SwiftUI View - viewDidLoad()?

Question

Trying to load an image after the view loads, the model object driving the view (see MovieDetail below) has a urlString. Because a SwiftUI View element has no life cycle methods (and there's not a view controller driving things) what is the best way to handle this?

The main issue I'm having is no matter which way I try to solve the problem (Binding an object or using a State variable), my View doesn't have the urlString until after it loads...

// movie object
struct Movie: Decodable, Identifiable {
    
    let id: String
    let title: String
    let year: String
    let type: String
    var posterUrl: String
    
    private enum CodingKeys: String, CodingKey {
        case id = "imdbID"
        case title = "Title"
        case year = "Year"
        case type = "Type"
        case posterUrl = "Poster"
    }
}

// root content list view that navigates to the detail view
struct ContentView : View {
    
    var movies: [Movie]
    
    var body: some View {
        NavigationView {
            List(movies) { movie in
                NavigationButton(destination: MovieDetail(movie: movie)) {
                    MovieRow(movie: movie)
                }
            }
            .navigationBarTitle(Text("Star Wars Movies"))
        }
    }
}

// detail view that needs to make the asynchronous call
struct MovieDetail : View {
    
    let movie: Movie
    @State var imageObject = BoundImageObject()
    
    var body: some View {
        HStack(alignment: .top) {
            VStack {
                Image(uiImage: imageObject.image)
                    .scaledToFit()
                
                Text(movie.title)
                    .font(.subheadline)
            }
        }
    }
}

Answer 1

We can achieve this using view modifier.

1. Create ViewModifier:

struct ViewDidLoadModifier: ViewModifier {

    @State private var didLoad = false
    private let action: (() -> Void)?

    init(perform action: (() -> Void)? = nil) {
        self.action = action
    }

    func body(content: Content) -> some View {
        content.onAppear {
            if didLoad == false {
                didLoad = true
                action?()
            }
        }
    }

}

2. Create View extension:

extension View {

    func onLoad(perform action: (() -> Void)? = nil) -> some View {
        modifier(ViewDidLoadModifier(perform: action))
    }

}

3. Use like this:

struct SomeView: View {
    var body: some View {
        VStack {
            Text("HELLO!")
        }.onLoad {
            print("onLoad")
        }
    }
}

Answer 2

I hope this is helpful. I found a blogpost that talks about doing stuff onAppear for a navigation view.

Idea would be that you bake your service into a BindableObject and subscribe to those updates in your view.

struct SearchView : View {
    @State private var query: String = "Swift"
    @EnvironmentObject var repoStore: ReposStore

    var body: some View {
        NavigationView {
            List {
                TextField($query, placeholder: Text("type something..."), onCommit: fetch)
                ForEach(repoStore.repos) { repo in
                    RepoRow(repo: repo)
                }
            }.navigationBarTitle(Text("Search"))
        }.onAppear(perform: fetch)
    }

    private func fetch() {
        repoStore.fetch(matching: query)
    }
}

import SwiftUI
import Combine

class ReposStore: BindableObject {
    var repos: [Repo] = [] {
        didSet {
            didChange.send(self)
        }
    }

    var didChange = PassthroughSubject<ReposStore, Never>()

    let service: GithubService
    init(service: GithubService) {
        self.service = service
    }

    func fetch(matching query: String) {
        service.search(matching: query) { [weak self] result in
            DispatchQueue.main.async {
                switch result {
                case .success(let repos): self?.repos = repos
                case .failure: self?.repos = []
                }
            }
        }
    }
}

Credit to: Majid Jabrayilov

Answer 3

Fully updated for Xcode 11.2, Swift 5.0

I think the viewDidLoad() just equal to implement in the body closure.
SwiftUI gives us equivalents to UIKit’s viewDidAppear() and viewDidDisappear() in the form of onAppear() and onDisappear(). You can attach any code to these two events that you want, and SwiftUI will execute them when they occur.

As an example, this creates two views that use onAppear() and onDisappear() to print messages, with a navigation link to move between the two:

struct ContentView: View {
    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: DetailView()) {
                    Text("Hello World")
                }
            }
        }.onAppear {
            print("ContentView appeared!")
        }.onDisappear {
            print("ContentView disappeared!")
        }
    }
}

ref: https://www.hackingwithswift.com/quick-start/swiftui/how-to-respond-to-view-lifecycle-events-onappear-and-ondisappear

Answer 4

I'm using init() instead. I think onApear() is not an alternative to viewDidLoad(). Because onApear is called when your view is being appeared. Since your view can be appear multiple times it conflicts with viewDidLoad which is called once.

Imagine having a TabView. By swiping through pages onApear() is being called multiple times. However viewDidLoad() is called just once.