一些观察：
但所有这些都是学术性的。最重要的是，您看不到使用download(for:delegate:) 或download(from:delegate:) 报告的进度。因此，如果您想在下载过程中看到进度，您有以下几种选择：
-
将bytes(from:) 实现为suggested by Won 并在字节进入时更新您的进度。
顺便说一句，我可能会建议将其流式传输到文件（例如，OutputStream）而不是将其附加到Data，以反映下载任务的内存特性。但是，他的回答说明了基本思想。
-
回退到基于委托的downloadTask(with:) 解决方案。
如果您想编写自己的版本来报告进度，您可以执行以下操作：
extension URLSession {
func download(from url: URL, delegate: URLSessionTaskDelegate? = nil, progress parent: Progress) async throws -> (URL, URLResponse) {
try await download(for: URLRequest(url: url), progress: parent)
}
func download(for request: URLRequest, delegate: URLSessionTaskDelegate? = nil, progress parent: Progress) async throws -> (URL, URLResponse) {
let progress = Progress()
parent.addChild(progress, withPendingUnitCount: 1)
let bufferSize = 65_536
let estimatedSize: Int64 = 1_000_000
let (asyncBytes, response) = try await bytes(for: request, delegate: delegate)
let expectedLength = response.expectedContentLength // note, if server cannot provide expectedContentLength, this will be -1
progress.totalUnitCount = expectedLength > 0 ? expectedLength : estimatedSize
let fileURL = URL(fileURLWithPath: NSTemporaryDirectory())
.appendingPathComponent(UUID().uuidString)
guard let output = OutputStream(url: fileURL, append: false) else {
throw URLError(.cannotOpenFile)
}
output.open()
var buffer = Data()
if expectedLength > 0 {
buffer.reserveCapacity(min(bufferSize, Int(expectedLength)))
} else {
buffer.reserveCapacity(bufferSize)
}
var count: Int64 = 0
for try await byte in asyncBytes {
try Task.checkCancellation()
count += 1
buffer.append(byte)
if buffer.count >= bufferSize {
try output.write(buffer)
buffer.removeAll(keepingCapacity: true)
if expectedLength < 0 || count > expectedLength {
progress.totalUnitCount = count + estimatedSize
}
progress.completedUnitCount = count
}
}
if !buffer.isEmpty {
try output.write(buffer)
}
output.close()
progress.totalUnitCount = count
progress.completedUnitCount = count
return (fileURL, response)
}
}
与：
extension OutputStream {
/// Write `Data` to `OutputStream`
///
/// - parameter data: The `Data` to write.
func write(_ data: Data) throws {
try data.withUnsafeBytes { (buffer: UnsafeRawBufferPointer) throws in
guard var pointer = buffer.baseAddress?.assumingMemoryBound(to: UInt8.self) else {
throw OutputStreamError.bufferFailure
}
var bytesRemaining = buffer.count
while bytesRemaining > 0 {
let bytesWritten = write(pointer, maxLength: bytesRemaining)
if bytesWritten < 0 {
throw OutputStreamError.writeFailure
}
bytesRemaining -= bytesWritten
pointer += bytesWritten
}
}
}
}
注意：
-
这使用一个小缓冲区来避免尝试将整个资产一次加载到内存中。它会在执行过程中将结果写入文件。
如果资产可能很大（这通常是我们使用download 而不是data 的原因），这一点很重要。
-
请注意，expectedContentLength 有时可以为 -1，在这种情况下，我们不知道正在下载的文件的大小。以上处理了这种情况。
在资产大小未知时估算进度的逻辑是个人喜好问题。上面我使用了估计的资产规模并调整了进度。它不会非常准确，但至少它反映了下载过程中的一些进度。
-
我包含一个try Task.checkCancellation()，以便下载任务可以取消。
-
我使用Progress 将进度报告给父级。您可以根据需要连接并显示它，但如果您使用的是UIProgressView，则特别简单。
无论如何，您可以执行以下操作：
func startDownloads(_ urls: [URL]) async throws {
let cachesFolder = try! FileManager.default.url(for: .cachesDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
let progress = Progress()
progressView.observedProgress = progress // assuming you are just updating a `UIProgressView` with the overall progress of all the downloads
try await withThrowingTaskGroup(of: Void.self) { group in
progress.totalUnitCount = Int64(urls.count)
for url in urls {
group.addTask {
let destination = cachesFolder.appendingPathComponent(url.lastPathComponent) // obviously, put the resulting file wherever you want
let (url, _) = try await URLSession.shared.download(from: url, progress: progress)
try? FileManager.default.removeItem(at: destination)
try FileManager.default.moveItem(at: url, to: destination)
}
}
try await group.waitForAll()
}
}