一、前言

近来在开发时，经常使用到inflate方法加载视图布局，并且回调onFinishInflate方法进行一些初始化的操作。

顿时心血来潮，想要探究一下Layoutinflater的原理，怎么就把XML格式的布局文件加载为布局的实例对象，对于一些特殊标签，例如<merge>，<include>如何处理的，所以带着以下👇问题探究一下：

1. LayoutInflater源码解析

   • view的加载流程

   • 特殊标签<merge>，<include>的处理

   • view实际创建过程，从xml定义到内存的视图实例（onCreateView， createView）

2. onFinishInflate调用机制和时机

3. inflate方法中的root和attachRoot参数的作用

二、LayoutInflater源码解析

通常我们动态加载一个布局文件是通过LayoutInflater的inflate方法来完成，其实在Activity中，setContentView方法底层也是调用的该方法完成。相关代码在PhoneWindow类：

 //PhoneWindow.java
 @Override
 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();
 } 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;
 }

好了话不多说，进入正题

2.1 View加载流程

在Android中，我们只需写好各种布局文件，使用Inflate方法去加载就好，通常使用LayoutInflater.from()方法获取LayoutInflater实例，然后调研inflate方法加载布局，它有如下重载方法

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root)
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
public View inflate(XmlPullParser parser, @Nullable ViewGroup root)
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)</pre>

这里root和attachToRoot参数作用下面分析源码时详细说明。
既然是源码分析，也就不得不摆出大段大段代码，已将无用代码省略掉，尽量让大家看起显得简洁。inflate方法代码如下，相关说明注释在代码中：

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 {
 // Look for the root node.
 int type;
 while ((type = parser.next()) != XmlPullParser.START_TAG &&
 type != XmlPullParser.END_DOCUMENT) {
 // Empty
 }
 //没有开始标签，说明布局文件语法错误，解析不出来
 if (type != XmlPullParser.START_TAG) {
 throw new InflateException(parser.getPositionDescription()
 + ": No start tag found!");
 }
​
 final String name = parser.getName();
 ......
 ①// TAG_MERGE就是merge标签，条件体是merge的加载
 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
 ②final View temp = createViewFromTag(root, name, inflaterContext, attrs);
 ViewGroup.LayoutParams params = null;
 if (root != null) {
 ......
 // Create layout params that match root, if supplied
 params = root.generateLayoutParams(attrs);
 ③//这里也能看出，只有root不为null，attachToRoot才起作用
 if (!attachToRoot) {
 // Set the layout params for temp if we are not
 // attaching. (If we are, we use addView, below)
 temp.setLayoutParams(params);
 }
 }
 ......
 ④// Inflate all children under temp against its context.
 rInflateChildren(parser, temp, attrs, true);
 ......
 // 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) {
 ......
 }
 return result;
 }
 }

加载时，首先判断是否是<merge>布局，是的话走到该布局的加载流程，否则先加载根视图，然后递归加载根视图下所有子视图。②处，createViewFromTag方法就是讲布局加载为实例对象的方法，此处暂不深究内部实现，下面有具体分析。加载完根视图后，代码走到④处，rInflateChildren开始递归加载所有子视图，进入方法内部：

final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
 boolean finishInflate) throws XmlPullParserException, IOException {
 rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
 }

可见，视图递归加载是通过rInflate方法来完成。

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)) {①
 if (parser.getDepth() == 0) {
 throw new InflateException("<include /> cannot be the root element");
 }
 parseInclude(parser, context, parent, attrs);
 } else if (TAG_MERGE.equals(name)) {②
 throw new InflateException("<merge /> must be the root element");
 } else {③
 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();④
 }
}

在循环体内，会根据标签名称来加载对应视图，由①知<include>不能作为根元素，而 <merge>必须为根节点。③处就是普通视图的加载逻辑，分别获取对应View的实例对象，父View的LayoutParams，然后继续递归加载子View。

<include>布局的加载在parseInclude方法中，只能在ViewGroup中使用该标签且必须指定layout属性，否则会抛InflateException。除了layout属性，加载<include>时，还会对id、visibility等属性做出处理，然后就是调用rInflateChildren方法去递归加载子布局。

