网络响应处理异常：Error状态无法触发问题求助

Loading和Success状态可正常工作，但Error状态无法触发。模拟错误场景时，触发的是Loading和Success状态，而非Error状态。

相关代码如下：

Repository类 #

fun getMoviesStream(genreId: String?, sortBy: String): Flow<PagingData<Result>> {
    return Pager(
        config = PagingConfig(pageSize = MOVIES_PAGE_SIZE, enablePlaceholders = false),
        pagingSourceFactory = { MoviesPagingSource(api, genreId, sortBy) }
    ).flow
}

ViewModel类 #

private val _moviesResponse = MutableStateFlow<Resource<PagingData<Result>>>(Resource.Loading())
val moviesResponse = _moviesResponse.asStateFlow()

fun getMovies(genreId: String?, sortBy: String) {
    viewModelScope.launch {
        _moviesResponse.value = Resource.Loading()
        moviesRepository.getMoviesStream(genreId, sortBy).cachedIn(viewModelScope)
            .catch { throwable ->
                _moviesResponse.value = Resource.Error(
                    throwable.localizedMessage ?: "An unexpected error occurred"
                )
            }
            .collect { result ->
                delay(500)
                _moviesResponse.value = Resource.Success(result)
            }
    }
}

Fragment类 #

private fun requestMoviesData(genreId: String?, sortBy: String) {
    viewLifecycleOwner.lifecycleScope.launch {
        repeatOnLifecycle(Lifecycle.State.STARTED) {
            moviesViewModel.getMovies(genreId, sortBy)
            moviesViewModel.moviesResponse.collect { response ->
                when (response) {
                    is Resource.Error -> {
                        Toast.makeText(
                            requireContext(),
                            response.message,
                            Toast.LENGTH_SHORT
                        ).show()
                    }
                    is Resource.Loading -> {
                        binding.spinnerLoading.visibility = View.VISIBLE
                    }
                    is Resource.Success -> {
                        binding.spinnerLoading.visibility = View.GONE
                        response.data?.let { moviesAdapter.submitData(it) }
                    }
                }
            }
        }
    }
}

问题原因 #

1. Paging3的Flow特性：Pager.flow不会直接抛出异常，网络错误会被封装到PagingData的LoadState中，不会触发catch操作符。
2. 逻辑覆盖：当前collect块会在任何情况下（包括错误场景返回空PagingData）执行，覆盖了catch中设置的Error状态。
3. cachedIn位置错误：放在catch之后会导致异常被缓存流程吞掉，无法被捕获。

修复方案 #

1. 修改ViewModel的状态处理逻辑 #

监听PagingData的loadStateFlow来捕获错误，替换原有的catch逻辑：

private val _moviesResponse = MutableStateFlow<Resource<PagingData<Result>>>(Resource.Loading())
val moviesResponse = _moviesResponse.asStateFlow()

fun getMovies(genreId: String?, sortBy: String) {
    viewModelScope.launch {
        _moviesResponse.value = Resource.Loading()
        moviesRepository.getMoviesStream(genreId, sortBy)
            .cachedIn(viewModelScope)
            .collectLatest { pagingData ->
                _moviesResponse.value = Resource.Success(pagingData)
                // 监听加载状态，捕获错误
                pagingData.loadStateFlow.collect { loadStates ->
                    val refreshError = loadStates.refresh as? LoadState.Error
                    refreshError?.let {
                        _moviesResponse.value = Resource.Error(
                            it.error.localizedMessage ?: "An unexpected error occurred"
                        )
                    }
                }
            }
    }
}

2. 确保PagingSource正确抛出异常 #

检查MoviesPagingSource的load方法，网络请求失败时必须返回LoadResult.Error，而非空数据：

class MoviesPagingSource(private val api: MovieApi, ...) : PagingSource<Int, Result>() {
    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Result> {
        return try {
            val currentPage = params.key ?: 1
            val response = api.getMovies(genreId, sortBy, currentPage)
            
            if (!response.isSuccessful) {
                throw IOException("Request failed with code ${response.code()}")
            }
            
            val movies = response.body()?.results ?: emptyList()
            LoadResult.Page(
                data = movies,
                prevKey = if (currentPage == 1) null else currentPage - 1,
                nextKey = if (movies.isEmpty()) null else currentPage + 1
            )
        } catch (e: Exception) {
            // 将异常封装为LoadResult.Error
            LoadResult.Error(e)
        }
    }
}

3. 可选：优化Fragment的分页错误处理 #

如果需要处理分页加载时的错误，可以监听Adapter的loadStateFlow：

private fun requestMoviesData(genreId: String?, sortBy: String) {
    viewLifecycleOwner.lifecycleScope.launch {
        repeatOnLifecycle(Lifecycle.State.STARTED) {
            moviesViewModel.getMovies(genreId, sortBy)
            moviesViewModel.moviesResponse.collect { response ->
                when (response) {
                    is Resource.Error -> {
                        Toast.makeText(requireContext(), response.message, Toast.LENGTH_SHORT).show()
                        binding.spinnerLoading.visibility = View.GONE
                    }
                    is Resource.Loading -> {
                        binding.spinnerLoading.visibility = View.VISIBLE
                    }
                    is Resource.Success -> {
                        binding.spinnerLoading.visibility = View.GONE
                        response.data?.let { moviesAdapter.submitData(it) }
                        // 监听分页加载错误
                        moviesAdapter.loadStateFlow.collect { loadStates ->
                            val appendError = loadStates.append as? LoadState.Error
                            appendError?.let {
                                Toast.makeText(requireContext(), it.error.localizedMessage, Toast.LENGTH_SHORT).show()
                            }
                        }
                    }
                }
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Anubis