Question

我正试图重写图像缓存以摆脱使用Alamofire的麻烦，这样做时我遇到了错误。以前，我的图片缓存代码是：

let imageCache = NSCache<NSString, UIImage>()

extension UIImageView {

    func loadImageUsingCacheWithUrlString(_ urlString: String) {

        self.image = nil

        // Check cache for image first
        if let cachedImage = imageCache.object(forKey: urlString as NSString) {
            self.image = cachedImage
            return
        }

        // Otherwise fire off a new download
        Alamofire.request(urlString)
            .responseImage { response in

                if let downloadedImage = response.result.value {
                    // image is here.
                    imageCache.setObject(downloadedImage, forKey: urlString as NSString)
                    self.image = downloadedImage
                }
        }
    }
}

示例用法如下：

navBarCell.avatarImageView.loadImageUsingCacheWithUrlString(avatars[indexPath.row])

现在，我已经像这样重写了缓存：

class ImageService {

    static let cache = NSCache<NSString, UIImage>()

    static func downloadImage(url: URL, completion: @escaping (_ image: UIImage?) -> (Void)) {

        URLSession.shared.dataTask(with: url) { (data, response, error) in

            guard let data = data else {return}
            var downloadedImage: UIImage?
            downloadedImage = UIImage(data: data)

            if downloadedImage != nil {
                cache.setObject(downloadedImage!, forKey: url.absoluteString as NSString)
            }
            DispatchQueue.main.async {
                completion(downloadedImage)
            }
        }.resume()
    }

    static func getImage(url: URL, completion: @escaping (_ image: UIImage?) -> (Void)) {

        if let image = cache.object(forKey: url.absoluteString as NSString) {
            completion(image)
        } else {
            downloadImage(url: url, completion: completion)
        }
    }

}

并在与上述相同的情况下使用它：

        if let avatarImageUrlString = self.chatVC?.avatarDictionary.allValues as! [String], let imageUrl = URL(string: avatarImageUrlString) {
            ImageService.getImage(url: imageUrl, completion: { (image) -> (Void) in
                navBarCell.avatarImageView.image = image
            })
        }

我在Initializer for conditional binding must have Optional type, not '[String]'语句中收到错误if let。

我不确定如何处理-我需要[String]数组，但是如果我正确理解错误，它需要是可选的，而不是用as! [String]强制展开？如果这是一个新手问题，我深表歉意，但似乎无法正常工作。

感谢您的指导！

编辑：

我尝试将let avatarImageUrlString....放在if let之外：

        let avatarImageUrlString = self.chatVC?.avatarDictionary.allValues as! [String]

        if let imageUrl = URL(string: avatarImageUrlString) {
            ImageService.getImage(url: imageUrl, completion: { (image) -> (Void) in
                navBarCell.avatarImageView.image = image
            })
        }

但是在Cannot convert value of type '[String]' to expected argument type 'String'行出现错误if let。

我还尝试了删除!并收到错误'[Any]?' is not convertible to '[String]'; did you mean to use 'as!' to force downcast?，并提出了将!放回Replace 'as' with 'as!'

的修复建议。

编辑2：

        if let avatarImageUrlString = self.chatVC?.avatarDictionary.allValues as? [String], let imageUrl = URL(string: avatarImageUrlString[0]) {
            ImageService.getImage(url: imageUrl, completion: { (image) -> (Void) in
                navBarCell.avatarImageView.image = image
            })
        }

编辑3：

        let avatars = self.chatVC?.avatarDictionary.allValues[indexPath.row] as? [String]

        if let avatarImageUrlString = avatars, let imageUrl = URL(string: avatarImageUrlString[0]) {
            ImageService.getImage(url: imageUrl, completion: { (image) -> (Void) in
                navBarCell.avatarImageView.image = image
            })
        }

Answer 1

您实际上是将allValues强制展开为[String]，所以没有什么可以降低的。还将类型从字符串数组更改为单个字符串

因此，而不是在可选绑定中

as! [String]

使用此

as? String

所以...

if let avatarImageUrlString = self.chatVC?.avatarDictionary.allValues[indexPath.row] as? String, let imageUrl = URL(string: avatarImageUrlString) {
    ...
}

使用图像缓存时解开[String]的问题

1 个答案: