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下載多個(gè)文件

使用 "async let "來模擬并發(fā)下載多個(gè)文件的情況。

上面的代碼可以被改進(jìn)，以并行地執(zhí)行多個(gè)下載，因?yàn)槊總€(gè)任務(wù)都是獨(dú)立于其他任務(wù)的。在Swift并發(fā)中，這是用async let實(shí)現(xiàn)的，它用一個(gè)承諾立即給一個(gè)變量賦值，允許代碼執(zhí)行下一行代碼。然后，代碼等待這些承諾，等待最終結(jié)果的完成。

async/await：

func downloadFiles() async {
        for index in files.indices {
            let num = await downloadFile(index)
            await MainActor.run {
                fileCount += num
            }
        }
    }

async let：

func downloadFiles() async {
        async let num1 = await downloadFile(0)
        async let num2 = await downloadFile(1)
        async let num3 = await downloadFile(2)
        
        let (result1, result2, result3) = await (num1, num2, num3)
        await MainActor.run {
            fileCount = result1 + result2 + result3
        }
    }

ViewModel：

class DataFileViewModel4: ObservableObject {
    @Published private(set) var files: [DataFile]
    @Published private(set) var fileCount = 0
    
    init() {
        files = [
            DataFile(id: 1, fileSize: 10),
            DataFile(id: 2, fileSize: 20),
            DataFile(id: 3, fileSize: 5)
        ]
    }
    
    var isDownloading : Bool {
        files.filter { $0.isDownloading }.count > 0
    }
    
    func downloadFiles() async {
        async let num1 = await downloadFile(0)
        async let num2 = await downloadFile(1)
        async let num3 = await downloadFile(2)
        
        let (result1, result2, result3) = await (num1, num2, num3)
        await MainActor.run {
            fileCount = result1 + result2 + result3
        }
    }
    
    private func downloadFile(_ index: Array.Index) async -> Int {
        await MainActor.run {
            files[index].isDownloading = true
        }
        
        for _ in 0..

View：

struct TestView4: View {
    @ObservedObject private var dataFiles: DataFileViewModel4
    
    init() {
        dataFiles = DataFileViewModel4()
    }
    
    var body: some View {
        VStack {
            TitleView(title: ["Parallel", "(multiple Files)"])
            
            Button("Download All") {
                Task {
                    await dataFiles.downloadFiles()
                }
            }
            .buttonStyle(BlueButtonStyle())
            .disabled(dataFiles.isDownloading)
            
            Text("Files Downloaded: \(dataFiles.fileCount)")
            
            ForEach(dataFiles.files) { file in
                HStack(spacing: 10) {
                    Text("File \(file.id):")
                    ProgressView(value: file.progress)
                        .frame(width: 180)
                    Text("\((file.progress * 100), specifier: "%0.0F")%")
                    
                    ZStack {
                        Color.clear
                            .frame(width: 30, height: 30)
                        if file.isDownloading {
                            ProgressView()
                                .progressViewStyle(CircularProgressViewStyle(tint: .blue))
                        }
                    }
                }
            }
            .padding()
            
            Spacer().frame(height: 150)
            
            Button("Reset") {
                dataFiles.reset()
            }
            .buttonStyle(BlueButtonStyle())
            
            Spacer()
        }
        .padding()
    }
}

使用 "async let "來模擬并行下載多個(gè)文件的情況