tableview中单元格内部的自动布局图像。正确的布局，但只有向下和向上滚动一次？

Question

我正在尝试通过自动调整图像大小来获取tableview单元格。基本上，我希望单元格中的图像宽度总是相同，并且高度要根据图像的长宽比而变化。

我创建了一个单元格类，该类仅具有标签的出口，imageView和用于图像高度的NSLayoutConstraint。我有一些异步方法来下载图像并将其设置为单元格imageView的图像。然后调用完成句柄，然后运行以下代码将高度约束调整为正确的高度：

cell.cellPhoto.loadImageFromURL(url: photos[indexPath.row].thumbnailURL, completion: {
            // Set imageView height to the width
            let imageSize = cell.cellPhoto.image?.size
            let maxHeight = ((self.tableView.frame.width-30.0)*imageSize!.height) / imageSize!.width
            cell.cellPhotoHeight.constant = maxHeight
            cell.layoutIfNeeded()
        })
        return cell

这是我编写的加载图像的UIImageView扩展：

func loadImageFromURL(url: String, completion: @escaping () -> Void) {
        let url = URL(string: url)
        makeDataRequest(url: url!, completion: { data in
            DispatchQueue.main.async {
                self.image = UIImage(data: data!)
                completion()
            }
        })
    }

以及它调用的makeDataRequest函数：

func makeDataRequest(url: URL, completion: @escaping (Data?) -> Void) {
    let session = URLSession.shared

    let task = session.dataTask(with: url, completionHandler: { data, response, error in
        if error == nil {
            let response = response as? HTTPURLResponse
            switch response?.statusCode {
                case 200:
                    completion(data)
                case 404:
                    print("Invalid URL for request")
                default:
                    print("Something else went wrong in the data request")
            }
        } else {
            print(error?.localizedDescription ?? "Error")
        }
    })

    task.resume()
}

这适用于所有超出帧的单元格，但是帧中的单元格中的图像视图很小。只有当我向下滚动然后再次向上备份时，它们的大小才正确。我该如何解决？我知道其他人也遇到了这个问题，但是尝试进行修复无济于事。

Answer 1

我不得不重新创建问题以了解发生了什么。基本上，您需要重新加载表视图。图片下载完成后，我将执行此操作。

在具有表视图var的视图控制器中。将此添加到viewDidLoad()功能。

   override func viewDidLoad() {
        super.viewDidLoad()
        tableView.delegate = self
        tableView.dataSource = self

        //Create a notification so we can update the list from anywhere in the app. Good if you are calling this from an other class.
         NotificationCenter.default.addObserver(self, selector: #selector(loadList), name: NSNotification.Name(rawValue: "loadList"), object: nil)

    }

   //This function updates the cells in the table view
   @objc func loadList(){
          //load data here
          self.tableView.reloadData()
      }

现在，在完成照片下载后，您可以使用以下方法通知视图控制器重新加载表格视图，

func loadImageFromURL(url: String, completion: @escaping () -> Void) {
        let url = URL(string: url)
        makeDataRequest(url: url!, completion: { data in
            DispatchQueue.main.async {
                self.image = UIImage(data: data!)
                completion()
                //This isn't the best way to do this as, if you have 25+ pictures,
                //the list will pretty much freeze up every time the list has to be reloaded.
                //What you could do is have a flag to check if the first 'n' number of cells 
                //have been loaded, and if so then don't reload the tableview.

                //Basically what I'm saying is, if the cells are off the screen who cares.
                NotificationCenter.default.post(name: NSNotification.Name(rawValue: "loadList"), object: nil)
            }
        })
    }

[为了提高异步性，我做了一些事情，请参见下文。

我的代码如下，我没有像您那样做缩放比例的事情，但是同样的想法适用。这就是您如何重新加载表格视图的方法。另外，我个人不喜欢编写自己的下载代码，状态代码和所有内容。这不好玩，为什么别人做完后重新发明轮子呢？

Podfile

pod 'SDWebImage',  '~> 5.0' 

mCell.swift

class mCell: UITableViewCell {
    //This keeps track to see if the cell has been already resized. This is only needed once.
    var flag = false
    @IBOutlet weak var cellLabel: UILabel!
    @IBOutlet weak var cell_IV: UIImageView!

    override func awakeFromNib() { super.awakeFromNib() }
}

viewController.swift（单击以查看完整的代码）我将在此处提供代码的重点。

//Set the image based on a url
//Remember this is all done with Async...In the backgorund, on a custom thread.
mCell.cell_IV.sd_setImage(with: URL(string: ViewController.cell_pic_url[row])) { (image, error, cache, urls) in
    // If failed to load image
    if (error != nil) {
        //Set to defult
         mCell.cell_IV.image = UIImage(named: "redx.png")
     }
    //Else we got the image from the web.
    else {
         //Set the cell image to the one we downloaded
         mCell.cell_IV.image = image

         //This is a flag to reload the tableview once the image is done downloading. I set a var in the cell class, this is to make sure the this is ONLY CALLED once. Otherwise the app will get stuck in an infinite loop.
         if (mCell.flag != true){
             DispatchQueue.main.asyncAfter(deadline: .now() + 0.025){ //Nothing wrong with a little lag.
                      NotificationCenter.default.post(name: NSNotification.Name(rawValue: "loadList"), object: nil)
                      mCell.flag = true
                   }
         }



    }
}