我最近看到一个奇怪的问题阻碍了我的项目。 设置实时数据值的多次调用不会在 View 中调用观察者。

似乎只有最后设置的值才真正调用 View 中的观察者。

这是评论的代码 fragment 。

主 Activity .kt

class MainActivity : AppCompatActivity() {

    private lateinit var viewModel: MainViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        viewModel = ViewModelProviders.of(this).get(MainViewModelImpl::class.java)

        viewModel.state().observe(this, Observer {
            onStateChange(it!!)
        })

        viewModel.fetchFirstThree()

    }

    private fun onStateChange(state: MainViewModel.State) {

        when (state) {
            is One -> {
                show(state.data)
            }
            is Two -> {
                show(state.data)
            }
            is Three -> {
                show(state.data)
            }
        }
    }

    private fun show(data: String) {
        Log.d("Response", data)
    }
}

主视图模型.kt

abstract class MainViewModel : ViewModel() {

    sealed class State {
        data class One(val data: String) : State()
        data class Two(val data: String) : State()
        data class Three(val data: String) : State()
    }

    abstract fun state(): LiveData<State>

    abstract fun fetchFirstThree()
}

MainViewModelImpl.kt

class MainViewModelImpl : MainViewModel() {

    private val stateLiveData: MediatorLiveData<State> = MediatorLiveData()

    override fun state(): LiveData<State> = stateLiveData

    override fun fetchFirstThree() {
        stateLiveData.value = State.One("One")
        stateLiveData.value = State.Two("Two")
        stateLiveData.value = State.Three("Three")
    }
}

预期输出:

Response: One
Response: Two
Response: Three

实际输出:

Response: Three

根据上面的输出，没有为前两个值调用 Observer。

最佳答案

我做了一些科学研究，重新实现了 LiveData 和 MutableLiveData 以记录一些数据。

检查 source code here .

setValue value=Test1
dispatchingValue mDispatchingValue=false mDispatchInvalidated=false
considerNotify
Returned at !observer.active
setValue value=Test2
dispatchingValue mDispatchingValue=false mDispatchInvalidated=false
considerNotify
Returned at !observer.active
setValue value=Test3
dispatchingValue mDispatchingValue=false mDispatchInvalidated=false
considerNotify
Returned at !observer.active
dispatchingValue mDispatchingValue=false mDispatchInvalidated=false
considerNotify
ITEM: Test3

当您发送初始值时，观察者似乎尚未达到 Activity 状态。

private void considerNotify(LifecycleBoundObserver observer) {
    // <-- Three times it fails here. This means that your observer wasn't ready for any of them.
    if (!observer.active) {
        return;
    }

一旦观察者达到 Activity 状态，它就会发送最后一个设定值。

void activeStateChanged(boolean newActive) {
    if (newActive == active) {
        return;
    }
    active = newActive;
    boolean wasInactive = LiveData.this.mActiveCount == 0;
    LiveData.this.mActiveCount += active ? 1 : -1;
    if (wasInactive && active) {
        onActive();
    }
    if (LiveData.this.mActiveCount == 0 && !active) {
        onInactive();
    }
    if (active) {
        // <--- At this point you are getting a call to your observer!
        dispatchingValue(this);
    }
}

关于android - 未观察到对设置 LiveData 的多次调用，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50582682/