浅析LayoutInflater
LayoutInflater用来加载一个布局，首先使用下面的方法获取LayoutInflater对象
LayoutInflater LayoutInflater =(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);这里的context通常是Activity，跟进Activity
@Override
public Object getSystemService(@ServiceName @NonNull String name) {
if (getBaseContext() == null) {
throw new IllegalStateException(
"System services not available to Activities before onCreate()");
}
if (WINDOW_SERVICE.equals(name)) {
return mWindowManager;
} else if (SEARCH_SERVICE.equals(name)) {
ensureSearchManager();
return mSearchManager;
}
return super.getSystemService(name);
}执行super.getSystemService(name)进入ContextThemeWrapper的getSystemService(String name)
@Override
public Object getSystemService(String name) {
if (LAYOUT_INFLATER_SERVICE.equals(name)) {
if (mInflater == null) {
mInflater = LayoutInflater.from(getBaseContext()).cloneInContext(this);
}
return mInflater;
}
return getBaseContext().getSystemService(name);
}在这里会拿到getBaseContext()然后再次调用LayoutInflater.from(Context context)，这里getBaseContext()是什么呢，点进去
public Context getBaseContext() {
return mBase;
}
返回的是mBase，mBase是在attachBaseContext(Context base)中赋值的
protected void attachBaseContext(Context base) {
if (mBase != null) {
throw new IllegalStateException("Base context already set");
}
mBase = base;
}Activity在attach中调用了attachBaseContext(context);
final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config, String referrer, IVoiceInteractor voiceInteractor,
Window window) {
attachBaseContext(context);
// 省略代码
}Activity的attach方法在Activity创建之后被调用
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// 省略代码
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
// 省略代码
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}
try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
// 省略代码
if (activity != null) {
Context appContext = createBaseContextForActivity(r, activity);
// 省略代码
// 调用Activity的attach()，传入appContext
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window);
// 省略代码
}
// 省略代码
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
// 省略代码
}
return activity;
}调用Activity的时候传入的appContext由createBaseContextForActivity(r, activity)得到
private Context createBaseContextForActivity(ActivityClientRecord r, final Activity activity) {
// 省略代码
ContextImpl appContext = ContextImpl.createActivityContext(
this, r.packageInfo, r.token, displayId, r.overrideConfig);
appContext.setOuterContext(activity);
Context baseContext = appContext;
final DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();
// For debugging purposes, if the activity's package name contains the value of
// the "debug.use-second-display" system property as a substring, then show
// its content on a secondary display if there is one.
String pkgName = SystemProperties.get("debug.second-display.pkg");
if (pkgName != null && !pkgName.isEmpty()
&& r.packageInfo.mPackageName.contains(pkgName)) {
for (int id : dm.getDisplayIds()) {
if (id != Display.DEFAULT_DISPLAY) {
Display display =
dm.getCompatibleDisplay(id, appContext.getDisplayAdjustments(id));
// createDisplayContext的返回值依然是一个ContextImpl
baseContext = appContext.createDisplayContext(display);
break;
}
}
}
// 所以这里返回的一定是一个ContextImpl
return baseContext;
}所以回到上面所说的mBase，就是一个ContextImpl 对象，看一下它的getSystemService(String name)
@Override
public Object getSystemService(String name) {
return SystemServiceRegistry.getSystemService(this, name);
}SystemServiceRegistry在加载的时候会执行下面的代码
static {
// 省略代码
registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
new CachedServiceFetcher<LayoutInflater>() {
@Override
public LayoutInflater createService(ContextImpl ctx) {
return new PhoneLayoutInflater(ctx.getOuterContext());
}});
// 省略代码
}
注册Context.LAYOUT_INFLATER_SERVICE服务，registerService很简单，只是往两个map中添加数据
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);
}
map的定义
private static final HashMap<Class<?>, String> SYSTEM_SERVICE_NAMES =
new HashMap<Class<?>, String>();
private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS =
new HashMap<String, ServiceFetcher<?>>();所以当执行SystemServiceRegistry.getSystemService的时候
public static Object getSystemService(ContextImpl ctx, String name) {
ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
return fetcher != null ? fetcher.getService(ctx) : null;
}
fetcher就是类加载的时候通过static代码块注册的一个匿名类，这个匿名类是一个CachedServiceFetcher，看一下它
static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
private final int mCacheIndex;
public CachedServiceFetcher() {
mCacheIndex = sServiceCacheSize++;
}
@Override
@SuppressWarnings("unchecked")
public final T getService(ContextImpl ctx) {
final Object[] cache = ctx.mServiceCache;
synchronized (cache) {
// Fetch or create the service.
