使用dispatch_async的TableView单元格显示图像显示“void函数中的意外非空返回值”。问题 [英] TableView Cell show image with dispatch_async shows "unexpected non-void return value in void function" issue

问题描述

我想在TableViewCell中显示图像。有代码：

I want to show the image in the TableViewCell. There are the codes:

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cellIdentifier = "myCell"
        let cell = tableView.dequeueReusableCellWithIdentifier(cellIdentifier, forIndexPath: indexPath) as! DIYSquareALLCell
        cell.titles!.text = titles[indexPath.row]
        cell.leftImages!.image = getPic(leftImages[indexPath.row])
        return cell
    }

func getPic(PicURL: String) -> UIImage! {
        let image = self.imageCache[PicURL] as UIImage?
        if image == nil {
            let url = NSURL(string: PicURL.stringByAddingPercentEncodingWithAllowedCharacters(NSCharacterSet.URLQueryAllowedCharacterSet())!)
            if let data = NSData(contentsOfURL: url!) {
                imageCache[PicURL] = UIImage(data: data)
                return UIImage(data: data)!
            }
        } else {
            return image
        }
        return nil
    }

但滚动TableView非常滞后所以我更改了函数并在其中添加了一些dispatch_async功能。

But scrolling the TableView is very lag so I change the function and add some dispatch_async feature in it.

它在我的getPic函数中显示了void函数中意外的非空返回值的问题。

It shows the issue "unexpected non-void return value in void function" in my getPic function.

更改后，有代码：

func getPic(PicURL: String) -> UIImage! {
        let image = self.imageCache[PicURL] as UIImage?
        if image == nil {
            let url = NSURL(string: PicURL.stringByAddingPercentEncodingWithAllowedCharacters(NSCharacterSet.URLQueryAllowedCharacterSet())!)
            let priority = DISPATCH_QUEUE_PRIORITY_HIGH
            dispatch_async(dispatch_get_global_queue(priority, 0)) {
                let data = NSData(contentsOfURL: url!)
                if data != nil {
                    dispatch_async(dispatch_get_main_queue(), { () -> Void in
                        self.imageCache[PicURL] = UIImage(data: data!)
                        return UIImage(data: data!)// here is the issue
                    })
                }
            }
        } else {
            return image
        }
        return nil
    }

任何人都可以告诉我如何解决它？谢谢！

Anyone can tell me how to fix it? Thanks!

推荐答案

使用异步任务时，无法返回对象的值，正在运行的函数主线程，它不会等待你的异步任务完成。

You can't return a value r an object when using the Asynchronous task, The function which is running on the main thread and it won't wait for your async task to be finish.

让我们用关闭。

你的代码应该是这样的：

Your code should be like this:

 typealias CompletionHandler = (image: UIImage) -> Void
    override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell: testCell = tableView.dequeueReusableCellWithIdentifier("cell", forIndexPath: indexPath) as! testCell
        downloadFileFromURL(NSURL(string: "http://img.youtube.com/vi/PCwL3-hkKrg/sddefault.jpg")!, completionHandler:{(img) in
             dispatch_async(dispatch_get_main_queue(), { () -> Void in
                cell.imgView.image = img
             })

            })
        return cell
    }

    func downloadFileFromURL(url1: NSURL?,completionHandler: CompletionHandler) {
        // download code.
        if let url = url1{
        let priority = DISPATCH_QUEUE_PRIORITY_HIGH
        dispatch_async(dispatch_get_global_queue(priority, 0)) {
            let data = NSData(contentsOfURL: url)
            if data != nil {
                print("image downloaded")
                completionHandler(image: UIImage(data: data!)!)
            }
            }
        }
    }

上传到 GIT 。

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