单例模式－LayoutInflater

前言

在Android系统中，我们通常会通过上下文对象Context获取一些系统级别的服务。而这些服务会在适当的时候以单例的形式注册在系统中，那么我们就可以通过Context的getSystemService(String name)获取。

以下的源码是Android8.0 API 26。

LayoutInflater

今天我们来看一下Android源码中的设计模式之单例模式。我们来讲一讲我们经常用到的单例类LayoutInflater。在我们使用RecylerView时，经常会用到，我们写RecyclerView的适配器时就会用到，如：

public class RvAdapter extends RecyclerView.Adapter<RvViewHolder> { ... @NonNull @Override public RvViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) { View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.rv_item,parent,false); return new RvViewHolder(itemView); } ...
}


  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
 

我们使用LayoutInflater.from(parent.getContext())来获取我们的LayoutInflater服务。我们来看看LayoutInflater的from方法：

/**
 * Obtains the LayoutInflater from the given context.
 */
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;
}

  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
 

从上面的代码，结合我们在前言中提到的，可知，LayoutInflater已在某个时刻以单例的形式注册在系统中了。我们现在通过上下文对象Context去获取这个对象。那么我们去看看Context对象的getSystemService方法是如何获取系统服务的:

/**
 * Interface to global information about an application environment.  This is
 * an abstract class whose implementation is provided by
 * the Android system.  It
 * allows access to application-specific resources and classes, as well as
 * up-calls for application-level operations such as launching activities,
 * broadcasting and receiving intents, etc.
 */
public abstract class Context {
  public abstract @Nullable Object getSystemService(@ServiceName @NonNull String name); /** * Return the handle to a system-level service by class. * <p> * Currently available classes are: * {@link android.view.LayoutInflater} * ... */
}

  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
  
12
  
13
  
14
  
15
  
16
  
17
  
18
  
19
 

从上面的信息，我们可以知道Context是一个抽象类，它的实现是由Android系统提供的。Context提供了关于应用环境的全局信息的接口，即我们可以通过Context提供的接口获取关于应用的全局信息。我们看看Android系统对Context的实现，如果想知道Context的实现类，我们就要找到Context在哪里被初始化，我们可以从Activity的初始化开始查找，一个Activity的入口是ActivityThread的main函数：
ActivityThread对象管理应用进程的主线程的执行，如计划和执行activity、广播和其他activity管理者请求的操作。

/**
 * This manages the execution of the main thread in an
 * application process, scheduling and executing activities,
 * broadcasts, and other operations on it as the activity
 * manager requests.
 *
 * {@hide}
 */
public final class ActivityThread extends ClientTransactionHandler {
public static void main(String[] args) { ... /** * Initialize the current thread as a looper, marking it as an * application's main looper. The main looper for your application * is created by the Android environment, so you should never need * to call this function yourself. * 将当前线程初始化为一个循环，作为应用程序的主循环，这个主循环是Android环境创建，我 * 们不能亲自去调用这个prepareMainLooper方法 */ Looper.prepareMainLooper(); ... /** * 创建ActivityThread对象 */ ActivityThread thread = new ActivityThread(); thread.attach(false); ... /**
		* 初始化主线程的Handler
		*/ if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } ... /** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. * 在这个线程运行消息队列 */ Looper.loop(); ... }

}

  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
  
12
  
13
  
14
  
15
  
16
  
17
  
18
  
19
  
20
  
21
  
22
  
23
  
24
  
25
  
26
  
27
  
28
  
29
  
30
  
31
  
32
  
33
  
34
  
35
  
36
  
37
  
38
  
39
  
40
  
41
  
42
  
43
  
44
  
45
  
46
 

我再来看看thread.attach()这个方法：

private void attach(boolean system) { sCurrentActivityThread = this; mSystemThread = system; if (!system) { ViewRootImpl.addFirstDrawHandler(new Runnable() { @Override public void run() { ensureJitEnabled(); } }); android.ddm.DdmHandleAppName.setAppName("<pre-initialized>", UserHandle.myUserId()); RuntimeInit.setApplicationObject(mAppThread.asBinder()); final IActivityManager mgr = ActivityManager.getService(); try { mgr.attachApplication(mAppThread); } catch (RemoteException ex) { throw ex.rethrowFromSystemServer(); } // Watch for getting close to heap limit. // 监视是否接近堆的限制 BinderInternal.addGcWatcher(new Runnable() { @Override public void run() { if (!mSomeActivitiesChanged) { return; } Runtime runtime = Runtime.getRuntime(); long dalvikMax = runtime.maxMemory(); long dalvikUsed = runtime.totalMemory() - runtime.freeMemory(); if (dalvikUsed > ((3*dalvikMax)/4)) { if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024) + " total=" + (runtime.totalMemory()/1024) + " used=" + (dalvikUsed/1024)); mSomeActivitiesChanged = false; try { mgr.releaseSomeActivities(mAppThread); } catch (RemoteException e) { throw e.rethrowFromSystemServer(); } } } }); } else { // Don't set application object here -- if the system crashes, // we can't display an alert, we just want to die die die. android.ddm.DdmHandleAppName.setAppName("system_process", UserHandle.myUserId()); try { mInstrumentation = new Instrumentation(); ContextImpl context = ContextImpl.createAppContext( this, getSystemContext().mPackageInfo); mInitialApplication = context.mPackageInfo.makeApplication(true, null); mInitialApplication.onCreate(); } catch (Exception e) { throw new RuntimeException( "Unable to instantiate Application():" + e.toString(), e); } } // add dropbox logging to libcore DropBox.setReporter(new DropBoxReporter()); ViewRootImpl.ConfigChangedCallback configChangedCallback = (Configuration globalConfig) -> { synchronized (mResourcesManager) { // We need to apply this change to the resources immediately, because upon returning // the view hierarchy will be informed about it. if (mResourcesManager.applyConfigurationToResourcesLocked(globalConfig, null /* compat */)) { updateLocaleListFromAppContext(mInitialApplication.getApplicationContext(), mResourcesManager.getConfiguration().getLocales()); // This actually changed the resources! Tell everyone about it. if (mPendingConfiguration == null || mPendingConfiguration.isOtherSeqNewer(globalConfig)) { mPendingConfiguration = globalConfig; sendMessage(H.CONFIGURATION_CHANGED, globalConfig); } } } }; ViewRootImpl.addConfigCallback(configChangedCallback); }
 }

