swift - 呈现 Controller 时呈现加载图像的延迟

Question

我正在重新创建一个简单版本的 snapchat 应用程序，但遇到了一个恼人的小问题，我似乎无法找到解决办法。

我查看了一些答案，例如 this one但没有运气。

总体概念与 snapchat 用户单击单元格相同，图像显示 x 时间然后消失。

以下是当前发生的情况:

正如您所看到的，单击和实际呈现的图像之间存在轻微的延迟。

这是我用来执行实际图像加载的 CustomeIvageView:

    import UIKit

    class CustomImageView: UIImageView {

        func loadImage(urlString: String){

            guard let url = URL(string: urlString) else {return}
            URLSession.shared.dataTask(with: url) { (data, response, err) in
                if let err = err {
                    print("failed to fetch user profile photoes", err)
                }

                guard let imageData = data else {return}
                let photoImage = UIImage(data: imageData)


                DispatchQueue.main.async {
                    self.image = photoImage
                }
            }.resume()
        }
    }

然后在我的 View Controller 中，我有这个:

class PictureViewController: UIViewController {

    var snap: Snap? {
        didSet{
            usernameLabel.text = snap?.fromUser?.username
            guard let imageUrl = snap?.imageUrl else {return}
            previewImageView.loadImage(urlString: imageUrl)
        }
    }

    lazy var previewImageView: CustomImageView = {
        let iv = CustomImageView()
        iv.contentMode = .scaleAspectFill
        return iv
    }()


    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(previewImageView)
        previewImageView.anchor(top: view.topAnchor, left: view.leftAnchor, bottom: view.bottomAnchor, right: view.rightAnchor, paddingTop: 0, paddingLeft: 0, paddingBottom: 0, paddingRight: 0, width: 0, height: 0)


        guard let timer = snap?.timer else {return}

        let delayTime = Double(timer)
        delay(delayTime){
            self.handleCancel()
        }
    }

    func delay(_ delay:Double, closure:@escaping ()->()) {
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Double(Int64(delay * Double(NSEC_PER_SEC))) / Double(NSEC_PER_SEC), execute: closure)
    }

    func handleCancel(){
        self.dismiss(animated: false) {
        }
    }

编辑:显示我设置 PictureViewController 的位置。为了简洁起见，我省略了代码。

import UIKit
import Firebase

class ChatViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout{

    var recievedSnaps = [Snap]()
    var mappedSnaps = [String: [Snap]]()
    let cellId = "cellId"

    let picMessageView = PictureViewController()

    override func viewDidLoad() {
        super.viewDidLoad()

        //Removed code for brevity

    }

     //OMITTED CODE FOR BREVITY


    override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        let cell = collectionView.cellForItem(at: indexPath) as! ChatCell

        picMessageView.snap = cell.snap

        self.present(picMessageView, animated: false, completion: nil)


    }

}

Answer 1

我尝试了您的代码，除了您关闭 PictureViewController 时的部分之外，它工作正常。您需要在主UI线程中调用dismiss，这样它才能正确执行。

func handleCancel(){
    DispatchQueue.main.async {
      self.dismiss(animated: false, completion: nil)      
    }
}