Room 返回的 Livedata GC 后不会触发

Question

我们的团队正在构建一个受益于 Android Jetpack 的项目。

有演示代码展示了我们面临的问题。这些代码可以在 https://github.com/viseator/TestRoomLivedata

找到

我创建一个 UserDao:

@Dao
interface UserDao {
    @Query("SELECT * FROM user WHERE uid = :uid LIMIT 1")
    fun findUserById(uid: String?): Single<User>

    @Query("SELECT * FROM user WHERE state = 1 LIMIT 1")
    fun findLoginUserWithObserve(): LiveData<User>

    @Query("SELECT * FROM user WHERE state =1 LIMIT 1")
    fun findLoginUser(): Single<User>

    @Update
    fun update(vararg user: User)
}

我还创建了一个 kotlin 对象来管理用户状态。

我正在观察 findLoginUserWithObserve() 返回的实时数据，以便在登录用户更改时收到通知：

object AccountManager {
    private const val DATA_BASE_NAME = "users"
    val TAG = "AccountManager"
    fun init(context: Application) {
        sDb = databaseBuilder(context, UserDataBase::class.java, DATA_BASE_NAME).build()
        sDao = sDb.userDao()
        sDao.findLoginUserWithObserve().observeForever {
            Log.d(TAG, "notified: $it")
        }
    }

    private lateinit var sDb: UserDataBase
    private lateinit var sDao: UserDao

    fun findLoginUserWithObserve() = sDao.findLoginUserWithObserve()

    fun logoutFlowable(): Single<Boolean> = sDao.findLoginUser().subscribeOn(
            Schedulers.io()).map { user ->
        user.state = User.STATE_NOT_LOGIN
        sDao.update(user)
        true
    }

    fun login(user: User) = logoutFlowable().subscribe({ doLogin(user) }, { doLogin(user) })

    private fun doLogin(user: User) = sDao.findUserById(user.uid).subscribeOn(
            Schedulers.io()).subscribe({ origin ->
        origin.userName = user.userName
        origin.state = User.STATE_HAVE_LOGIN
        sDao.update(origin)
        user.state = User.STATE_HAVE_LOGIN
    }, {
        user.state = User.STATE_HAVE_LOGIN
        sDao.insert(user)
    })

}

我通过调用它的 init 方法在 Applicaiton 中初始化 AccountManager 并创建一个演示 activity:

class MainActivity : AppCompatActivity() {
    private var i = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        AccountManager.login(User().apply {
            userName = "user1"
            uid = System.currentTimeMillis().toString()
        })
        button.setOnClickListener {
            AccountManager.login(User().apply {
                userName = "user${i++}"
                uid = System.currentTimeMillis().toString()
            })
        }
    }
}

我想一旦 AccountManager.login() 被调用，我就会收到通知并打印一条日志消息。但是我们发现GC之后就不会再通知了。（我们通过 Android Studio Profiler 触发 GC）

Log message

在探索房间生成的 UserDao_Impl class 之后，我们发现它通过调用 addWeakObserver():

创建了一个观察者和 link 数据库

  @Override
  public LiveData<User> findLoginUserWithObserve() {
    final String _sql = "SELECT * FROM user WHERE state = 1 LIMIT 1";
    final RoomSQLiteQuery _statement = RoomSQLiteQuery.acquire(_sql, 0);
    return new ComputableLiveData<User>(__db.getQueryExecutor()) {
      private Observer _observer;

      @Override
      protected User compute() {
        if (_observer == null) {
          _observer = new Observer("user") {
            @Override
            public void onInvalidated(@NonNull Set<String> tables) {
              invalidate();
            }
          };
          __db.getInvalidationTracker().addWeakObserver(_observer);
        }

所以我们想知道为什么房间在这里使用WeakObserver，这使得房间返回的实时数据不可靠？

PS：我们正在使用 Flowable 在它的 onNext() 中发射实时数据来解决这个问题，onNext() 将按预期每次触发。

Answer 1

在 post 这个问题到 google 问题跟踪器（https://issuetracker.google.com/issues/114833188）之后，我得到了回复：

We don't want to leak the LiveData if it is not used anymore. We could technically keep adding and removing the observer when LiveData is in use / not in use; but that might mean missing some events that happens when LiveData is inactive. We used to do that in the initial prototypes but became harder to maintain. You should keep a reference to the LiveData to keep using it. This is the patter we use in all examples.

因此，只需保留对 room 返回的实时数据的引用，而不仅仅是观察它，现在一切正常。

Room 返回的 Livedata GC 后不会触发

Livedata returned by Room won't be triggered after GC

android

android-room

android-architecture-components

android-jetpack