On my UIView, I have 3 UIImageViews to load 3 image's url. As we all know, 3 UIImageView load images has time difference , But I have a requirement: After 3 images all downloaded, then 3 UIImageViews show these 3 images at the same time, how to do?

CodePudding user response：

So if I understood correctly you want to firstly download all 3 images and only then display them? If so I'd suggest using a DispatchGroup.

let images: [URL] = []
    let dispatchGroup = DispatchGroup()
    
    for image in images {
        dispatchGroup.enter()
        URLSession.shared.dataTask(with: image) { (data, response, error) in
            // save image data somewhere
            dispatchGroup.leave()
        }.resume()
    }
    
    dispatchGroup.notify(queue: .main) {
        // display images on the image view
    }

Basically what this does is whenever starting a download you enter in a dispatch group and when the download is done you leave it. After every operation left the group you use the notify completion and display what you want.

CodePudding user response：

If you are have minimum support version of iOS 13 and using Xcode 13.2, you can use async and await to achieve this:

func loadImages() {
    Task {
        async let firstImage = loadImage(index: 1)
        async let secondImage = loadImage(index: 2)
        async let thirdImage = loadImage(index: 3)
        let images = await [firstImage, secondImage, thirdImage]
    }
}

func loadImage(index: Int) async -> UIImage {
    let imageURL = URL(string: urlPathString)!
    let request = URLRequest(url: imageURL)
    let (data, _) = try! await URLSession.shared.data(for: request, delegate: nil)
    print("Finished loading image \(index)")
    return UIImage(data: data)!
}

Source: SwiftLee: Async let explained: call async functions in parallel