前言

我们在开发过程中，在布局文件里添加TextView,代码运行起来就可以看到对应文字显示出来，那系统是如何把我们的TextView加载并显示出来的呢？

源码分析（这里版本对应30）

第一阶段

我们直接从Activity.setContentView()【为什么不是AppCompatActivity呢？其实最终继承Activity，只不过进行了高版本的适配】源码开始分析：

Activity.setContentView()

    public void setContentView(@LayoutRes int layoutResID) {getWindow().setContentView(layoutResID);initWindowDecorActionBar();}

getWindow()对应Window类，它是一个抽象类，我们知道它的唯一实现类是PhoneWindow：

PhoneWindow.setContentView()

    public void setContentView(int layoutResID) {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);}mContentParent.requestApplyInsets();final Callback cb = getCallback();if (cb != null && !isDestroyed()) {cb.onContentChanged();}mContentParentExplicitlySet = true;}

mContentParent是一个ViewGroup，一开始默认为null，我们先看下installDecor()方法都做了什么？

    private void installDecor() {mForceDecorInstall = false;if (mDecor == null) {mDecor = generateDecor(-1);...} else {mDecor.setWindow(this);}if (mContentParent == null) {mContentParent = generateLayout(mDecor);}...}

installDecor()方法代码比较多，我们看源码最忌讳一行行弄清楚，我们只关心我们需要关心的代码，这里重点方法为generateDecor(-1)和generateLayout(mDecor),我们继续跟进下：

    protected DecorView generateDecor(int featureId) {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());}

可以看到generateDecor()方法如其名，最终就是创建了一个DecorView对象；我们再看下generateLayout(mDecor)方法；

protected ViewGroup generateLayout(DecorView decor) {....// Inflate the window decor.int layoutResource;//下面会根据features不同的值给layoutResource赋值不同的布局文件，features就是对应不同的窗口样式int features = getLocalFeatures();if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {...} ...} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {layoutResource = R.layout.screen_simple_overlay_action_mode;} else {//默认加载R.layout.screen_simple布局layoutResource = R.layout.screen_simple;}mDecor.startChanging();//在这里将layoutResource添加到DecorView上mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);contentParent对应布局文件中ID_ANDROID_CONTENT的ViewViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);if (contentParent == null) {throw new RuntimeException("Window couldn't find content container view");}...return contentParent;}

在generateLayout方法中，会根据不同的features（窗口样式，比如带不带标题栏等等）加载不同的布局文件，默认采用R.layout.screen_simple布局文件，我们看下这个布局文件代码：

获取到需要加载的布局文件后，紧跟着调用mDecor.onResourcesLoaded(mLayoutInflater, layoutResource)方法：

 void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {...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();}

可以看到onResourcesLoaded方法就是将layoutResource布局添加到DecorView的根布局位置。添加完成后，最终generateLayout方法返回的就是ID对应ID_ANDROID_CONTENT的FrameLayout!!

到这里我们先简单画一下当前界面的显示内容：

第二阶段

分析完了installDecor(),接下来，我们就来分析mLayoutInflater.inflate(layoutResID, mContentParent)

在分析之前，我们先简单了解下mLayoutInflater,在PhoneWindow初始化时，会完成mLayoutInflater的初始化工作：

 public PhoneWindow(Context context) {super(context);mLayoutInflater = LayoutInflater.from(context);...

    public static LayoutInflater from(Context context) {LayoutInflater LayoutInflater =(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);if (LayoutInflater == null) {throw new AssertionError("LayoutInflater not found.");}return LayoutInflater;}

Context是一个抽象类，它对应的实现类为ContextImpl:

    public Object getSystemService(String name) {....return SystemServiceRegistry.getSystemService(this, name);}

public static Object getSystemService(ContextImpl ctx, String name) {if (name == null) {return null;}final ServiceFetcher fetcher = SYSTEM_SERVICE_FETCHERS.get(name);...final Object ret = fetcher.getService(ctx);...return ret;}

SYSTEM_SERVICE_FETCHERS是一个Map集合，那什么时候把LayoutInflater放进集合的呢？答案在SystemServiceRegistry类的静态代码块中：

....
static{registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,new CachedServiceFetcher() {@Overridepublic LayoutInflater createService(ContextImpl ctx) {return new PhoneLayoutInflater(ctx.getOuterContext());}});}
...### registerService方法private static  void registerService(@NonNull String serviceName,@NonNull Class serviceClass, @NonNull ServiceFetcher serviceFetcher) {SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);SYSTEM_SERVICE_CLASS_NAMES.put(serviceName, serviceClass.getSimpleName());}

从这里我们可以看出mLayoutInflater是一个单例，整个APP启动只会创建一个实例。

我们继续分析mLayoutInflater.inflate(layoutResID, mContentParent),会调用到LayoutInflater.inflate方法

 public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {final Resources res = getContext().getResources();...View view = tryInflatePrecompiled(resource, res, root, attachToRoot);if (view != null) {return view;}XmlResourceParser parser = res.getLayout(resource);try {return inflate(parser, root, attachToRoot);} finally {parser.close();}}

