How to use results from Swift completion handler?

Question

I'm new to Swift and SwiftUI.

In my macOS SwiftUI project, I'm trying to verify that a URL is reachable so I can present one of two views conditionally. One view which loads the image URL, another one which displays an error image if the URL is not reachable.

Here's my URL extension with completion:

import Foundation

extension URL {
    func isReachable(completion: @escaping (Bool) -> Void) {
        var request = URLRequest(url: self)
        request.httpMethod = "HEAD"
        request.timeoutInterval = 1.0
        
        URLSession.shared.dataTask(with: request) { data, response, error in
            if error != nil {
                DispatchQueue.main.async {
                    completion(false)
                }
                return
            }
            if let httpResp: HTTPURLResponse = response as? HTTPURLResponse {
                DispatchQueue.main.async {
                    completion(httpResp.statusCode == 200)
                }
                return
            } else {
                DispatchQueue.main.async {
                    completion(false)
                }
                return
            }
        }.resume()
    }
}

Elsewhere, I'm trying to use that in a model-view:

var imageURL: URL? {
    if let url = self.book.image_url {
        return URL(string: url)
    } else {
        return nil
    }
}

var imageURLIsReachable: Bool {
    if let url = self.imageURL {
        url.isReachable { result in
            return result  // Error: Cannot convert value of type 'Bool' to closure result type 'Void'
        }
    } else {
        return false
    }
}

Though Xcode is showing this error:

Cannot convert value of type 'Bool' to closure result type 'Void'

What am I doing wrong?

Answer 1

I got this to work after reading some of the comments here and doing more research/experimentation. Here's what I changed:

In the URL extension, I left it pretty much the same as I find it more readable this way. I did push the timeoutInterval to a parameter:

// Extensions/URL.swift


import Foundation

extension URL {
    func isReachable(timeoutInterval: Double, completion: @escaping (Bool) -> Void) {
        var request = URLRequest(url: self)
        request.httpMethod = "HEAD"
        request.timeoutInterval = timeoutInterval
        
        URLSession.shared.dataTask(with: request) { data, response, error in
            if error != nil {
                DispatchQueue.main.async {
                    completion(false)
                }
                return
            }
            if let httpResp: HTTPURLResponse = response as? HTTPURLResponse {
                DispatchQueue.main.async {
                    completion(httpResp.statusCode == 200)
                }
                return
            } else {
                DispatchQueue.main.async {
                    completion(false)
                }
                return
            }
        }.resume()
    }
}

I modified my BookViewModel to make two of the properties to @Published and used the URL extension there:

// View Models/BookViewModel.swift

import Foundation

class BookViewModel: ObservableObject {
    @Published var book: Book
    @Published var imageURLIsReachable: Bool
    @Published var imageURL: URL?
    
    init(book: Book) {
        self.book = book
        self.imageURL = nil
        self.imageURLIsReachable = false
        if let url = book.image_url {
            self.imageURL = URL(string: url)
            self.imageURL!.isReachable(timeoutInterval: 1.0) { result in
                self.imageURLIsReachable = result
            }
        }
    }
    
    // Rest of properties...
}

Now my BookThumbnailView can properly display the conditional views:

// Views/BookThumbnailView.swift

import SwiftUI
import Foundation
import KingfisherSwiftUI

struct BookThumbnailView: View {
    @ObservedObject var viewModel: BookViewModel
        
    private var book: Book {
        viewModel.book
    }
    
    @ViewBuilder
    var body: some View {
        if let imageURL = self.viewModel.imageURL {
            if self.viewModel.imageURLIsReachable {
                KFImage(imageURL)
                        .resizable()
                        .aspectRatio(contentMode: .fit)
                        .frame(maxWidth: 70)
                        .cornerRadius(8)
            } else {
                ErrorBookThumbnailView()
            }
        } else {
            DefaultBookThumbnailView()
        }
    }
}

Whew, that was quite the learning experience. Thanks to everyone who commented with suggestions and provided hints on where to look!