1、LayoutInfalter的作用
查看源码注释，可以知道其作用为将布局XML文件实例化为其相应的View对象。
Instantiates a layout XML file into its corresponding View objects.
并且可以使用以下三种方式获取布局解析服务，其实最后都是使用的第三种：
Activity.getLayoutInflater();
LayoutInflater.from(context);
Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
2、LayoutInfalter的使用
ViewDrawActivity.java
public class ViewDrawActivity extends AppCompatActivity {
private LinearLayout mLinearLayoutMain;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_view_draw);
mLinearLayoutMain = findViewById(R.id.ll_main);
//获取XML对应的View对象
View viewRootNull = LayoutInflater.from(this).inflate(R.layout.layout_button, null);
//将View对象添加到其他布局中
mLinearLayoutMain.addView(viewRootNull);
}
}
activity_view_draw.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/ll_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
</LinearLayout>
layout_button.xml
<?xml version="1.0" encoding="utf-8"?>
<Button xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@string/app_name">
</Button>
运行结果：
3、LayoutInfalter的源码解析
3.1、尝试获取已经提前编译好的View对象
LayoutInfalter.java
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
//使用反射尝试获取已经提前编译好的View对象
View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
if (view != null) {
return view;
}
//根据资源ID获取布局的XML资源解析器
XmlResourceParser parser = res.getLayout(resource);
try {
//使用XML资源解析器对XML进行解析
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
先直接使用反射尝试获取已经提前编译好的View对象，如果获取到直接返回View对象，如果获取不到再根据资源ID获取布局的XML资源解析器，然使用XML资源解析器对XML进行解析。
3.2、将XML解析并实例化后的View对象进行添加返回操作
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
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 (TAG_MERGE.equals(name)) {
//如果布局根标签为merge，那么要求传入的root不能为空，且attachToRoot需为true
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid ViewGroup root and attachToRoot=true");
}
//继续解析XML
rInflate(parser, root, inflaterContext, attrs, false);
} else {
//根据XML中的根标签生成根View对象
// 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) {
//如果布局提供参数，则创建匹配root的布局参数
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
//如果attachToRoot==false，则当使用addView的时候再设置布局参数
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
//解析子布局
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
//如果root!=null，且attachToRoot==true，则自动将布局添加到root中，并设置布局参数
// 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.
//当root==null或者attachToRoot==false，则直接返回View对象
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;
}
return result;
}
}
1、布局根标签为merge
首先判断根标签是否是merge标签，如果布局根标签为merge，那么要求传入的root不能为空，且attachToRoot需为true，然后继续解析XML，也就是rInflate()方法，否则会报以下异常：
Caused by: android.view.InflateException: Binary XML file line #2: <merge /> can be used only with a valid ViewGroup root and attachToRoot=true
如果布局根标签不是merge，首先根据XML中的根标签生成根View对象，也就是createViewFromTag()方法，然后解析子布局，也就是rInflateChildren()方法。然后分以下几种情况进行View对象的添加操作：
2、root==null
直接返回生成的View对象，需要使用addView添加到其他布局中，添加的时候会忽略布局最外层的参数。
也就是说，修改了Button布局为固定大小，最后的加载结果还是不变：layout_button.xml
<?xml version="1.0" encoding="utf-8"?>
<Button xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="360dp"
android:layout_height="360dp"
android:text="@string/app_name">
</Button>
3、attachToRoot==true
如果root!=null，默认情况下attachToRoot==true，那么给加载的布局文件指定root为父布局，并且直接添加到root布局中，添加时不忽略布局最外层的参数，然后返回View对象。
layout_width和layout_height是用于设置View在布局中的大小的，但如果还没有依附在父布局中，也就是在Root为null的情况下，这两个的设置将会无效。平时在Activity中指定布局文件的时候，最外层的那个布局是可以指定大小的，layout_width和layout_height都是有作用的，这主要是因为，在setContentView()方法中，Android是使用root!=null和attachToRoot==true的方式进行添加布局的，自动将布局添加到FrameLayout布局contentParent中，所以layout_width和layout_height属性才会有效果。
AppCompatDelegateImpl.java
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mAppCompatWindowCallback.getWrapped().onContentChanged();
}
我们修改添加Button的方式为root!=null，attachToRoot==true，就可以使得修改Button布局为固定大小产生效果：ViewDrawActivity.java
public class ViewDrawActivity extends AppCompatActivity {
private LinearLayout mLinearLayoutMain;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_view_draw);
mLinearLayoutMain = findViewById(R.id.ll_main);
//解析获取View对象的同时，添加View到root布局中
LayoutInflater.from(this).inflate(R.layout.layout_button, mLinearLayoutMain);
}
}
4、attachToRoot==false
如果root!=null，且如果attachToRoot==false，那么给加载的布局文件指定root为父布局，但无法直接添加到root布局中，需要使用addView添加到其他布局中，但不一定非要添加到所传的root布局中，添加时不忽略布局最外层的参数，然后返回View对象。
3.3、使用XML资源解析器对XML进行解析
在3.2中想要获取XML解析并实例化的View对象，就必须先进行XML解析得到AttributeSet， 也就是rInflate()、rInflateChildren()等方法；
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;
//遍历节点
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)) {
//request_foucs
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
//tag
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
//include
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
//merge
throw new InflateException("<merge /> must be the root element");
} else {
//生成View对象，然后继续循环解析子布局
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();
}
}
然后使用反射根据tag对解析后的AttributeSet进行实例化得到View对象，也就是createViewFromTag()、createView()等方法。
@Nullable
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 {
//创建构造器
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 {
//使用反射创建View的实例对象
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
//如果是ViewStub类型，当稍后解析ViewStub时使用同个上下文解析
// 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;
}
} catch (NoSuchMethodException e) {
final InflateException ie = new InflateException(
getParserStateDescription(viewContext, attrs)
+ ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (ClassCastException e) {
// If loaded class is not a View subclass
final InflateException ie = new InflateException(
getParserStateDescription(viewContext, attrs)
+ ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (ClassNotFoundException e) {
// If loadClass fails, we should propagate the exception.
throw e;
} catch (Exception e) {
final InflateException ie = new InflateException(
getParserStateDescription(viewContext, attrs) + ": Error inflating class "
+ (clazz == null ? "<unknown>" : clazz.getName()), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
附上LayoutInflater脑图：
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