一.inflate的三参数重载方法else里面逻辑
我们先看到setContentView里面的inflate的调用链：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
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);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
重点看三参数的方法：
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);
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("<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
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
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.
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;
}
}在这个方法里面，先拿到我们xml的根布局对象，如果是merge的话，直接取merge的子view添加。我们先看一下不是merge会怎么样。
创建根布局View
先执行createViewFromTag方法，这个传入的name是根布局的name，这个方法就是创建布局的根View：
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) {
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;
}
}重点看到这段代码：
if (-1 == name.indexOf('.')) {
view = onCreateView(context, parent, name, attrs);
} else {
view = createView(context, name, null, attrs);
}这里分为两种情况了，一种是有点，一种是全路径没有点的。
全路径名称有点(自定义的view)
先看看看有点是什么情况：
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);
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;
}
}
......
}
可以看到有点的直接反射创建出这个view了。
全路径名称没有点(android系统的view)
再来看看没有点的情况：
public View onCreateView(@NonNull Context viewContext, @Nullable View parent,
@NonNull String name, @Nullable AttributeSet attrs)
throws ClassNotFoundException {
return onCreateView(parent, name, attrs);
}
protected View onCreateView(View parent, String name, AttributeSet attrs)
throws ClassNotFoundException {
return onCreateView(name, attrs);
}
实际上LayoutInflater是一个抽象类，它的实现类是PhoneLayoutInflater：
public class PhoneLayoutInflater extends LayoutInflater {
private static final String[] sClassPrefixList = {
"android.widget.",
"android.webkit.",
"android.app."
};
/**
* Instead of instantiating directly, you should retrieve an instance
* through {@link Context#getSystemService}
*
* @param context The Context in which in which to find resources and other
* application-specific things.
*
* @see Context#getSystemService
*/
public PhoneLayoutInflater(Context context) {
super(context);
}
protected PhoneLayoutInflater(LayoutInflater original, Context newContext) {
super(original, newContext);
}
/** Override onCreateView to instantiate names that correspond to the
widgets known to the Widget factory. If we don't find a match,
call through to our super class.
*/
@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);
}
public LayoutInflater cloneInContext(Context newContext) {
return new PhoneLayoutInflater(this, newContext);
}
}所以在LayoutInflater里面调用的两参方法最后会执行PhoneLayoutInflater的两参方法：
@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);
}这个方法先进行三个参数的处理，如果这三个参数对不上，会执行super的两参方法，再进行一个参数的处理。实际上是进行了四个参数的处理：
private static final String[] sClassPrefixList = {
"android.widget.",
"android.webkit.",
"android.app."
};
protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException {
return createView(name, "android.view.", attrs);
}从没有点的情况来看，先把全路径给补全了，然后再和有点的情况一样，反射创建出View。
总结
其实创建xml的根布局的时候，如果是全路径名称有点的话，就是我们自己创建的view，比如：
androidx.constraintlayout.widget.ConstraintLayout这个是谷歌自定义的根布局，直接反射创建view就行。
如果在布局文件里面根布局没有点：
LinearLayout这就是android系统自带的根布局，我们无法反射创建它，所以补全全路径后再反射创建就行了。
创建根布局包裹的子View
在inflate的三参重载方法里面，我们通过：
final View temp = createViewFromTag(root, name, inflaterContext, attrs);创建出了根布局view。接下来我们执行下面代码创建子view：
rInflateChildren(parser, temp, attrs, true); 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("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> 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();
}
}从这个函数我们可以看到循环解析我们的xml文件，如果是include不能定义在xml的根里面，merge又只能定义在xml的根里面。
这几种情况都除外的话，接下来又是进入和xml根view一样的解析流程了。
二.inflate函数几个参数的作用
根布局是merge
主要文件
R.layout.activity_main：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/ll"
tools:context=".MainActivity2">
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button" />
</LinearLayout>R.layout.merge_layout：
<merge xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button" />
</merge>我在mainActivty里面通过LayoutInflater把R.layout.merge_layout加载到R.layout.activity_main里面。
第一种方式：
//第一种
LayoutInflater.from(this).inflate(R.layout.merge_layout, layout, true);
可以看到第一种方式完美加入。
