How to update data model using DispatchQueue.main.async/Block/Closures in Swift 3

Question

I am learning Swift. I have one Problem.

Problem- I have DataModel with image url, So first time will download image from url and of course second time won't. So when I am getting image in my block I want to update my data model with image. But its not working.

• I have tried with inout function
• Also completionHander call back

ViewController.swift

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

    let cell = tableView.dequeueReusableCell(withIdentifier: "OffersTableCell", for: indexPath) as! OffersTableCell

    var model = offersArray[indexPath.row] as! OffersModel
    cell.updateOfferCellWith(model: model, completionHandler: { image in
        model.image = image
    })///// I am getting image here, but model is not getting update
    return cell
}

Cell.Swift

func updateOfferCellWith(model: OffersModel, completionHandler:@escaping (UIImage) -> Void) {

    if (model.image != nil) { //Second time this block should be execute, but model image is always nil
        DispatchQueue.main.async { [weak self] in
            self?.offerImageView.image = model.image
        }
    }
    else{
        //First time- Download image from URL
        ImageDownloader.downloadImage(imageUrl: model.offerImageURL) { [weak self] image in
            DispatchQueue.main.async {[model] in
                var model = model
                self?.offerImageView.image = image
                //model.image = image*//This is also not working*
                completionHandler(image)*//Call back*
            }
        }
    }
}

Answer 1

From the comments model is a struct.

When passing a struct (or any value type) to a method or assigning it to a variable, the struct is copied.

Therefore model.image = image is actually assigning image to a brand new model and not to the original model.

Using a reference type (a class) will fix it.

Answer 2

Below is a potential solution based on your code, however, this is as vadain said in the comments, a bit dangerous because the image loading is asynchronous and could return the image at a time when the cell no longer exists.

Ideally what you want to do is set up your model in such a way that it is persistent and it should send the async request to download it's image. Then, when the image comes back it update the model and the model broadcasts a message saying that it's image has been update. The cells can listen and respond to updates to the model while they exist.

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

    let cell = tableView.dequeueReusableCell(withIdentifier: "OffersTableCell", for: indexPath) as! OffersTableCell

    var model = offersArray[indexPath.row] as! OffersModel

    if image = model.image {
        // setting directly here... you could call an image on the cell if you want more data encapuslation
        cell.offerImageView.image = model.image
    } else {
        ImageDownloader.downloadImage(imageUrl: model.offerImageURL) { [weak self] image in
            DispatchQueue.main.async {
                model.image = image
                cell.offerImageView.image = image
            }
        }
    }

    return cell
}