  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
  
12
  
13
  
14
  
15
  
16
  
17
  
18
  
19
  
20
  
21
  
22
  
23
  
24
  
25
  
26
  
27
  
28
  
29
  
30
  
31
  
32
  
33
  
34
  
35
  
36
  
37
  
38
  
39
  
40
  
41
  
42
  
43
  
44
  
45
  
46
  
47
  
48
  
49
  
50
  
51
  
52
  
53
  
54
  
55
  
56
  
57
  
58
  
59
  
60
  
61
  
62
  
63
  
64
  
65
  
66
  
67
  
68
  
69
  
70
  
71
  
72
  
73
  
74
  
75
  
76
  
77
  
78
  
79
  
80
  
81
  
82
  
83
  
84
 

从上面的代码我们看到Context的实现类，可能是ContextImpl，我们来看看ContextImpl.java:
ContextImpl为Context API的一般实现，它为Activity和其他应用组件提供了基本的上下文对象。

/**
 * Common implementation of Context API, which provides the base
 * context object for Activity and other application components.
 */
class ContextImpl extends Context {
	... @Override public Object getSystemService(String name) { return SystemServiceRegistry.getSystemService(this, name); } ...
}

  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
  
12
 

SystemServiceRegistry类管理所有系统服务，可以通过Context对象的getSystemService方法获得服务。

/**
 * Manages all of the system services that can be returned by {@link Context#getSystemService}.
 * Used by {@link ContextImpl}.
 */
final class SystemServiceRegistry {
	... /** * Gets a system service from a given context. */ public static Object getSystemService(ContextImpl ctx, String name) { ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name); return fetcher != null ? fetcher.getService(ctx) : null; } ...
}

  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
  
12
  
13
  
14
  
15
 

SYSTEM_SERVICE_FETCHERS是一个HashMap：

private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS = new HashMap<String, ServiceFetcher<?>>();

  

 

  
1
  
2
 

SystemServiceRegistry还有一段这样的代码：

 /** * Base interface for classes that fetch services. * These objects must only be created during static initialization. */ static abstract interface ServiceFetcher<T> { T getService(ContextImpl ctx); } /** * Override this class when the system service constructor needs a * ContextImpl and should be cached and retained by that context. */ 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) { try { service = createService(ctx); cache[mCacheIndex] = service; } catch (ServiceNotFoundException e) { onServiceNotFound(e); } } return (T)service; } } public abstract T createService(ContextImpl ctx) throws ServiceNotFoundException; }

  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
  
12
  
13
  
14
  
15
  
16
  
17
  
18
  
19
  
20
  
21
  
22
  
23
  
24
  
25
  
26
  
27
  
28
  
29
  
30
  
31
  
32
  
33
  
34
  
35
  
36
  
37
  
38
  
39
  
40
 

CachedServiceFetcher的mCacheIndex会保存单例在数组中的位置，数组的初始化也就是单例的初始化是在下面这个静态代码块里完成。
SystemServiceRegistry有一个静态块：

 static { // 注册各种各样的服务，都是单例 ... registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class, new CachedServiceFetcher<LayoutInflater>() { @Override public LayoutInflater createService(ContextImpl ctx) { return new PhoneLayoutInflater(ctx.getOuterContext()); }}); ... } /** * Statically registers a system service with the context. * This method must be called during static initialization only. */ 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); }


  

 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
  
11
  
12
  
13
  
14
  
15
  
16
  
17
  
18
  
19
  
20
  
21
 

至此，我们终于从SystemServiceRegistry对象中把LayoutInflater单例取回来了。
核心服务以单例形式存在，减少了资源的消耗。

谢谢

文章来源: blog.csdn.net，作者：WongKyunban，版权归原作者所有，如需转载，请联系作者。

原文链接：blog.csdn.net/weixin_40763897/article/details/103841322