android - Dagger 2 – 我应该使用单例 Realm 实例吗？

Question

我正在使用 Dagger 2为整个应用程序提供一个单独的 Realm 实例(所有数据访问对象都使用一个单一的 Realm )。但是，据我所知，Realm 可以使用 Realm.getInstance() 拥有多个实例，我们必须在完成每个实例后关闭它，如 Realm 文档所示:

    /**
     * Closes the Realm instance and all its resources.
     * <p>
     * It's important to always remember to close Realm instances when you're done with it in order not to leak memory,
     * file descriptors or grow the size of Realm file out of measure.
     *
     * @throws IllegalStateException if attempting to close from another thread.
     */
    @Override
    public void close() {
        if (this.threadId != Thread.currentThread().getId()) {
            throw new IllegalStateException(INCORRECT_THREAD_CLOSE_MESSAGE);
        }

        if (realmCache != null) {
            realmCache.release(this);
        } else {
            doClose();
        }
    }

我的问题是:我应该像以前一样使用单例 Realm 实例，还是为每个 Activity/Fragment 创建一个 realm 实例并使用 realm.close() 在 onDestroy( )?

Answer 1

只有在给定线程上至少有 1 个打开的 Realm 实例时，Managed RealmObjects(它们在访问时是延迟加载的)才可以访问，但是在非循环后台线程上不关闭 Realm 实例是非常严重的问题。

如果您从 Dagger 模块提供线程本地单例 Realm，那么该 Realm 实例将只能在创建它的线程上访问。并且会导致从其他任何地方访问时崩溃。

一种可能性是提供一个你自己的单例类，它可以打开 Realm 实例，比如 this :

@Singleton
public class RealmManager {
    private final ThreadLocal<Realm> localRealms = new ThreadLocal<>();

    @Inject
    public RealmManager() {
    }

    /**
     * Opens a reference-counted local Realm instance.
     *
     * @return the open Realm instance
     */
    public Realm openLocalInstance() {
        checkDefaultConfiguration();
        Realm realm = Realm.getDefaultInstance(); // <-- maybe this should be configurable
        if(localRealms.get() == null) {
            localRealms.set(realm);
        }
        return realm;
    }

    /**
     * Returns the local Realm instance without adding to the reference count.
     *
     * @return the local Realm instance
     * @throws IllegalStateException when no Realm is open
     */
    public Realm getLocalInstance() {
        Realm realm = localRealms.get();
        if(realm == null) {
            throw new IllegalStateException(
                    "No open Realms were found on this thread.");
        }
        return realm;
    }

    /**
     * Closes local Realm instance, decrementing the reference count.
     *
     * @throws IllegalStateException if there is no open Realm.
     */
    public void closeLocalInstance() {
        checkDefaultConfiguration();
        Realm realm = localRealms.get();
        if(realm == null) {
            throw new IllegalStateException(
                    "Cannot close a Realm that is not open.");
        }
        realm.close();
        // noinspection ConstantConditions
        if(Realm.getLocalInstanceCount(Realm.getDefaultConfiguration()) <= 0) {
            localRealms.set(null);
        }
    }

    private void checkDefaultConfiguration() {
        if(Realm.getDefaultConfiguration() == null) {
            throw new IllegalStateException("No default configuration is set.");
        }
    }
}

但即便如此，you'd need to manage the local instances for the given threads where you need them.

public class MainActivity
        extends AppCompatActivity {
    RealmManager realmManager;

    ...

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        realmManager = Injector.get().realmManager();
        realmManager.openLocalInstance();
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
        ...
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        realmManager.closeLocalInstance();
    }