如何在协程中通过循环调用API实现Flickr应用分页加载？

方案选择原则 #

分页逻辑的分层设计要遵循关注点分离：Repository负责封装数据获取逻辑，ViewModel负责UI状态管理和用户交互响应。针对你的需求（自动逐页加载并实时更新UI），两种分层实现都可行，各有优劣，具体选择取决于后续需求的灵活性。

方案一：Repository层封装全量分页加载（推荐用于自动全量加载场景） #

将逐页调用API的逻辑封装在Repository层，返回一个Flow来逐页发射数据，ViewModel只需收集这个Flow并更新UI状态，职责更清晰。

Repository层修改 #

override fun getRecentAllPhotos(): Flow<FlickrResult<FlickrPhotosList?>> = flow {
    var currentPage = 1
    var totalPages = 1

    do {
        val apiResponse = flickrApiHelper.getRecentPhotos(currentPage)
        val result = when (apiResponse) {
            is FlickrApiResult.OnSuccess -> {
                val photosList = FlickrMapper.fromRetrofitToFlickrPhotosList(apiResponse.data)
                totalPages = photosList?.pages ?: 1
                FlickrResult.OnSuccess(photosList)
            }
            is FlickrApiResult.OnError -> FlickrResult.OnError(apiResponse.exception)
        }
        // 发射当前页数据，ViewModel会收到并更新UI
        emit(result)
        currentPage++
    } while (currentPage <= totalPages)
}.flowOn(Dispatchers.IO) // 统一指定IO线程，避免重复切换

ViewModel层修改 #

fun loadAllRecentPhotos() {
    viewModelScope.launch {
        _flickerPhotoState.value = FlickrState.Loading
        flickrUseCases.getRecentAllPhotos().collect { result ->
            when (result) {
                is FlickrResult.OnSuccess -> {
                    result.data?.photo?.let { newPhotos ->
                        if (newPhotos.isNotEmpty()) {
                            // 获取当前已展示的照片列表，追加新页数据
                            val currentPhotos = (_flickerPhotoState.value as? FlickrState.DisplayPhotos)?.photos ?: emptyList()
                            _flickerPhotoState.value = FlickrState.DisplayPhotos(
                                currentPhotos + Mapper.fromListFlickrPhotoItemToListPhotoDetails(newPhotos)
                            )
                        } else {
                            // 如果当前没有任何数据，显示无照片状态
                            if ((_flickerPhotoState.value as? FlickrState.DisplayPhotos)?.photos.isNullOrEmpty()) {
                                _flickerPhotoState.value = FlickrState.NoPhotos
                            }
                        }
                    } ?: run {
                        if ((_flickerPhotoState.value as? FlickrState.DisplayPhotos)?.photos.isNullOrEmpty()) {
                            _flickerPhotoState.value = FlickrState.NoPhotos
                        }
                    }
                }
                is FlickrResult.OnError -> {
                    _flickerPhotoState.value = FlickrState.Error(result.exception)
                    // 可选：遇到错误是否停止加载后续页？这里取消协程终止加载
                    cancel()
                }
            }
        }
    }
}

方案二：ViewModel层处理分页逻辑（适合按需加载场景） #

如果后续需要支持滚动加载更多（而非一次性全量加载），可以让ViewModel维护当前页码，在需要时触发单页请求，这种方式更灵活。

Repository层保持单页请求逻辑 #

无需修改原有getRecentPhotos(page: Int)方法，保持只获取单页数据的职责。

ViewModel层修改 #

private var currentPage = 1
private var totalPages = 1

fun loadNextPage() {
    if (currentPage > totalPages) return
    viewModelScope.launch {
        // 可选：显示加载更多状态
        _flickerPhotoState.value = FlickrState.LoadingMore
        when(val result = flickrUseCases.getRecentPhotos(currentPage)) {
            is FlickrResult.OnSuccess -> {
                result.data?.let { photosList ->
                    totalPages = photosList.pages ?: 1
                    photosList.photo?.let { newPhotos ->
                        if (newPhotos.isNotEmpty()) {
                            val currentPhotos = (_flickerPhotoState.value as? FlickrState.DisplayPhotos)?.photos ?: emptyList()
                            _flickerPhotoState.value = FlickrState.DisplayPhotos(
                                currentPhotos + Mapper.fromListFlickrPhotoItemToListPhotoDetails(newPhotos)
                            )
                            currentPage++
                        } else {
                            if ((_flickerPhotoState.value as? FlickrState.DisplayPhotos)?.photos.isNullOrEmpty()) {
                                _flickerPhotoState.value = FlickrState.NoPhotos
                            }
                        }
                    } ?: run {
                        if ((_flickerPhotoState.value as? FlickrState.DisplayPhotos)?.photos.isNullOrEmpty()) {
                            _flickerPhotoState.value = FlickrState.NoPhotos
                        }
                    }
                }
            }
            is FlickrResult.OnError -> {
                _flickerPhotoState.value = FlickrState.Error(result.exception)
            }
        }
    }
}

// 初始化加载第一页
fun initLoadRecentPhotos() {
    currentPage = 1
    totalPages = 1
    viewModelScope.launch {
        _flickerPhotoState.value = FlickrState.Loading
        loadNextPage()
    }
}

两种方案对比 #

注意事项 #

1. 错误处理：根据需求决定加载失败时是否继续加载后续页，比如在Repository的Flow中可以选择emit错误后继续循环，或者终止流。
2. 线程管理：Repository层用flowOn(Dispatchers.IO)统一指定IO线程，ViewModel无需额外切换线程，collect默认在Main线程执行。
3. 状态合并：每次加载新页时要与已有数据合并，避免覆盖之前的UI内容。

内容的提问来源于stack exchange，提问作者Seb