我正在使用 viewModelScope
在 ViewModel
它调用存储库中的挂起函数,如下所示:
查看型号
class DeepFilterViewModel(val repo: DeepFilterRepository) : ViewModel() {
var deepFilterLiveData: LiveData<Result>? = null
fun onImageCompressed(compressedImage: File): LiveData<Result>? {
if (deepFilterLiveData == null) {
viewModelScope.launch {
deepFilterLiveData = repo.applyFilter(compressedImage)
}
}
return deepFilterLiveData
}
}
存储库
class DeepFilterRepository {
suspend fun applyFilter(compressedImage: File): LiveData<Result> {
val mutableLiveData = MutableLiveData<Result>()
mutableLiveData.value = Result.Loading
withContext(Dispatchers.IO) {
mutableLiveData.value = Result.Success("Done")
}
return mutableLiveData
}
}
我正在观察 fragment 中的 LiveData,如下所示:
viewModel.onImageCompressed(compressedImage)?.observe(this, Observer { result ->
when (result) {
is Result.Loading -> {
loader.makeVisible()
}
is Result.Success<*> -> {
// Process result
}
}
})
问题是我没有从 LiveData 中获得任何值(value)。如果我不使用
viewModelScope.launch {}
如下所示,那么一切正常。class DeepFilterViewModel(val repo: DeepFilterRepository) : ViewModel() {
var deepFilterLiveData: LiveData<Result>? = null
fun onImageCompressed(compressedImage: File): LiveData<Result>? {
if (deepFilterLiveData == null) {
deepFilterLiveData = repo.applyFilter(compressedImage)
}
return deepFilterLiveData
}
}
我不知道我错过了什么。任何帮助将不胜感激。
最佳答案
这段代码:
viewModelScope.launch {
deepFilterLiveData = repo.applyFilter(compressedImage)
}
立即返回,所以当您第一次调用
onImageCompressed()
您返回的方法null
如deepFilterLiveData
.因为在你的 UI 中你使用 ?.
在 null
onImageCompressed()
的返回值when
条款将无法达成。没有协程的代码可以工作,因为在这种情况下你有顺序代码,你的 ViewModel 等待存储库调用。为了解决这个问题,您可以保留 ViewModel-UI 交互的 LiveData 并直接从存储库方法返回值:
class DeepFilterRepository {
suspend fun applyFilter(compressedImage: File) = withContext(Dispatchers.IO) {
Result.Success("Done")
}
}
和 View 模型:
class DeepFilterViewModel(val repo: DeepFilterRepository) : ViewModel() {
private val _backingLiveData = MutableLiveData<Result>()
val deepFilterLiveData: LiveData<Result>
get() = _backingLiveData
fun onImageCompressed(compressedImage: File) {
// you could also set Loading as the initial state for _backingLiveData.value
_backingLiveData.value = Result.Loading
viewModelScope.launch {
_backingLiveData.value = repo.applyFilter(compressedImage)
}
}
}
关于android - 将 viewModelScope 与 LiveData 一起使用时出现问题,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/55932799/