其中tryInflatePrecompiled是Android 10（Android Q）中新增的方法，用来根据布局文件的xml预编译生成dex，然后通过反射来生成对应的View，从而减少XmlPullParser解析Xml的时间。它是一个编译优化选项。

我们重点看下inflate(parser, root, attachToRoot)方法：

    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {synchronized (mConstructorArgs) {...if (TAG_MERGE.equals(name)) {//有merge标签的解析rInflate(parser, root, inflaterContext, attrs, false);} else {//普通布局解析final View temp = createViewFromTag(root, name, inflaterContext, attrs);// 解析Children，最终调用rInflate方法，一层层解析rInflateChildren(parser, temp, attrs, true);//添加创建的temp到root中，root即对应上面的FrameLayout,这里就完成了整个界面的解析if (root != null && attachToRoot) {root.addView(temp, params);}if (root == null || !attachToRoot) {result = temp;}}return result;}}

我们重点看下createViewFromTag(root, name, inflaterContext, attrs)

 View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,boolean ignoreThemeAttr) {if (name.equals("view")) {name = attrs.getAttributeValue(null, "class");}...try {//先去创建ViewView view = tryCreateView(parent, name, context, attrs);if (view == null) {//创建不成功，则直接通过反射去创建View，并做缓存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;.....}}

我们先看下tryCreateView(parent, name, context, attrs)方法：

    public final View tryCreateView(@Nullable View parent, @NonNull String name,@NonNull Context context,@NonNull AttributeSet 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;}

我们看到创建View又交给了mFactory2处理【二者都是LayoutInflater类内部定义的接口。Factory2继承自Factory接口，Factory2比Factory多增加了一个onCreateView(View parent, String name, Context context, AttributeSet attrs)，该方法多了一个parent，用来存放构建出的View。】

然后会交给AppCompatDelegateImpl.createView来处理：

public View createView(View parent, final String name, @NonNull Context context,@NonNull AttributeSet attrs) {....return mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext,IS_PRE_LOLLIPOP, true, VectorEnabledTintResources.shouldBeUsed() );}

mAppCompatViewInflater.createView方法如下：

   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;View view = null;switch (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;....//匹配其他Viewdefault:             view = createView(context, name, attrs);}//没有匹配成功if (view == null && originalContext != context) {view = createViewFromTag(context, name, attrs);}if (view != null) {checkOnClickListener(view, attrs);backportAccessibilityAttributes(context, view, attrs);}return view;}

我们就看下TextView是如何创建的：

    protected AppCompatTextView createTextView(Context context, AttributeSet attrs) {return new AppCompatTextView(context, attrs);}

就是直接new了一个AppCompatTextView返回，对于没有匹配成功的View（如自定义的View）,会调用createViewFromTag方法进行创建：

 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;//表示name里不包含.如LinearLayout/RetiveLayout等，就是拼上sClassPrefixList前缀，如android.widget.LinearLayoutif (-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) {...}}

我们看下createViewByPrefix方法：

    private View createViewByPrefix(Context context, String name, String prefix)throws ClassNotFoundException, InflateException {//先从缓存map中获取，减少反射带来的开销Constructor constructor = sConstructorMap.get(name);try {//缓存中没有则通过反射根据类的全名去创建Viewif (constructor == null) {Class clazz = Class.forName(prefix != null ? (prefix + name) : name,false,context.getClassLoader()).asSubclass(View.class);constructor = clazz.getConstructor(sConstructorSignature);//存放到缓存集合中sConstructorMap.put(name, constructor);}constructor.setAccessible(true);//这里是调用两参的构造方法return constructor.newInstance(mConstructorArgs);} catch (Exception e) {return null;}}

小结

布局解析主要以下几个步骤：

1. 先会调用tryInflatePrecompiled进行解析添加到FrameLayout中【它会根据布局文件的xml预编译生成的dex文件，然后通过反射来生成对应的View，从而减少XmlPullParser解析Xml的时间。它是一个编译优化】，如果添加完成直接返回。

2. 否则调用inflate(parser, root, attachToRoot)方法进行解析加载，会调用createViewFromTag方法进行根View创建，先调用tryCreateView()方法，最终会调用到AppCompatViewInflater.createView方法，对于TextView、ImageView、Button这类View，直接调用两参的构造方法完成创建，对于LinearLayout或自定义View则通过反射进行创建，并进行了缓存处理。

3. 如果上述tryCreateView()方法创建的根View返回为null，则会直接调用createView方法使用反射进行创建，同样进行了缓存处理。

4. 根布局创建完成会调用rInflateChildren进行子View的创建，一层层创建添加到根布局View中；

5. 最后将根布局View添加到FrameLayout中，完成整个界面View的解析。

总结

通过对setContentView的源码分析，了解了View是如何添加到当前界面上的，对于插件换肤方案有很大的帮助！

结语

如果以上文章对您有一点点帮助，希望您不要吝啬的点个赞加个关注，您每一次小小的举动都是我坚持写作的不懈动力！ღ( ´･ᴗ･` )