我们常用LayoutInflater加载xml布局文件，adapter中没少写，下面看看源码流程。
LayoutInflater.from()
public static LayoutInflater from(Context context) {
LayoutInflater LayoutInflater =
(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (LayoutInflater == null) {
throw new AssertionError("LayoutInflater not found.");
}
return LayoutInflater;
}
context.getSystemService(LAYOUT_INFLATER_SERVICE)，LayoutInflater本质上是一个系统服务。
LayoutInflater.inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
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) + ")");
}
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
adapter中加载布局，root-->parent，attachToRoot-->false。
XmlResourceParser继承自XmlPullParser-->解析xml的工具类。
LayoutInflater.inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
...
View result = root;
try {
...
//处理merge标签，没有父View，或者attachToRoot为false没有意义，抛出异常
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 {
//创建xml布局中的顶层View
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
temp.setLayoutParams(params);
}
}
//创建子View
rInflateChildren(parser, temp, attrs, true);
//父View不为null且attachToRoot为true-->xml布局需要添加到父View
if (root != null && attachToRoot) {
root.addView(temp, params);
}
//父View为null或attachToRoot为false-->xml布局无父View需要关联
if (root == null || !attachToRoot) {
result = temp;
}
}
}
...
return result;
}
}
inflate()最终有两个流程分支
- root != null && attachToRoot
将xml布局View树当做子View添加到root，并返回root - root == null || !attachToRoot
直接返回xml布局View树
inflate()方法调用链，LayoutInflater.createViewFromTag()
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);
}
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
//view标签
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
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();
}
//官方彩蛋
if (name.equals(TAG_1995)) {
return new BlinkLayout(context, attrs);
}
try {
View view;
//各种Factory都可以对View进行扩展
//可自行设置Factory，拦截创建View过程。
if (mFactory2 != null) {
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);
}
//无Factory直接生成View
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
//名字包含'.'是自定义View，否则为系统原生View
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
}
}
LayoutInflater.onCreateView()
protected View onCreateView(View parent, String name, AttributeSet attrs)
throws ClassNotFoundException {
return onCreateView(name, attrs);
}
protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException {
return createView(name, "android.view.", attrs);
}
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
//判断view的构造器是否已缓存
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> clazz = null;
try {
//构造器不存在，反射获取构造器并缓存到map
if (constructor == null) {
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
//过滤器
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
} else {
//构造器已存在，loadclass加载view的class对象
if (mFilter != null) {
Boolean allowedState = mFilterMap.get(name);
if (allowedState == null) {
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
mFilterMap.put(name, allowed);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
} else if (allowedState.equals(Boolean.FALSE)) {
failNotAllowed(name, prefix, attrs);
}
}
}
...
//构造器创建view对象
final View view = constructor.newInstance(args);
...
return view;
}
}
LayoutInflater.rInflateChildren(...)
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
LayoutInflater.rInflate(...)
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)) {
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);
}
}
...
}
rInflate()对各种标签进行处理，调用createViewFromTag()生成View实例。如果View是ViewGroup则继续调用rInflateChildren()生成子View实例并添加到View，直到View树全部实例化。
最后捋一捋流程：首先通过LayoutInflater.from()获取系统服务LAYOUT_INFLATER_SERVICE--->LayoutInflater实例；调用LayoutInflater.inflate()解析xml文件，创建xml中顶层View；调用createViewFromTag()->createView()反射生成View实例，createView()会缓存View构造器，优化反射的频繁调用；ViewGroup继续调用rInflateChildren()->createView()创建子View并添加到父View。递归调用，直到View树中所有View创建完毕。
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