0. 前言
LayoutInflater(布局填充器) 在安卓开发中，可以说是扮演着相当重要的角色，它让我们的 ListView 、 RecyclerView 等很容易变得多姿多彩，也正是它如此容易的操作，让它不由地多出了一份神秘…这篇博文将基于 Android 6.0 对 LayoutInflater 的源码进行一定分析。
1. 获取实例
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| protected LayoutInflater(Context context) {
mContext = context;
}
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保护权限的构造方法使得获取 LayoutInflater 并不能直接使用 new 关键字，需要使用静态方法 from(context) 来从 SystemService 取得:
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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;
}
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getSystemService(...) 的最终实现在 ContextImpl 调用的 SystemServiceRegistry 类中:
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| public static Object getSystemService(ContextImpl ctx, String name) {
ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
return fetcher != null ? fetcher.getService(ctx) : null;
}
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也就是从 SYSTEM_SERVICE_FETCHERS 这个常量 HashMap 中获取，那么这些系统服务是什么时候 put 进去的呢？答案就在类开头的静态代码块:
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static {
registerService(Context.ACCESSIBILITY_SERVICE, AccessibilityManager.class,
new CachedServiceFetcher<AccessibilityManager>() {
@Override
public AccessibilityManager createService(ContextImpl ctx) {
return AccessibilityManager.getInstance(ctx);
}});
...
registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
new CachedServiceFetcher<LayoutInflater>() {
@Override
public LayoutInflater createService(ContextImpl ctx) {
return new PhoneLayoutInflater(ctx.getOuterContext());
}
});
...
}
private static <T> void registerService(String serviceName, Class<T> serviceClass,
ServiceFetcher<T> serviceFetcher) {
SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}
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既然放进去了，那么如何取出呢？
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| return fetcher != null ? fetcher.getService(ctx) : null;
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取出服务是通过 Fetcher 取出的， Fetcher 又是什么呢？
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static abstract interface ServiceFetcher<T> {
T getService(ContextImpl ctx);
}
static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
...
@Override
public final T getService(ContextImpl ctx) {
final Object[] cache = ctx.mServiceCache;
synchronized (cache) {
Object service = cache[mCacheIndex];
if (service == null) {
service = createService(ctx);
cache[mCacheIndex] = service;
}
return (T)service;
}
}
public abstract T createService(ContextImpl ctx);
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好了，这下终于找到 LayoutInflater 的实例化入口了，就在 CachedServiceFetcher<T> 接口的实现中，也就是 registerService(...) 的第三个参数:
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| new CachedServiceFetcher<LayoutInflater>() {
@Override
public LayoutInflater createService(ContextImpl ctx) {
return new PhoneLayoutInflater(ctx.getOuterContext());
}
}
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这里 new 出了 PhoneLayoutInflater 实例，原来 PhoneLayoutInflater 才是我们使用到 LayoutInflater :
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public class PhoneLayoutInflater extends LayoutInflater {
...
}
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2. 填充视图
inflate(...) 有多个方法重载，此处以参数较为全面的，也是常用的一个进行分析:
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| 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();
}
}
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接下来就来到那个含有XmlPullParser参数的重载方法:
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| 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 {
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
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 {
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);
}
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
rInflateChildren(parser, temp, attrs, true);
if (DEBUG) {
System.out.println("-----> done inflating children");
}
if (root != null && attachToRoot) {
root.addView(temp, params);
}
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
InflateException ex = new InflateException(e.getMessage());
ex.initCause(e);
throw ex;
} catch (Exception e) {
InflateException ex = new InflateException(
parser.getPositionDescription()
+ ": " + e.getMessage());
ex.initCause(e);
throw ex;
} finally {
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
return result;
}
}
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思路就是解析 xml ，对参数进行合理的应用，然后遍历子 view ，从而带领出两条路: 一个是填充根 view group ，一个是遍历子 view ，分别是以下两个后面将进行分析的方法:
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createViewFromTag(root, name, inflaterContext, attrs);
rInflate(parser, root, inflaterContext, attrs, false);
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还有一个重点是，这段代码让我们更加理解了 `inflate(...)` 后面两个参数 `ViewGroup root` 和 `boolean attachToRoot` 的联系:
root 指的是需要将填充好的view所放在的父容器attachToRoot 指的是是否链接到root父容器，如果为true，则调用父容器的addView()方法，否则应用父容器的LayoutParams
另外这里补充几个标签的知识:
<merge/> 、<include/> 和 <View Stub/> 标签的使用与区别
<merge/> 将视图组合，减少UI层次，但只能用于根节点，比如include一个layout，这个layout中就可以用merge作为根节点，防止include后造成多层嵌套<include/> 包含一个layout布局，减少代码重复<View Stub/> 用于需要时才加载的布局，比如错误信息的展示
2.1 填充根布局
填充根布局调用 createViewFromTag(...) 来获取 View
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| private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);
}
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其实调用的是含包访问权限的其重载方法:
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View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
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();
}
if (name.equals(TAG_1995)) {
return new BlinkLayout(context, attrs);
}
try {
View view;
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);
}
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
throw e;
} catch (ClassNotFoundException e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;
}
}
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经过一番折腾，原来真正的创建 View 在 createView(name, null, attrs) 方法中:
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| public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructor<? extends View> constructor = sConstructorMap.get(name);
Class<? extends View> clazz = null;
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
if (constructor == null) {
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
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 {
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);
}
}
}
Object[] args = mConstructorArgs;
args[1] = attrs;
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
return view;
} catch (NoSuchMethodException e) {
...
}
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至此，根布局的填充算是完成了。
2.2 遍历子布局
接下来便是遍历子布局:
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void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
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)) {
parseRequestFocus(parser, parent);
} 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 (finishInflate) {
parent.onFinishInflate();
}
}
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遍历子布局就是一个不断递归的过程，递归完毕parent就被充满了内容，这时返回到 inflate() 方法中，各种绚丽花哨的效果就被填充完毕，就可以随时进行展示了。
总结
LayoutInflater 是一个神奇有力的工具，将它用好，相信你的App一定会更加绚丽多姿，通过此文，希望你对 LayoutInflater 有更近一步的了解，对安卓源码的精妙设计也更感兴趣！
