Android MVVM-如何使LiveData发出其拥有的数据(强制触发观察者) [英] Android MVVM - How to make LiveData emits the data it has (forcing to trigger the observer)

问题描述

我有这个ViewModel从网络获取列表，我用数据填充RecyclerView(MyAvailabilityRepository返回MutableLiveData，这就是我使用Transformations.switchMap的原因):

I have this ViewModel that gets a list from the network and I populate a RecyclerView with the data (MyAvailabilityRepository returns a MutableLiveData, that's why i'm using Transformations.switchMap):

class MyAvailabilityViewModel : ViewModel() {
    private val getListsParams = MutableLiveData<String>()
    private val getListsObservable = Transformations.switchMap(getListsParams) { 
        organizationId -> MyAvailabilityRepository.getSectionedLists(organizationId) 
    }

    fun getListsObservable() : LiveData<Resource<MutableList<SectionedAvailabilityList>>> {
        return getListsObservable
    }

    fun fetchLists(organizationId: String, forceRefresh: Boolean = false) {
        if (getListsParams.value == null || forceRefresh) {
            getListsParams.value = organizationId
        }
    }
}

片段的onActivityCreated:

override fun onActivityCreated(savedInstanceState: Bundle?) {
    ...
    viewModel.getListsObservable().observe(this, Observer { // populate RecyclerView })
    viewModel.fetchLists(organizationId)
}

由于getListParams.value第一次为空，它将设置getListsParams.value = organizationId并触发switchMap并调用存储库以从网络获取列表.

Since getListParams.value is null the first time, it will set getListsParams.value = organizationId and trigger the switchMap and call the repository to get the list from the network.

当我想强制刷新(通过刷新")并再次呼叫网络时，可以使用forceRefresh = true:

When I want to force a refresh (by pull-to-refresh) and call the network again, I can use forceRefresh = true:

override fun onRefresh() {
    viewModel.fetchLists(organizationId, forceRefresh = true)
}

它将设置organizationId的值并触发转换，然后调用网络.

It will set the value of organizationId and trigger the Transformations that will then call network.

但是，在我的场景中，我从RecyclerView的适配器中清除了数据.如果在那之后，用户单击一个按钮，我想再次触发观察者，以便我用getListsObservable已经获取的数据重新填充适配器.我不想在这一点上调用forceRefresh，因为我确定我已经有数据了，我只想再次触发观察者，以便用现有数据更新我的UI.由于此时getListParams.value不为null，所以以后再调用fetchLists(organizationId)时什么也没有发生.

But, I have a scenario where I clear the data from my RecyclerView's adapter. If after that, the user click a button, I would like to trigger the observer again so that I re-populate the adapter with the data that the getListsObservable has already fetched. I don't want to call forceRefresh on this one cause i'm sure I already have the data and I would just like to trigger the observer again so that my UI is updated with the existing data. Since getListParams.value is not null at that point, then nothing happens when I call fetchLists(organizationId) later on.

任何关于如何使用当前设置实现此目标的想法吗?

Any idea of how I could achieve that with my current setup?

推荐答案

是的，洪端给出的答案是完美的. 我只是在这里扩展答案，

Yes, the answer given by Hong Duan is perfect. I am just going to extend that answer here,

更好的方法是具有扩展功能.

The better way of doing is having an extension function.

我的扩展功能如下:

fun <T> MutableLiveData<T>.forceRefresh() {
    this.value = this.value
}

呼叫者函数如下所示，

mutableLiveDataObject.forceRefresh()

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