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：
- 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）
- onFinishInflate调用机制和时机
- 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)
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) {
......
}
}
可以看出，View的创建是通过反射，调用View的构造方法完成的（①）。对于ViewStub，会为其设置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
在自定义View时，通常在onFinishInflate回调中进行一些初识化操作或者判断View是否创建完成，该方法的调用位置在rInflate方法中：
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>
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>
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
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;
}
}
......
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。
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。
今天的文章Android源码解析之LayoutInflater分享到此就结束了，感谢您的阅读。
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