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TableView单元格显示带有dispatch_async的图像,显示“void函数中意外的非void返回值”问题

dispatch-async tableview image swift

-1

YungCheng Su · 技术社区 · 8 年前

我想在TableViewCell中显示图像。代码如下:

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
    }

但是滚动TableView非常滞后,所以我更改了函数并在其中添加了一些dispatch_async特性。

它在我的getPic函数中显示了“void函数中意外的非void返回值”问题。

在我更改后,代码如下:

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
    }

谁能告诉我怎么修理它?谢谢

1 回复 | 直到 8 年前

Kumar KL ganeshIOS 8 年前

使用异步任务时,不能返回值r对象。该函数在主线程上运行,它不会等待异步任务完成。

让我们用 Closure .

你的代码应该是这样的:

 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!)!)
            }
            }
        }
    }

示例项目已上载到 GIT .