android - 如何管理 Realm 实例？

Question

我有一个管理多个 fragment 和嵌套 fragment 的 Activity，就像这样:

Activity -> RootFragment1 -> NestedFragment1、NestedFragment2
-> RootFragment2 -> NestedFragment3、NestedFragment4...

我用来获取一个 Realm 实例并在 onStart、onStop 方法中的每个嵌套 fragment 中关闭它，但有时我会遇到此异常:

Fatal Exception: java.lang.IllegalStateException: This Realm instance has already been closed, making it unusable.

是否有推荐的方法来获取 Realm 实例并关闭它？就我而言，我应该在 Activity 中获取一个实例并将其传递给我的 fragment 吗？

Answer 1

文档说您应该在 onCreateView()/onDestroyView() 中打开/关闭 Realm，但根据我的经验， fragment 生命周期异常不稳定，因此我可以向您展示另外两种方法。

1.) 在Activity.onCreate()和Activity.onDestroy()中打开/关闭Realm，然后共享到fragments(甚至向下 View 层次结构!)使用 getSystemService()。

public class MyActivity extends AppCompatActivity {
    Realm realm;

    @Override
    protected void onCreate(Bundle bundle) {
        // ...
        realm = Realm.getDefaultInstance();
    }

    @Override
    protected void onDestroy() {
        realm.close();
        realm = null;
        // ...
    }

    // -----------------------------
    private static final String REALM_TAG = "__REALM__";

    public static Realm getRealm(Context context) {
        // noinspection ResourceType
        return (Realm)context.getSystemService(REALM_TAG);
    }

    @Override
    public Object getSystemService(@NonNull String name) {
        if(REALM_TAG.equals(name)) {
            return realm;
        }
        return super.getSystemService(name);
    }
}

然后在 fragment 中你可以做

Realm realm = MyActivity.getRealm(getActivity());

在 View 中你可以做

Realm realm = MyActivity.getRealm(getContext());

---

2.) 使用保留的 fragment 作为生命周期监听器/Activity 引用计数器，为 UI 线程全局管理 Realm 生命周期。

/**
 * Created by Zhuinden on 2016.08.16..
 */
public class RealmManager {
    private static final String TAG = "RealmManager";

    static Realm realm;

    static RealmConfiguration realmConfiguration;

    public static void init(Context context) {
        Realm.init(context);
    }

    public static void initializeRealmConfig(Context appContext) {
        if(realmConfiguration == null) {
            Log.d(TAG, "Initializing Realm configuration.");
            setRealmConfiguration(new RealmConfiguration.Builder(appContext).initialData(new RealmInitialData())
                    .deleteRealmIfMigrationNeeded()
                    .inMemory()
                    .build());
        }
    }

    public static void setRealmConfiguration(RealmConfiguration realmConfiguration) {
        RealmManager.realmConfiguration = realmConfiguration;
        Realm.setDefaultConfiguration(realmConfiguration);
    }

    private static int activityCount = 0;

    public static Realm getRealm() { // use on UI thread only!
        return realm;
    }

    public static void incrementCount() {
        if(activityCount == 0) {
            if(realm != null) {
                if(!realm.isClosed()) {
                    Log.w(TAG, "Unexpected open Realm found.");
                    realm.close();
                }
            }
            Log.d(TAG, "Incrementing Activity Count [0]: opening Realm.");
            realm = Realm.getDefaultInstance();
        }
        activityCount++;
        Log.d(TAG, "Increment: Count [" + activityCount + "]");
    }

    public static void decrementCount() {
        activityCount--;
        Log.d(TAG, "Decrement: Count [" + activityCount + "]");
        if(activityCount <= 0) {
            Log.d(TAG, "Decrementing Activity Count: closing Realm.");
            activityCount = 0;
            realm.close();
            if(Realm.compactRealm(realmConfiguration)) {
                Log.d(TAG, "Realm compacted successfully.");
            }
            realm = null;
        }
    }
}

连同

public class RealmScopeListener
        extends Fragment {
    public RealmScopeListener() {
        setRetainInstance(true);
        RealmManager.incrementCount();
    }

    @Override
    public void onDestroy() {
        RealmManager.decrementCount();
        super.onDestroy();
    }
}

和

/**
 * Created by Zhuinden on 2016.09.04..
 */
public class RealmActivity extends AppCompatActivity {
    protected Realm realm;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        RealmManager.init(this);
        RealmManager.initializeRealmConfig(getApplicationContext());

        super.onCreate(savedInstanceState);
        RealmScopeListener realmScopeListener = (RealmScopeListener)getSupportFragmentManager().findFragmentByTag("SCOPE_LISTENER");
        if(realmScopeListener == null) {
            realmScopeListener = new RealmScopeListener();
            getSupportFragmentManager().beginTransaction().add(realmScopeListener, "SCOPE_LISTENER").commit();
        }
        realm = RealmManager.getRealm();
    }
}

这允许您调用 RealmManager.getRealm() UI 线程，它的生命周期由保留 fragment 管理。