结合下载进度

Question

我尝试使用 combine 和 Alamofire 显示文件下载进度。我有下载器 class

class DataManager: NSObject, DataManagerProtocol {
    private(set) var value = 0.0 {
        didSet { subject.send(value) }
    }

    private let subject = PassthroughSubject<Double, Never>()

    func increment(by value: Double) {
        self.value = value
    }
    
    func saveFile(urlString: String, fileName: String) -> AnyPublisher<Double, Never> {
        download(urlString: urlString, fileName: fileName)
        return subject.eraseToAnyPublisher()
    }
    
    private func download(urlString: String, fileName: String) {
        AF.download(urlString)
            .downloadProgress { [self] progress in
                print("Download Progress: \(progress.fractionCompleted)")
                increment(by: progress.fractionCompleted)
            }
            .responseData { response in
                if let data = response.value {
                    print("data recieved")
                    self.writeToFile(data: data, fileName: fileName)
                }
            }
    }
    
    func writeToFile(data: Data, fileName: String) {
        // get path of directory
        
        guard let directory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).last else {
            return
        }
        // create file url
        let fileurl =  directory.appendingPathComponent(filename)
        if FileManager.default.fileExists(atPath: fileurl.path) {
            if let fileHandle = FileHandle(forWritingAtPath: fileurl.path) {
                print("FileExist")
            } else {
                print("Can't open file to write.")
            }
        } else {
            // if file does not exist write data for the first time
            do {
                try data.write(to: fileurl, options: .atomic)
            } catch {
                print("Unable to write in new file.")
            }
        }
    }
}

在控制台中我看到文件下载成功

Download Progress: 0.2707762694368025 Download Progress: 0.30361701168087313 Download Progress: 0.45961053734020857 Download Progress: 0.5088716507063145 Download Progress: 0.5827633207554733 Download Progress: 0.615604062999544 Download Progress: 0.6484448052436146 Download Progress: 0.7798077742198971 Download Progress: 0.8783300009521089 Download Progress: 1.0 data recieved

但是在我的 ViewModel 中我没有看到进度变化的发布

import Combine

final class RecordsListViewModel: ObservableObject {
    private var cancellable: AnyCancellable?
    private(set) var progress = PassthroughSubject<Double, Never>()
    private let dataManager: DataManagerProtocol
    
    init(dataManager: DataManagerProtocol) {
        self.dataManager = dataManager
    }
    
    func downloadFile() {
        cancellable = dataManager.saveFile(urlString: "https://i.artfile.ru/2880x1800_1455670_[www.ArtFile.ru].jpg", fileName: "filename.jpg")
            .receive(on: DispatchQueue.main)
            .sink { [weak self] completion in
                print(completion, "completion")
            } receiveValue: { progress in
                print(progress, "progress")
            }
    }
}

Answer 1

问题是在 saveFile 方法中，您在 ViewModel 获取主题之前调用了下载方法。我建议将 saveFile 方法一分为二。

在数据管理器中：

func getSubscription() -> AnyPublisher<Double, Never> {
    return subject.eraseToAnyPublisher()
}
    
func saveFile(urlString: String, fileName: String) {
    download(urlString: urlString, fileName: fileName)
    // or just put the contents of the download method here
}

在ViewModel中，先获取DataManager的主题，准备下载：

init(dataManager: DataManagerProtocol) {
    self.dataManager = dataManager
    
    cancellable = self.dataManager.getSubscription()
        .receive(on: DispatchQueue.main)
        .sink { completion in
            print(completion, "completion")
        } receiveValue: { progress in
            print(progress, "progress")
        }
}
    
func downloadFile() {
    dataManager.saveFile(urlString: "https://i.artfile.ru/2880x1800_1455670_[www.ArtFile.ru].jpg", fileName: "filename.jpg")
}

我在 Playground 中尝试过，使用 Timer Publisher 来模拟下载。它工作得很好。当然，你需要把DataManagerProtocol改成这样：

protocol DataManagerProtocol {
    func saveFile(urlString: String, fileName: String)
    func getSubscription() -> AnyPublisher<Double, Never>
}

编辑： 我刚刚找到了另一种解决问题的方法。您只需要更改您提供的代码中的一行：

func saveFile(urlString: String, fileName: String) -> AnyPublisher<Double, Never> {
    defer { download(urlString: urlString, fileName: fileName) }
    return subject.eraseToAnyPublisher()
}

延迟块将在函数完成之前直接执行，即使在 return 之后。我不确定这是否会使您的代码难以阅读。但我想为了完整起见我应该提到这个选项。如果有人对 defer 有更多经验，请在评论中告诉我这是否属于误用。

顺便说一句，我想你想将增量方法的内容更改为：

self.value += value

结合下载进度

downloading progress with combine

swift

alamofire

combine