ios - Swift 延迟加载属性 Alamofire 请求

Question

我正在尝试使用 Alamofire 请求延迟加载属性 (UIImage)。这就是我所在的位置:

public lazy var theImage: UIImage = MyCalass.loadImage { (imageTest) -> Void in
    return imageTest
}

class public func loadImage(completion: (imageTest: UIImage) -> Void){
    Alamofire.request(.GET, "http://pathtoimage/image.jpg").response { (request, response, data, error) in
        var tempImage = UIImage(data: data!, scale:1)
        completion(imageTest: tempImage!)
    }
}

错误:“()”无法转换为“UIImage”

Answer 1

嘿伙计，用 GCD 很容易解决...

像这样构建一个类:

class AsynchImageLoader {
    class func loadImageAsync (imageURL: NSURL, completionHandler: (downloadedImage: UIImage?) -> Void) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), { () -> Void in
            if let imageData: NSData = NSData(contentsOfURL: imageURL) {
                if let imageImage: UIImage = UIImage(data: imageData) {
                    dispatch_async(dispatch_get_main_queue(), { () -> Void in
                        completionHandler(downloadedImage: imageImage)
                    })
                } else {
                    dispatch_async(dispatch_get_main_queue(), { () -> Void in
                        completionHandler(downloadedImage: nil)
                    })
                }
            } else {
                dispatch_async(dispatch_get_main_queue(), { () -> Void in
                    completionHandler(downloadedImage: nil)
                })
            }
        })
    }
}

然后像这样加载你的图片...

let imageView: UIImageView = UIImageView()
        AsynchImageLoader.loadImageAsync(NSURL(string: "yourURL")!, completionHandler: { (downloadedImage) -> Void in
            imageView.image = downloadedImage
        })

downloadedImage 可以是 nil，如果出现问题...否则您可以将其放入您的 View 、数组或任何您想要的...

顺便说一下...惰性修饰符并不意味着图像加载为“惰性/异步”...在 Swift 中，您可以将某些东西声明/初始化为惰性，这意味着它将在其初始化时声明类，但在您第一次尝试在代码中访问它之前不久，它会首先被初始化……这可能是任何时候，甚至永远不会:-)

或者从这里更改您的代码...

public lazy var theImage: UIImage = MyCalass.loadImage { (imageTest) -> Void in
    return imageTest
}

为此...

var theImage: UIImage? 
MyCalass.loadImage { (imageTest) -> Void in
    theImage = imageTest
}