2.2 View的创建

从XML中定义的View到内存中储存的对应View对象，是通过createViewFromTag方法来完成。具体代码👇：

View 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('.')) {
 view = onCreateView(context, parent, name, attrs);
 } else {
 view = createView(context, name, null, attrs);
 }
 } finally {
 mConstructorArgs[0] = lastContext;
 }
 }
 return view;
 } catch (InflateException e) {
 ......
 }

首先是添加Theme资源（若有），然后进入tryCreateView方法。

public final View tryCreateView(@Nullable View parent, @NonNull String name,
 @NonNull Context context,
 @NonNull AttributeSet attrs) {
 if (name.equals(TAG_1995)) {①
 // Let's party like it's 1995!
 return new BlinkLayout(context, attrs);
 }
 View view;
 if (mFactory2 != null) {②
 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;
 }

①处是1个彩蛋，blink意为眨眼、闪烁，相关渊源感兴趣的可以自己查查。从②处起，增加了几个接口去拦截创建视图的流程，开发者可以自定义Factory2和Factory接口方法onCreateView 去拦截加载流程，针对不同业务场景实现想要的效果。有个例子，当在一些特殊纪念日（例如9.18），可能需要APP界面显示灰色。我们不可能去替换所有图片，然后改变各个View的颜色。但我们知道，Activity的根布局是content view，它是FrameLayout。我们只用写一套显示增加了灰度效果的FrameLayout，然后在加载时重写Factory2和Factory接口方法onCreateView 去拦截，就能实现相应的效果。具体例子可看大佬的文章：App 黑白化实现探索，有一行代码实现的方案吗？
正常情况下，没有拦截View创建过程，会进入到

...... 
 if (view == null) {
 final Object lastContext = mConstructorArgs[0];
 mConstructorArgs[0] = context;
 try {
 if (-1 == name.indexOf('.')) {①
 view = onCreateView(context, parent, name, attrs);②
 } else {
 view = createView(context, name, null, attrs);③
 }
 } finally {
 mConstructorArgs[0] = lastContext;
 }
 }
 return view;
 ......

①处判断是否有「.」号，通常是自定义View（自定义 View的形式为<package_name>.customViewName），自定义View直接调用③处方法createView，代码如下👇(部分代码已省略)：

public 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
 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);
 }
 }
 constructor = clazz.getConstructor(mConstructorSignature);
 constructor.setAccessible(true);
 sConstructorMap.put(name, constructor);
 } else {
 // If we have a filter, apply it to cached constructor
 if (mFilter != null) {
 // Have we seen this name before?
 Boolean allowedState = mFilterMap.get(name);
 if (allowedState == null) {
 // New class -- remember whether it is allowed
 clazz = 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 {
 final View view = constructor.newInstance(args);①
 if (view instanceof ViewStub) {②
 // Use the same context when inflating ViewStub later.
 final ViewStub viewStub = (ViewStub) view;
 viewStub.setLayoutInflater(cloneInContext((Context) args[0]));③
 }
 return view;
 } finally {
 mConstructorArgs[0] = lastContext;
 }
 } catch (NoSuchMethodException e) {
 ......
 }
 }

LayoutInflater对象。

至此，inflate方法加载View的流程和相关代码已梳理一遍。

总结一下：

• View的加载首先加载根布局，然后递归加载各个子View。
• 对于<merge>，<include>，<ViewStub>等标签有特殊的处理。
• View对象的创建实际通过反射方式，调用该View的构造方法完成。

三、onFinishInflate调用机制和时机

void rInflate(XmlPullParser parser, View parent, Context context,
 AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
 ......
 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)) {
 ......
 } 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();
 }
 }

