Android 源码中的单例模式

经常使用 Context.getSystemService(String name) 获取一些系统服务。 其实这些服务都是以单例的方式注册在系统中的。 获取到的都是单例对象。

LayoutInflater 为例

在 View 中经常通过 LayoutInflater.from(Context context) 获取 LayoutInflater 对象,查看方法实现:

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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;
}

内部是调用的 Context 的 getSystemService(String key) 方法,查看 Context 类发现是个抽象方法,需要子类实现:

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public abstract @Nullable Object getSystemService(@ServiceName @NonNull String name);

View 中的 Context 通常就是 Activity,Android 中在 Application,Activity,Service 中都存在 Context。一个应用中: Context 总数 = Activity 数 + Service 数 + 1 ( Application )。

所以从 Activity 的 Context 中分析 getSystemService 实现,先分析 Activity 的 Context 是在哪创建的。

Activity 的入口是 ActivityThread 的 main 函数:

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public static void main(String[] args) {
...
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread"));
}
...
Looper.loop();

throw new RuntimeException("Main thread loop unexpectedly exited");
}

创建 MainLooper,创建 ActivityThread 对象 ,启动 Looper 消息循环。ActivityThread 的 attach 方法:

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private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
RuntimeInit.setApplicationObject(mAppThread.asBinder());
final IActivityManager mgr = ActivityManager.getService();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
......
}

不是系统应用(boolean system)会通过 Binder 机制与 ActivityManagerService 通信,最终调用 handleLaunchActivity,里面又会调用 performLaunchActivity 方法:

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private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
ActivityInfo aInfo = r.activityInfo;
...
ContextImpl appContext = createBaseContextForActivity(r);
...
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, r.configCallback);
...
}

ActivityThread createBaseContextForActivity 方法:

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private ContextImpl createBaseContextForActivity(ActivityClientRecord r) {
final int displayId;
try {
displayId = ActivityManager.getService().getActivityDisplayId(r.token);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
ContextImpl appContext = ContextImpl.createActivityContext(this, r.packageInfo, r.activityInfo, r.token, displayId, r.overrideConfig);
...
}

performLaunchActivity 里创建了 ContextImpl 对象,Context 的具体实现就是 ComtextImpl 了。查看 ContextImpl 里 getSystemService 实现:

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public Object getSystemService(String name) {
return SystemServiceRegistry.getSystemService(this, name);
}

ContextImpl 里通过 SystemServiceRegistry 的类方法获取服务,查看 SystemServiceRegistry 类:

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final class SystemServiceRegistry {
private static final String TAG = "SystemServiceRegistry";

// Service registry information.
// This information is never changed once static initialization has completed.
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<?>>();
private static int sServiceCacheSize;

private SystemServiceRegistry() { }

// 静态代码块,类加载时就执行
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());
}});
...
}

// 获取服务
public static Object getSystemService(ContextImpl ctx, String name) {
ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
return fetcher != null ? fetcher.getService(ctx) : null;
}
}

静态代码块里注册了很多服务, LayoutInflater 也是在这里注册的, 看到 LayoutInflater 的实现类是 PhoneLayoutInflater

PhoneLayoutInflater

获得了 LayoutInflater 之后,就该调用 inflate 加载,查看源码:

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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();
}
}

继续查看:

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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 {
// 查找根节点
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
final String name = parser.getName();
// 解析 merge 标签
if (TAG_MERGE.equals(name)) {
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// 从 xml 中构造 View
final View temp = createViewFromTag(root, name, inflaterContext, attrs);

ViewGroup.LayoutParams params = null;
...
if (root != null && attachToRoot) {
root.addView(temp, params);
}
if (root == null || !attachToRoot) {
result = temp;
}
...
rInflateChildren(parser, temp, attrs, true);
...
}

}
...
return result;
}
}

调用 createViewFromTag :

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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 (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
//内置控件解析
view = onCreateView(parent, name, attrs);
} else {
//自定义控件解析
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
...
}

  • 解析内置控件:调用 onCreateView
  • 解析自定义控件:直接调用 createView(name, null, attrs);

前面分析了,Activity 的 Context 返回的 LayoutInflater 对象的具体实现是 PhoneLayoutInflater,查看源码,发现其复写了 onCreateView 方法:

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public class PhoneLayoutInflater extends LayoutInflater {
private static final String[] sClassPrefixList = {
"android.widget.",
"android.webkit.",
"android.app."
};
...
/** Override onCreateView to instantiate names that correspond to the
widgets known to the Widget factory. If we don't find a match,
call through to our super class.
复写 LayoutInflater 的 onCreateView
*/
@Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {
for (String prefix : sClassPrefixList) {
try {
View view = createView(name, prefix, attrs);
if (view != null) {
return view;
}
} catch (ClassNotFoundException e) {
// In this case we want to let the base class take a crack
// at it.
}
}

return super.onCreateView(name, attrs);
}
}

所以,在 PhoneLayoutInflater 解析内置控件时,会使用3种前缀。

createView 在 LayoutInflater 中实现:

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public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
//先根据类名从缓存中获取构造器
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> clazz = null;
try {
// 如果构造函数不存在,就通过反射获取构造函数
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);
}
}
...
//构造View
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
return view;
}

这样就成功从 xml 构造了一个根 View,接着就会调用 rInflateChildren 创建子 View。 rInflateChildren 调用 rInflate

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void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
//获得布局深度
final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;

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();
//requestFocus 标签
if (TAG_REQUEST_FOCUS.equals(name)) {
pendingRequestFocus = true;
consumeChildElements(parser);
//tag 标签
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
//include 标签
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
//merge 标签
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
//普通 View 标签
} else {
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
//递归调用
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);
}
}
if (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}
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