建view的流程
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {return createViewFromTag(parent, name, context, attrs, false);} @UnsupportedAppUsageView createViewFromTag(View parent, String name, Context context, AttributeSet attrs,boolean ignoreThemeAttr) {if (name.equals("view")) {name = attrs.getAttributeValue(null, "class");} // Apply a theme wrapper, if allowed and one is specified.if (!ignoreThemeAttr) {final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);final int themeResId = ta.getResourceId(0, 0);if (themeResId != 0) {context = new ContextThemeWrapper(context, themeResId);}ta.recycle();} try {View view = tryCreateView(parent, name, context, attrs); if (view == null) {final Object lastContext = mConstructorArgs[0];mConstructorArgs[0] = context;try {if (-1 == name.indexOf('.')) { //⭐重点view = onCreateView(context, parent, name, attrs); //⭐重点} else {view = createView(context, name, null, attrs); //⭐重点}} finally {mConstructorArgs[0] = lastContext;}} return view;} catch (InflateException e) {throw e; } catch (ClassNotFoundException e) {final InflateException ie = new InflateException(getParserStateDescription(context, attrs)+ ": Error inflating class " + name, e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie; } catch (Exception e) {final InflateException ie = new InflateException(getParserStateDescription(context, attrs)+ ": Error inflating class " + name, e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;}}我们分析重点:
if (view == null) {final Object lastContext = mConstructorArgs[0];mConstructorArgs[0] = context;try {if (-1 == name.indexOf('.')) {view = onCreateView(context, parent, name, attrs);} else {view = createView(context, name, null, attrs);}} finally {mConstructorArgs[0] = lastContext;}}return view;if (-1 == name.indexOf(‘.’))这个判断是标签是不是自定义View，带”.”的是自定义View，如果不是自定义View，通过onCreateView创建View，如果是自定义View，通过createView创建View
所以我们比较一下为啥不是自定义View要通过onCreateView创建而是自定义VIew的用createView创建
先说一个知识点，LayoutInflater的实现类是PhoneLayoutInflater，onCreateView方法也被重写了:
private static final String[] sClassPrefixList = {"android.widget.","android.webkit.","android.app."}; @Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {for (String prefix : sClassPrefixList) {try {View view = createView(name, prefix, attrs);if (view != null) {return view;}} catch (ClassNotFoundException e) {// In this case we want to let the base class take a crack// at it.}} return super.onCreateView(name, attrs);} 最后还调用了super:protected View onCreateView(String name, AttributeSet attrs)throws ClassNotFoundException {return createView(name, "android.view.", attrs);}可以看到onCreateView最后还是调用了createVIew，所以onCreateView和createView主要的差别就是这个prefix的前缀，不是自定义View需要有前缀，想想LinearLayout这个类的全类名是”android.widget.LinearLayout”，这下知道了这些SDK自带的不是自定义View标签都会在这里补全全类名，最后看一下createView
(2.2.4).createView方法
static final Class" : clazz.getName()), e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;} finally {Trace.traceEnd(Trace.TRACE_TAG_VIEW);}}这么一大堆代码其实就是通过反射去创建View，注意里面的注释
b.再分析AppCompatActivity的createViewFromTag创建view的流程
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {return createViewFromTag(parent, name, context, attrs, false);} @UnsupportedAppUsageView createViewFromTag(View parent, String name, Context context, AttributeSet attrs,boolean ignoreThemeAttr) {if (name.equals("view")) {name = attrs.getAttributeValue(null, "class");} // Apply a theme wrapper, if allowed and one is specified.if (!ignoreThemeAttr) {final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);final int themeResId = ta.getResourceId(0, 0);if (themeResId != 0) {context = new ContextThemeWrapper(context, themeResId);}ta.recycle();} try {View view = tryCreateView(parent, name, context, attrs);//⭐重点 if (view == null) {final Object lastContext = mConstructorArgs[0];mConstructorArgs[0] = context;try {if (-1 == name.indexOf('.')) { view = onCreateView(context, parent, name, attrs);} else {view = createView(context, name, null, attrs); }} finally {mConstructorArgs[0] = lastContext;}} return view;} catch (InflateException e) {throw e; } catch (ClassNotFoundException e) {final InflateException ie = new InflateException(getParserStateDescription(context, attrs)+ ": Error inflating class " + name, e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie; } catch (Exception e) {final InflateException ie = new InflateException(getParserStateDescription(context, attrs)+ ": Error inflating class " + name, e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;}}这个tryCreatView方法如果尝试创建View失败之后才轮到Activity的创建方式，我们看一下tryCreateView方法:
public final View tryCreateView(@Nullable View parent, @NonNull String name,@NonNull Context context,@NonNull AttributeSet attrs) {if (name.equals(TAG_1995)) {// Let's party like it's 1995!return new BlinkLayout(context, attrs);} View view;if (mFactory2 != null) {view = mFactory2.onCreateView(parent, name, context, attrs);} else if (mFactory != null) {view = mFactory.onCreateView(name, context, attrs);} else {view = null;} if (view == null && mPrivateFactory != null) {view = mPrivateFactory.onCreateView(parent, name, context, attrs);} return view;}发现有几个变量不认识，mFactory2和mFactory还有mPrivateFactory，我们的AppCompatActivity就是使用的mFactory2，看AppCompatActivity的onCreate方法:
@Overrideprotected void onCreate(@Nullable Bundle savedInstanceState) {final AppCompatDelegate delegate = getDelegate();delegate.installViewFactory(); // ⭐delegate.onCreate(savedInstanceState);super.onCreate(savedInstanceState);}我们继续跟踪这个方法:
public void installViewFactory() {LayoutInflater layoutInflater = LayoutInflater.from(mContext);if (layoutInflater.getFactory() == null) {LayoutInflaterCompat.setFactory2(layoutInflater, this);} else {if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImpl)) {Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"+ " so we can not install AppCompat's");}}} //Factory2是一个接口public interface Factory2 extends Factory {@NullableView onCreateView(@Nullable View parent, @NonNull String name,@NonNull Context context, @NonNull AttributeSet attrs);}//LayoutInflater实现了这个接口class AppCompatDelegateImpl extends AppCompatDelegateimplements MenuBuilder.Callback, LayoutInflater.Factory2 这下看到了，AppCompatActivity的oncreate方法中调用installViewFactory方法，获取到layoutInflater对象，AppCompatDelegateImpl实现了Factory2的接口，
LayoutInflaterCompat.setFactory2(layoutInflater, this);
public static void setFactory2(@NonNull LayoutInflater inflater, @NonNull LayoutInflater.Factory2 factory) {inflater.setFactory2(factory); // ⭐ if (Build.VERSION.SDK_INT < 21) {final LayoutInflater.Factory f = inflater.getFactory();if (f instanceof LayoutInflater.Factory2) {// The merged factory is now set to getFactory(), but not getFactory2() (pre-v21).// We will now try and force set the merged factory to mFactory2forceSetFactory2(inflater, (LayoutInflater.Factory2) f);} else {// Else, we will force set the original wrapped Factory2forceSetFactory2(inflater, factory);}}}//LayoutInflater类中的方法public void setFactory2(Factory2 factory) {if (mFactorySet) {throw new IllegalStateException("A factory has already been set on this LayoutInflater");}if (factory == null) {throw new NullPointerException("Given factory can not be null");}mFactorySet = true;if (mFactory == null) {mFactory = mFactory2 = factory;} else {mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);}}可以看到吧Factory2传进了infalter中，所以Inflater的对象中Factory2不是null了。
Factory2不是null了，在执行创建createViewFromTag方法的tryCreateView时:
