NSCache第一次加载时不适用于所有图像 [英] NSCache Doesn't work with all images when loading for the first time

问题描述

我正在使用swift 3.0中的一个项目，我通过使用NSCache来缓存来自服务器的响应，以便在UITableView中填充它们。然而由于某种原因，我只看到第一次加载应用程序时加载的图片很少，但是如果我滚动并返回我看到的一切（从服务器检索响应的结束我也重新加载我的tableview，但似乎不是这样）。我不确定我在这里错过了什么，下面的代码显示我如何缓存图像。

I'm woking on a project in swift 3.0 where I cache the response from the server by using NSCache as to populate them in a UITableView. However for some reason I'm only seeing few images loading when the app loads for the first time, but if If i scroll and come back I see everything (end of retrieving the response from the server I reload my tableview too, but seems that not the case). I'm not sure what I''m exactly missing here, the code as bellow as to show how I cache the images.

let imageCache = NSCache<AnyObject, AnyObject>()
var imageURLString : String?

extension UIImageView {


    public func imageFromServerURL(urlString: String) {
        imageURLString = urlString

        if let url = URL(string: urlString) {

            image = nil


            if let imageFromCache = imageCache.object(forKey: urlString as AnyObject) as? UIImage {

                self.image = imageFromCache

                return
            }

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

                if error != nil{
                    print(error as Any)


                    return
                }

                DispatchQueue.main.async(execute: {

                    if let imgaeToCache = UIImage(data: data!){

                        if imageURLString == urlString {
                            self.image = imgaeToCache
                        }

                        imageCache.setObject(imgaeToCache, forKey: urlString as AnyObject)// calls when scrolling
                    }
                })
            }) .resume()
        }
    }
}

推荐答案

这里是图片s正在下载并存储在缓存中就好了。问题在于tableview单元格的更新。

Here the images are downloading and stored in cache just fine. The problem lies in the updation of tableview cells.

当表格视图将单元格加载到表格时，图像尚未下载。但是一旦下载了图像，我们必须有选择地更新单元格，以便立即显示图像。

When the table view is loading the cells on to the table the images are not downloaded yet. But once the image is downloaded we have to selectively update the cell so that the image is displayed instantly.

由于您正在滚动，因此tableview再次调用'cellForRowatIndexpath'更新滚动时显示下载图像的单元格。

Since you are scrolling , the tableview calls 'cellForRowatIndexpath' again which updates the cell showing the downloaded images while scrolling.

如果您仍希望使用扩展名，我建议您添加tableView和indexpath作为参数，以便我们可以调用reload特定行并立即更新视图。

If you still wish to use the extension , I suggest you add the tableView and indexpath as the parameters so that we can call reload specific row and have the view updated instantly.

我更新了表重新加载代码和扩展中定义的函数结构。让我知道它是怎么回事。

I have updated the table reload code and structure of the function defined in extension. Let me know how it goes.

let imageCache = NSCache<AnyObject, AnyObject>()
var imageURLString : String?

extension UIImageView {


public func imageFromServerURL(urlString: String, tableView : UITableView, indexpath : IndexPath)) {
    imageURLString = urlString

    if let url = URL(string: urlString) {

        image = nil


        if let imageFromCache = imageCache.object(forKey: urlString as AnyObject) as? UIImage {

            self.image = imageFromCache

            return
        }

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

            if error != nil{
                print(error as Any)


                return
            }

            DispatchQueue.main.async(execute: {

                if let imgaeToCache = UIImage(data: data!){

                    if imageURLString == urlString {
                        self.image = imgaeToCache
                    }

                    imageCache.setObject(imgaeToCache, forKey: urlString as AnyObject)// calls when scrolling

    tableView.reloadRows(at: [indexpath], with: .automatic)

                }
            })
        }) .resume()
    }
}

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