第二种，第三种方式：
//第二种
LayoutInflater.from(this).inflate(R.layout.merge_layout, layout, false);
//第三种
LayoutInflater.from(this).inflate(R.layout.merge_layout, null, true);
第二种和第三种方式会报错，这是为什么呢？
在inflate的三参构造方法里面：
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);
}如果xml的根布局是merge的话，root为空和attachToRoot是false都会报错。
根布局不是merge
涉及到inflate的参数的几处地方分别是在下面：
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// 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);
}
}if (root != null && attachToRoot) {
root.addView(temp, params);
}if (root == null || !attachToRoot) {
result = temp;
}这里的root就是我们传入的root，temp是xml的根布局。
R.layout.inflater_activity：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/ll">
</LinearLayout>R.layout.inflater_layout：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="200dp"
android:layout_height="200dp"
android:background="@color/teal_200">
<Button
android:layout_gravity="center"
android:background="@color/cardview_dark_background"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
</LinearLayout>第一种方式：
public class InflaterActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
supportRequestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.inflater_activity);
LinearLayout layout = findViewById(R.id.ll);
//第一种方式成功添加
LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, true);
}
第一种方式成功添加。
第二种方式：
public class InflaterActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
supportRequestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.inflater_activity);
LinearLayout layout = findViewById(R.id.ll);
//第一种方式成功添加
//LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, true);
//第二种方式,报错The specified child already has a parent. You must call removeView() on the child's parent first.
View view = LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, true);
layout.addView(view);
}
}第二种方式会报错，因为root不为空，attachToRoot为true的情况下，会执行：
if (root != null && attachToRoot) {
root.addView(temp, params);
}第三种方式：
public class InflaterActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
supportRequestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.inflater_activity);
LinearLayout layout = findViewById(R.id.ll);
//第一种方式成功添加
//LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, true);
//第二种方式,报错The specified child already has a parent. You must call removeView() on the child's parent first.
//View view = LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, true);
//layout.addView(view);
//第三种方式，布局成功
//想让R.layout.inflater_layout的根节点的属性(宽高)有效，又不想让其处于某一个容器里面
View view = LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, false);
layout.addView(view);
}
}
第三种方式布局有效，会执行：
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);
}
}第四种方式：
public class InflaterActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
supportRequestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.inflater_activity);
LinearLayout layout = findViewById(R.id.ll);
//第一种方式成功添加
//LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, true);
//第二种方式,报错The specified child already has a parent. You must call removeView() on the child's parent first.
//View view = LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, true);
//layout.addView(view);
//第三种方式，布局成功
//想让R.layout.inflater_layout的根节点的属性(宽高)有效，又不想让其处于某一个容器里面
//View view = LayoutInflater.from(this).inflate(R.layout.inflater_layout, layout, false);
//layout.addView(view);
//第四种方式，布局成功
//root为空的时候，不管第三个参数是什么，效果都是一样的
//R.layout.inflater_layout根布局的宽高失效，只是包裹子View
//但是子View（button）有效，因为Button是在容器下的
View view = LayoutInflater.from(this).inflate(R.layout.inflater_layout, null, false);
layout.addView(view);
}
}
总结：
当xml的根布局是merge的话，添加到root里面。三个参数必须都填且最后一个参数是true。
当xml的根布局不是merge的话：
当root为空时，不管第三个参数是true还是false，xml的根布局的宽高都无效，且需要手动addView到root里面；
当root不为空时，第三个参数为true的话，xml根布局宽高有效，且自动帮我们添加到root里面；第三个参数为false的话，xml根布局宽高有效，需要我们手动addView到root里面。
三.标签特性
merge
1.优化布局，使用merge减少一次循环解析
2.必须用作更布局
include
1.如果include的标签设置了id，findViewById去查找通过include标签映入的xml的根布局的id是找不到的，会被include的标签的id给覆盖
2.不能用作根布局
ViewStub
1.与include差不多
2.viewStub构造函数里面会隐藏viewStub，具有懒加载作用
（在调用inflate()或者setVisibility()时，ViewStub才会加载真正的布局资源并在控件层级结构中替换为真正的控件，同时ViewStub从控件层级结构中移除，这是“懒加载”的核心思想）