加载完视图后finishInflate为true，调用parent.onFinishInflate()。例如我们有如下布局文件：

<LinearLayout>
 <TextView>
 ......
 </TextView>
</LinearLayout></pre>

首先加载根节点<LinearLayout>，然后进入rInflate，进入循环体，遇到<TextView>开始解析，创建TextView对象并添加到父视图中（LinearLayout）。然后递归创建子视图，rInflateChildren会把finishInflate参数设为true，然后还是会走到rInflate中，由于是结束标签，不会进入循环体。最后会调用onFinishInflate，所以如果一个View重写了onFinishInflate方法，那么在该View创建完成后便会调用该方法。

写个demo来验证一下，自定义一个简单View布局，重写onFinishInflate，打印一句话。

public class customTextView extends AppCompatTextView {
 public customTextView(Context context) {
 super(context);
 }
​
 public customTextView(Context context, AttributeSet attrs) {
 super(context, attrs);
 }
​
 public customTextView(Context context, AttributeSet attrs, int defStyleAttr) {
 super(context, attrs, defStyleAttr);
 }
​
 @Override
 protected void onFinishInflate() {
 super.onFinishInflate();
 Log.d(InflateTestView.TAG, "customTextView onFinishInflate");
 }
​
 @Override
 protected void onDraw(Canvas canvas) {
 super.onDraw(canvas);
 }
}

public class InflateTestView extends LinearLayout {
 public static final String TAG = "InflateTestView";
 public InflateTestView(Context context) {
 this(context, null);
 }
​
 public InflateTestView(Context context, @Nullable AttributeSet attrs) {
 this(context, attrs, 0);
 }
​
 public InflateTestView(Context context, @Nullable AttributeSet attrs, int defStyleAttr)    {
 super(context, attrs, defStyleAttr);
 }

 @Override
 protected void onFinishInflate() {
 super.onFinishInflate();
 Log.d(TAG, "onFinishInflate");
 }
​
}

主界面的布局文件：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 xmlns:tools="http://schemas.android.com/tools"
 android:id="@+id/main_layout"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical"
 tools:context=".MainActivity">
 <Button
 android:id="@+id/button"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="Inflate测试"
 app:layout_constraintBottom_toBottomOf="parent"
 app:layout_constraintEnd_toEndOf="parent"
 app:layout_constraintHorizontal_bias="0.3"
 app:layout_constraintStart_toStartOf="parent"
 app:layout_constraintTop_toTopOf="parent"
 app:layout_constraintVertical_bias="0.499" />
</LinearLayout>

inflateView布局文件：

<com.example.servicedemo.InflateTestView
 android:id="@+id/inflate_view"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:orientation="vertical"
 android:background="@android:color/holo_orange_light"
 xmlns:android="http://schemas.android.com/apk/res/android">
 <com.example.servicedemo.customTextView
 android:text="customTextView in inflateView"
 android:textSize="20sp"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"/>
</com.example.servicedemo.InflateTestView></pre>

点击按钮，动态加载InflateTestView视图到Activity。

......
 View inflateTestView =  LayoutInflater.from(this).inflate(R.layout.inflate_view, null);
 LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
 mainLayout.addView(inflateTestView, params);
......

然后看下打印信息

image-20201101222600134.png

customTextView和InflateTestView都重写了onFinishInflate方法，所以在加载布局文件时，创建对应View对象后就会调用onFinishInflate。

总结：只要重写了onFinishInflate，那么通过inflate加载布局时，每个View对象创建完成后都会调用onFinishInflate

四、root和attachRoot参数的作用

关于root和attachRoot参数的作用，还是要到代码中去找，在inflate方法中：

...... 
// Temp is the root view that was found in the xml
 final View temp = createViewFromTag(root, name, inflaterContext, attrs);
 ViewGroup.LayoutParams params = null;
 if (root != null) {
 ......
 // Create layout params that match root, if supplied
 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);
 }
 }
 // Inflate all children under temp against its context.
 rInflateChildren(parser, temp, attrs, true);
 // 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;
 }
 }
......

如果root参数不为null且attachToRoot参数为true，就把当前视图添加到root所代表的View中。
temp是root view对象，如果root参数不为null，那么会生成该View的LayoutParams对象（params）；此时，如果attachToRoot参数为false，那么会将params设置为根视图的layout params。