Proper way to set up a protocol in Swift 3 - ios

Question

I have a view controller that will show images from Flickr. I put Flickr API request methods in a separate class "FlickrAPIRequests", and now I need to update the image in the view controller when I get the data.
I choose to go with that using a protocol to eliminate the coupling of the two classes. How would I achieve that?

Answer 1

You could define a protocol and set up your FlickrAPIRequests class to take a delegate. I suggest another approach.
Set up your FlickrAPIRequests to have a method that takes a completion handler. It might look like this:
func downloadFileAtURL(_ url: URL, completion: #escaping DataClosure)
The FlickrAPIRequests function would take a URL to the file to download, as well as a block of code to execute once the file has downloaded.
You might use that function like this (In this example the class is called DownloadManager)
DownloadManager.downloadManager.downloadFileAtURL(
url,
//This is the code to execute when the data is available
//(or the network request fails)
completion: {
[weak self] //Create a capture group for self to avoid a retain cycle.
data, error in
//If self is not nil, unwrap it as "strongSelf". If self IS nil, bail out.
guard let strongSelf = self else {
return
}
if let error = error {
print("download failed. message = \(error.localizedDescription)")
strongSelf.downloadingInProgress = false
return
}
guard let data = data else {
print("Data is nil!")
strongSelf.downloadingInProgress = false
return
}
guard let image = UIImage(data: data) else {
print("Unable to load image from data")
strongSelf.downloadingInProgress = false
return
}
//Install the newly downloaded image into the image view.
strongSelf.imageView.image = image
}
)
I have a sample project on Github called Asyc_demo (link) that has uses a simple Download Manager as I've outlined above.
Using a completion handler lets you put the code that handles the completed download right in the call to start the download, and that code has access to the current scope so it can know where to put the image data once it's been downloaded.
With the delegation pattern you have to set up state in your view controller so that it remembers the downloads that it has in progress and knows what to do with them once they are complete.

Answer 2

Write a protocol like this one, or similar:
protocol FlickrImageDelegate: class {
func displayDownloadedImage(flickrImage: UIImage)
}
Your view controller should conform to that protocol, aka use protocol method(s) (there can be optional methods) like this:
class ViewController:UIViewController, FlickrImageDelegate {
displayDownloadedImage(flickrImage: UIImage) {
//handle image
}
}
Then in FlickrAPIRequests, you need to have a delegate property like this:
weak var flickrDelegate: FlickrImageDelegate? = nil
This is used in view controller when instantiating FlickrAPIRequests, set its instance flickrDelegate property to view controller, and in image downloading method,when you download the image, you call this:
self.flickrDelegate.displayDownloadedImage(flickrImage: downloadedImage)
You might consider using callback blocks (closures) in FlickrAPIRequests, and after you chew that up, look into FRP, promises etc :)
Hope this helps

Answer 3

I followed silentBob answer, and it was great except that it didn't work for me. I needed to set FlickrAPIRequests class as a singleton, so here is what I did:
protocol FlickrAPIRequestDelegate{
func showFlickrPhoto(photo: UIImage) }
class FlickrAPIRequests{
private static let _instance = FlickrAPIRequests()
static var Instance: FlickrAPIRequests{
return _instance
}
var flickrDelegate: FlickrAPIRequestDelegate? = nil
// Make the Flickr API call
func flickrAPIRequest(_ params: [String: AnyObject], page: Int){
Then in the view controller when I press the search button:
// Set flickrDelegate protocol to self
FlickrAPIRequests.Instance.flickrDelegate = self
// Call the method for api requests
FlickrAPIRequests.Instance.flickrAPIRequest(paramsDictionary as [String : AnyObject], page: 0)
And Here is the view controller's extension to conform to the protocol:
extension FlickrViewController: FlickrAPIRequestDelegate{
func showFlickrPhoto(photo: UIImage){
self.imgView.image = photo
self.prepFiltersAndViews()
self.dismissAlert()
}
}
When the api method returns a photo I call the protocol method in the main thread:
// Perform this code in the main UI
DispatchQueue.main.async { [unowned self] in
let img = UIImage(data: photoData as Data)
self.flickrDelegate?.showFlickrPhoto(photo: img!)
self.flickrDelegate?.setPhotoTitle(photoTitle: photoTitle)
}