Android Launcher-----MainThreadInitializedObject介绍

MainThreadInitializedObject 是 Android 开发中用于确保对象在主线程上初始化的一种设计模式

一、用途

MainThreadInitializedObject 通常用于确保那些需要在主线程上创建的对象（比如UI组件或依赖于主线程环境的对象）能够安全地进行初始化

二、优点

可以避免多线程环境下的同步问题，因为它限定了对象的初始化必须在主线程上完成。
实现了单例和懒加载，适用于那些需要访问UI组件或者依赖于应用程序上下文的单例对象，如全局配置管理器或资源管理器

三、使用

MainThreadInitializedObject 提供了一个 get(Context context) 方法，用于获取单例对象的实例。如果实例尚未创建，它将在调用线程上初始化，确保对象的创建发生在主线程上
以Launcher中的DisplayController为例：
初始化：

    public static final MainThreadInitializedObject<DisplayController> INSTANCE =new MainThreadInitializedObject<>(DisplayController::new);

获取对象：

    /*** Returns the current navigation mode*/public static NavigationMode getNavigationMode(Context context) {return INSTANCE.get(context).getInfo().navigationMode;}

通过get方法获取对象

四、实现

初始化

    public MainThreadInitializedObject(ObjectProvider<T> provider) {mProvider = provider;}

初始化传入一个ObjectProvider，ObjectProvider是一个内部接口，里面只有一个get方法

    public interface ObjectProvider<T> {T get(Context context);}

获取对象

    public T get(Context context) {if (context instanceof SandboxContext) {return ((SandboxContext) context).getObject(this, mProvider);}if (mValue == null) {if (Looper.myLooper() == Looper.getMainLooper()) {mValue = TraceHelper.allowIpcs("main.thread.object",() -> mProvider.get(context.getApplicationContext()));} else {try {return MAIN_EXECUTOR.submit(() -> get(context)).get();} catch (InterruptedException|ExecutionException e) {throw new RuntimeException(e);}}}return mValue;}

get会先检查context对象是否属于SandboxContext，SandboxContext是一个用于管理和控制在沙盒环境中创建和销毁的对象的抽象类。它提供了对象的缓存、创建和销毁机制，确保对象的生命周期和线程安全性。

    public static abstract class SandboxContext extends ContextWrapper {private static final String TAG = "SandboxContext";protected final Set<MainThreadInitializedObject> mAllowedObjects;protected final Map<MainThreadInitializedObject, Object> mObjectMap = new HashMap<>();protected final ArrayList<Object> mOrderedObjects = new ArrayList<>();private final Object mDestroyLock = new Object();private boolean mDestroyed = false;public SandboxContext(Context base, MainThreadInitializedObject... allowedObjects) {super(base);mAllowedObjects = new HashSet<>(Arrays.asList(allowedObjects));}.../*** Find a cached object from mObjectMap if we have already created one. If not, generate* an object using the provider.*/private <T> T getObject(MainThreadInitializedObject<T> object, ObjectProvider<T> provider) {synchronized (mDestroyLock) {if (mDestroyed) {Log.e(TAG, "Static object access with a destroyed context");}T t = (T) mObjectMap.get(object);if (t != null) {return t;}if (Looper.myLooper() == Looper.getMainLooper()) {t = createObject(provider);// Check if we've explicitly allowed the object or if it's a SafeCloseable,// it will get destroyed in onDestroy()if (!mAllowedObjects.contains(object) && !(t instanceof SafeCloseable)) {throw new IllegalStateException("Leaking unknown objects " + object + "  " + provider + " " + t);}mObjectMap.put(object, t);mOrderedObjects.add(t);return t;}}try {return MAIN_EXECUTOR.submit(() -> getObject(object, provider)).get();} catch (InterruptedException | ExecutionException e) {throw new RuntimeException(e);}}@UiThreadprotected <T> T createObject(ObjectProvider<T> provider) {return provider.get(this);}}

在SandboxContext和MainThreadInitializedObject中获取对象的方式相似，如果已经有了就直接获取，没有就通过MAIN_EXECUTOR在主线程上创建对象。MAIN_EXECUTOR是一个主线程的LooperExecutor，LooperExecutor继承自AbstractExecutorService，最终会在主线程执行ObjectProvider的get方法，也就是创建所需的对象，然后通过LooperExecutor的get方法获取任务执行的结果

public class LooperExecutor extends AbstractExecutorService {private final Handler mHandler;...@Overridepublic void execute(Runnable runnable) {if (getHandler().getLooper() == Looper.myLooper()) {runnable.run();} else {getHandler().post(runnable);}}
}