Feature response delegate

[onevcat]

A generic way to implement #2043

This adds a delegate method below:

protocol ImageDownloaderDelegate {
    func imageDownloader(
        _ downloader: ImageDownloader,
        didReceive response: URLResponse,
        completionHandler: @escaping (URLSession.ResponseDisposition) -> Void)
}

It is not possible to setup a delegate for the ImageDownloader and determine a response disposition when a response is received. Some example usage:

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        ImageDownloader.default.delegate = self
    }
}

extension ViewController: ImageDownloaderDelegate {
    func imageDownloader(_ downloader: ImageDownloader, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -> Void) {
        guard let httpResponse = response as? HTTPURLResponse else {
            completionHandler(.cancel)
            return
        }
        let value = httpResponse.value(forHTTPHeaderField: "content-type")

        // Only allow JPEG images, based on the response header.
        completionHandler(value == "image/jpeg" ? . allow : .cancel)
    }
}

Now, loading a PNG:

KF.url(somePNGImageURL)
    .onFailure { err in print("Error: \(err)") }

Will give you a ResponseErrorReason.cancelledByDelegate error.

[rztime]

guard let httpResponse = response as? HTTPURLResponse else {
    completionHandler(.cancel)
    return
}
let value = httpResponse.value(forHTTPHeaderField: "content-type")
completionHandler(value == "image/jpeg" ? . allow : .cancel)

这里建议直接使用

let value = response.mimeType

当url为本地file:///xxxx.jpeg的图片时，response 不是一个 HTTPURLResponse 会导致错误行为

[onevcat]

这里只是一个可行示例，ImageDownloaderDelegate 中的相关方法可由使用方进行具体实现。