Object service = cache[mCacheIndex];
if (service == null) {
service = createService(ctx);
cache[mCacheIndex] = service;
}
return (T)service;
}
}
public abstract T createService(ContextImpl ctx);
}
调用它的getService方法，然后调用createService方法，匿名类里重写了这个方法
return new PhoneLayoutInflater(ctx.getOuterContext());返回一个PhoneLayoutInflater对象，PhoneLayoutInflater继承了LayoutInflater，然后回来调用cloneInContext，这是一个抽象方法，实现在PhoneLayoutInflater里
public LayoutInflater cloneInContext(Context newContext) {
return new PhoneLayoutInflater(this, newContext);
}在这里重新创建了一个PhoneLayoutInflater对象，这里不是很明白，为什么要重新创建一个对象，之后同一个Activity每次from得到的都是同一个LayoutInflater。在得到对象之后，看一下是怎么把一个布局文件变成View的。
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
// 省略代码
View result = root;
// 省略try catch finally
// 省略代码
final String name = parser.getName();
// 省略代码
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) {
// 省略代码
// 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);
}
}
// 省略代码
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
// 省略代码
// 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;
}
}
return result;
}
}
安卓使用pull解析器解析布局，这个方法有三个参数
XmlPullParser parser是布局文件生成的解析器
@Nullable ViewGroup root是根布局
boolean attachToRoot表示是否把解析的View添加到根布局，当调用两个参数的方法时，attachToRoot = root != null
这里分两种情况，merge标签，非merge标签
merge标签的时候root不能为空，attachToRoot 必须为true，否则会抛出异常，然后执行rInflate。
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();
}
}
rInflate通过深度优先遍历，遍历每一个标签，每解析一个View都会调用rInflateChildren方法，rInflateChildren的实现就是再递归调用rInflate方法，然后再回来把自己加入到parent之中，所有标签遍历完成之后，整个View树就构建结束了。
View的解析通过createViewFromTag来处理。
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
// 省略代码
// 闪烁视图，不是很清楚，可以忽略
if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(context, attrs);
}
// 省略try catch
View view;
// 这一段默认都是为空的，可以通过LayoutInflaterCompat.setFactory(LayoutInflater inflater, LayoutInflaterFactory factory)设置自己的处理逻辑，比如换肤，系统各种AppCompatxxx控件就是这么处理的
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;
}通过标签名是否包含.来判断是系统View还是自定义View，最后都会调用createView(String name, String prefix, AttributeSet attrs)
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> clazz = null;
// 省略try catch
// 省略代码
if (constructor == null) {
// Class not found in the cache, see if it's real, and try to add it
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
// 省略代码
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 = 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) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
return view;
}首先拿到View的全路径，然后判断View的Constructor对象是否为空，为空先通过反射拿到Constructor对象，然后缓存起来，下次同样类型的View直接使用，最后通过反射创建View对象返回。
上面inflate方法中，非merge标签的处理也是这样，先通过createViewFromTag创建根布局，然后调用rInflateChildren，同样递归处理所有的子View。判断root是否为空，如果root为空，直接返回布局文件得到的根布局。root不为空判断attachToRoot的值，attachToRoot为true，设置params，把布局文件得到的根布局添加到root中，如果为false，只是设置params不添加。这样整个加载过程就结束了。