public final View tryCreateView(@Nullable View parent, @NonNull String name,@NonNull Context context,@NonNull AttributeSet attrs) {if (name.equals(TAG_1995)) {// Let's party like it's 1995!return new BlinkLayout(context, attrs);} View view;if (mFactory2 != null) {view = mFactory2.onCreateView(parent, name, context, attrs);} else if (mFactory != null) {view = mFactory.onCreateView(name, context, attrs);} else {view = null;} if (view == null && mPrivateFactory != null) {view = mPrivateFactory.onCreateView(parent, name, context, attrs);} return view;}mFactory2不是null了，就会执行 view = mFactory2.onCreateView(parent, name, context, attrs);
我们知道，这个mFactory2其实就是AppCompatDelegateImpl的实例对象，这个设计挺巧妙和上面ensureDecor有点像，添加了一些逻辑，使对view的创建逻辑转移到了AppCompatDelegateImpl中，所以我们下面看AppCompatDelegateImpl的onCreateView方法:
@Overridepublic final View onCreateView(View parent, String name, Context context, AttributeSet attrs) {return createView(parent, name, context, attrs);} @Overridepublic View createView(View parent, final String name, @NonNull Context context,@NonNull AttributeSet attrs) {if (mAppCompatViewInflater == null) {TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);String viewInflaterClassName =a.getString(R.styleable.AppCompatTheme_viewInflaterClass);if ((viewInflaterClassName == null)|| AppCompatViewInflater.class.getName().equals(viewInflaterClassName)) {// Either default class name or set explicitly to null. In both cases// create the base inflater (no reflection)mAppCompatViewInflater = new AppCompatViewInflater();} else {try {Class viewInflaterClass = Class.forName(viewInflaterClassName);mAppCompatViewInflater =(AppCompatViewInflater) viewInflaterClass.getDeclaredConstructor().newInstance();} catch (Throwable t) {Log.i(TAG, "Failed to instantiate custom view inflater "+ viewInflaterClassName + ". Falling back to default.", t);mAppCompatViewInflater = new AppCompatViewInflater();}}} boolean inheritContext = false;if (IS_PRE_LOLLIPOP) {inheritContext = (attrs instanceof XmlPullParser)// If we have a XmlPullParser, we can detect where we are in the layout? ((XmlPullParser) attrs).getDepth() > 1// Otherwise we have to use the old heuristic: shouldInheritContext((ViewParent) parent);}//⭐return mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext,IS_PRE_LOLLIPOP, /* Only read android:theme pre-L (L+ handles this anyway) */true, /* Read read app:theme as a fallback at all times for legacy reasons */VectorEnabledTintResources.shouldBeUsed() /* Only tint wrap the context if enabled */);}可以看到创建了一个mAppCompatViewInflater ，最后调用了mAppCompatViewInflater的createView方法:
final View createView(View parent, final String name, @NonNull Context context,@NonNull AttributeSet attrs, boolean inheritContext,boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {final Context originalContext = context; // We can emulate Lollipop's android:theme attribute propagating down the view hierarchy// by using the parent's contextif (inheritContext && parent != null) {context = parent.getContext();}if (readAndroidTheme || readAppTheme) {// We then apply the theme on the context, if specifiedcontext = themifyContext(context, attrs, readAndroidTheme, readAppTheme);}if (wrapContext) {context = TintContextWrapper.wrap(context);} View view = null; // We need to 'inject' our tint aware Views in place of the standard framework versionsswitch (name) {case "TextView":view = createTextView(context, attrs);verifyNotNull(view, name);break;case "ImageView":view = createImageView(context, attrs);verifyNotNull(view, name);break;case "Button":view = createButton(context, attrs);verifyNotNull(view, name);break;case "EditText":view = createEditText(context, attrs);verifyNotNull(view, name);break;case "Spinner":view = createSpinner(context, attrs);verifyNotNull(view, name);break;case "ImageButton":view = createImageButton(context, attrs);verifyNotNull(view, name);break;case "CheckBox":view = createCheckBox(context, attrs);verifyNotNull(view, name);break;case "RadioButton":view = createRadioButton(context, attrs);verifyNotNull(view, name);break;case "CheckedTextView":view = createCheckedTextView(context, attrs);verifyNotNull(view, name);break;case "AutoCompleteTextView":view = createAutoCompleteTextView(context, attrs);verifyNotNull(view, name);break;case "MultiAutoCompleteTextView":view = createMultiAutoCompleteTextView(context, attrs);verifyNotNull(view, name);break;case "RatingBar":view = createRatingBar(context, attrs);verifyNotNull(view, name);break;case "SeekBar":view = createSeekBar(context, attrs);verifyNotNull(view, name);break;case "ToggleButton":view = createToggleButton(context, attrs);verifyNotNull(view, name);break;default:// The fallback that allows extending class to take over view inflation// for other tags. Note that we don't check that the result is not-null.// That allows the custom inflater path to fall back on the default one// later in this method.view = createView(context, name, attrs);} if (view == null && originalContext != context) {// If the original context does not equal our themed context, then we need to manually// inflate it using the name so that android:theme takes effect.view = createViewFromTag(context, name, attrs);} if (view != null) {// If we have created a view, check its android:onClickcheckOnClickListener(view, attrs);} return view;}可以看到有一个switch，如果View是case中的一种，那就把布局参数传进去重新创建一个AppCompat的View，如果不是case中的一种，进default，这个createView方法直接返回null，
protected View createView(Context context, String name, AttributeSet attrs) {
return null;
}
然后走下面这句:
if (view == null && originalContext != context) {// If the original context does not equal our themed context, then we need to manually// inflate it using the name so that android:theme takes effect.view = createViewFromTag(context, name, attrs);}private View createViewFromTag(Context context, String name, AttributeSet attrs) {if (name.equals("view")) {name = attrs.getAttributeValue(null, "class");} try {mConstructorArgs[0] = context;mConstructorArgs[1] = attrs; if (-1 == name.indexOf('.')) {for (int i = 0; i < sClassPrefixList.length; i++) {final View view = createViewByPrefix(context, name, sClassPrefixList[i]);if (view != null) {return view;}}return null;} else {return createViewByPrefix(context, name, null);}} catch (Exception e) {// We do not want to catch these, lets return null and let the actual LayoutInflater// tryreturn null;} finally {// Don't retain references on context.mConstructorArgs[0] = null;mConstructorArgs[1] = null;}} private View createViewByPrefix(Context context, String name, String prefix)throws ClassNotFoundException, InflateException {Constructor constructor = sConstructorMap.get(name); try {if (constructor == null) {// Class not found in the cache, see if it's real, and try to add itClass clazz = Class.forName(prefix != null ? (prefix + name) : name,false,context.getClassLoader()).asSubclass(View.class); constructor = clazz.getConstructor(sConstructorSignature);sConstructorMap.put(name, constructor);}constructor.setAccessible(true);return constructor.newInstance(mConstructorArgs);} catch (Exception e) {// We do not want to catch these, lets return null and let the actual LayoutInflater// tryreturn null;}}这个方法和Activity的逻辑一致了
总结一下：AppCompatActivity通过LayoutInfater解析创建View时，会通过setFactory2拦截原有Activity的逻辑，去执行AppCompatDelegateImpl的逻辑，在View解析和创建的时候，会先检查，如果是AppCompat新设计的View就是case里的那一堆，就把这个View转换成AppCompat新设计的View，如果不是还是按照之前的逻辑来。
三、总结
1.setContentView的总结
setContentView总的来说就是创建DecorView，DecorView是一个FrameLayout，然后根据style选择系统自带的布局文件，(例如有没有title，这里说一下这个布局文件根布局是linearLayout，如果有title则是有一个viewStub和两个FrameLayout：title的和content的，如果没有title则是一个viewstub和一个content的FrameLayout)，添加到DecorView上，最后再把我们自己的的activity_mian这种layout添加到content这个FrameLyout上。
注意Activity和AppCompatActivity有一些区别，但总体上的逻辑是不变的。
2.inflate的总结
总结了inflate三个参数的作用，总结了解析到include、merge等标签时的注意点，总结了如何解析xml文件和如何创建View的重要流程。