swift mvvm architecture ios patterns

MVVM architecture pattern for iOS

Sofia Martinez Jan 2026

2 tabs

import SwiftUI

struct PostDetailView: View {
    @StateObject private var viewModel: PostDetailViewModel

    init(postId: Int) {
        _viewModel = StateObject(wrappedValue: PostDetailViewModel(postId: postId))
    }

    var body: some View {
        ScrollView {
            VStack(alignment: .leading, spacing: 16) {
                if viewModel.isLoading {
                    ProgressView()
                } else if let post = viewModel.post {
                    Text(post.title)
                        .font(.title)
                        .fontWeight(.bold)

                    HStack {
                        Image(systemName: "person.circle")
                        Text(post.author.name)
                            .font(.subheadline)
                        Spacer()
                        Text(post.createdAt, style: .date)
                            .font(.caption)
                            .foregroundColor(.secondary)
                    }

                    Divider()

                    Text(post.body)
                        .font(.body)

                    HStack(spacing: 20) {
                        Button(action: viewModel.toggleLike) {
                            Label("\(viewModel.likesCount)", systemImage: viewModel.isLiked ? "heart.fill" : "heart")
                        }
                        .foregroundColor(viewModel.isLiked ? .red : .primary)

                        Label("\(post.commentsCount)", systemImage: "bubble.left")
                    }
                    .padding(.top)

                    CommentsSection(comments: viewModel.comments)
                } else if let error = viewModel.errorMessage {
                    Text(error)
                        .foregroundColor(.red)
                }
            }
            .padding()
        }
        .navigationTitle("Post")
        .onAppear {
            viewModel.loadPost()
        }
    }
}

import Foundation
import Combine

class PostDetailViewModel: ObservableObject {
    @Published var post: Post?
    @Published var comments: [Comment] = []
    @Published var isLoading = false
    @Published var errorMessage: String?
    @Published var isLiked = false
    @Published var likesCount = 0

    private let postId: Int
    private let networkService: NetworkService
    private var cancellables = Set<AnyCancellable>()

    init(postId: Int, networkService: NetworkService = .shared) {
        self.postId = postId
        self.networkService = networkService
    }

    func loadPost() {
        isLoading = true
        errorMessage = nil

        Publishers.Zip(
            networkService.fetchPost(id: postId),
            networkService.fetchComments(postId: postId)
        )
        .sink(
            receiveCompletion: { [weak self] completion in
                self?.isLoading = false
                if case .failure(let error) = completion {
                    self?.errorMessage = error.localizedDescription
                }
            },
            receiveValue: { [weak self] post, comments in
                self?.post = post
                self?.comments = comments
                self?.isLiked = post.likedByCurrentUser
                self?.likesCount = post.likesCount
            }
        )
        .store(in: &cancellables)
    }

    func toggleLike() {
        guard let post = post else { return }

        // Optimistic update
        isLiked.toggle()
        likesCount += isLiked ? 1 : -1

        networkService.toggleLike(postId: post.id)
            .sink(
                receiveCompletion: { [weak self] completion in
                    if case .failure = completion {
                        // Rollback on error
                        self?.isLiked.toggle()
                        self?.likesCount += (self?.isLiked ?? false) ? -1 : 1
                    }
                },
                receiveValue: { _ in }
            )
            .store(in: &cancellables)
    }
}

2 files · swift Explain with highlit

MVVM (Model-View-ViewModel) separates concerns cleanly in iOS apps. Models hold data, Views display UI, and ViewModels mediate between them with business logic and state. Views bind to ViewModel properties using Combine or SwiftUI's property wrappers. ViewModels expose @Published properties that Views observe. This separation makes code testable—ViewModels can be unit tested without UI, and Views become thin presentation layers. I keep ViewModels framework-agnostic, importing only Foundation, not UIKit or SwiftUI. Navigation and coordination logic goes in Coordinators. This architecture scales well from small to large apps while maintaining clear boundaries.