LayoutInflater在Android开发中有大量的使用场景，例如：
那么你是否了解LayoutInflater在创建View的时候，是如何处理View的属性吗？
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;
}
//根据R.layout.xxx的资源Id，创建出可以读取xml文件的解析器parser
XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final Context inflaterContext = mContext;
//通过parser构造了AttributeSet对象，后者就是View的构造函数中返回的第二个参数，在自定义View时，我们通过他来
//找到我们在xml中给View定义的所有属性，例如宽高，visibility，padding，和其他自定义属性。
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;
}
}这里注意一点：AttributeSet对象是从XmlPullParser中构造来的，这个AttributeSet就是View类的构造函数的第二个参数。包含所有在xml中定义的View的属性。
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
//...
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
//...
}注意这里将atrrs传递，传递到通过attrs创建，并返回了一个View temp。
//Creates a view from a tag name using the supplied attribute set.
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) {
//提取theme，也是使用的TypeArray来提取的。
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 {
//给LayoutInflater创建View提供一个Hook，待会分析
View view = tryCreateView(parent, name, context, attrs);
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
//创建android源码中的View，即那些在xml中不需要包名就可以定义并使用的View。
view = onCreateView(context, parent, name, attrs);
} else {
//创建非android源码中的View，如自定义View或者第三方库的View。
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;
}
}可以看到创建View的两个语句：
通过这个if的判断，我们大致可以猜到这个是用来判断加载的是源码View还是非源码的View。
if (-1 == name.indexOf('.')) {
//创建android源码中的View，即那些在xml中不需要包名就可以定义并使用的View。
view = onCreateView(context, parent, name, attrs);
} else {
//创建非android源码中的View，如自定义View或者第三方库的View。
view = createView(context, name, null, attrs);
}这两个方法内部都调用的是一个方法，我们看下两个方法是如何调用的，第一个：
public View onCreateView(@NonNull Context viewContext, @Nullable View parent,
@NonNull String name, @Nullable AttributeSet attrs)
throws ClassNotFoundException {
return onCreateView(parent, name, attrs);
}
//-----往下调用
protected View onCreateView(View parent, String name, AttributeSet attrs)
throws ClassNotFoundException {
return onCreateView(name, attrs);
}
//-----往下调用
protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException {
//注意这里，将参数为String prefix的位置赋值成了：android.view，即为源码的view拼接上前缀。为的是后续能
//正确通过反射调用到该View类的构造方法。需要全路径名。
return createView(name, "android.view.", attrs);
}
//-----往下调用
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Context context = (Context) mConstructorArgs[0];
if (context == null) {
context = mContext;
}
return createView(context, name, prefix, attrs);
}第二个方法的内部调用和第一个方法的内部调用，最终都调用到了方法：
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);
}
}
}
//上面的代码做的事情是：判空和对反射构造器做缓存，方便下次再使用。
//主要看下面调用的方法：
//为View类的反射方法赋值参数，这里可以看到赋值了两个参数：Context和AttributeSet
Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = viewContext;
Object[] args = mConstructorArgs;
args[1] = attrs;
try {
//传入参数，调用反射并创建View对象
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;
}
//后续为异常处理
} 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);
}
}上述源码看50~70行的部分。
可以看到，通过反射，带上Context和AttributeSet参数，创建了View对象并返回。这里的AttributeSet对象则是LayoutInflater.inflate方法中通过xml解析器创建的。
因此一个View从xml文件解析并生成View对象到这里就结束了。
至于后面的View添加到ViewTree，View对提取AttributeSet中的属性（即xml中定义的属性），并根据业务逻辑重写自定义View三大方法，则是自定义View的内容，在此不做拓展。
子View在xml中定义自己的View的属性。
在LayoutInflator中，子View的属性被从xml中读取到内存中存为key-value。
父View根据子view的key-value属性决定所有的子View的LayoutParams的生成规则。
例如必要的宽和高，以及其他该ViewGroup支持的自定义的属性。
现在来分析在RecyclerView中调用LayoutInflater创建Item View的场景。
默认的LayoutParams？
默认的LayoutParams？
结论：
图：