Concurrency Demystified
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Now, you’ll go over the improvements you applied in this lesson.
Start Xcode and open the starter project in the folder 03-background-tasks-made-easy-with-async-await.
AsyncImage(url: URL(string: url)) { image in
image
.resizable()
.aspectRatio(contentMode: .fit)
.background(.clear)
.mask(RoundedRectangle(cornerRadius: 8))
} placeholder: {
ProgressView()
.frame(alignment: .center)
}
.frame(maxWidth: .infinity, alignment: .center)
image
.resizable()
.aspectRatio(contentMode: .fit)
.background(.clear)
.mask(RoundedRectangle(cornerRadius: 8))
To start downloading the news when the app is launched, you used the .task
modifier on the view where you want the task to start. .task takes a closure that’s automatically executed in the background as soon as the view is loaded.
@State private var isLoading = false
.overlay {
if isLoading {
ProgressView()
} else if shouldPresentContentUnavailable {
ContentUnavailableView {
Label("Latest News", systemImage: "newspaper.fill")
}
}
}
Button("Load Latest News") { newsViewModel.fetchLatestNews() }
.task {
isLoading = true
await newsViewModel.fetchLatestNews()
isLoading = false
}
@MainActor
func fetchLatestNews() async {
news.removeAll()
Task {
let news = try? await newsService.latestNews()
self.news = news ?? []
}
}
SwiftUI natively supports the pull-to-refresh gesture. To add this feature to your app, you just need to add the .refreshable
modifier to the view that you want to refresh.
.refreshable {
await newsViewModel.fetchLatestNews()
}
Open the file NewsView.swift, and make the following changes:
@Environment(\.openURL)
var openURL
.onTapGesture {
if let url = article.url {
openURL(url)
}
}
var body: some View {
VStack(alignment: .center)NavigationStack {
List {
ForEach(newsViewModel.news, id: \.url) { article in
ArticleView(article: article)
.listRowSeparator(.hidden)
.onTapGesture {
if let url = article.url {
openURL(url)
}
}
}
}
.navigationTitle("Latest Apple News")
.listStyle(.plain)
First, add the Persistence component in charge of downloading and saving the article’s image.
import OSLog
actor Persistence {
func saveToDisk(_ article: Article) {
guard let fileURL = fileName(for: article) else {
Logger.main.error("Can't build filename for article: \(article.title)")
return
}
guard let imageURL = article.urlToImage, let url = URL(string: imageURL) else {
Logger.main.error("Can't build image URL for article: \(article.title)")
return
}
Task.detached(priority: .background) {
guard let (downloadedFileURL, response) = try? await URLSession.shared.download(from: url) else {
Logger.main.error("URLSession error when downloading article's image at: \(imageURL)")
return
}
guard let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode == 200 else {
Logger.main.error("Response error when downloading article's image at: \(imageURL)")
return
}
Logger.main.info("File downloaded to: \(downloadedFileURL.absoluteString)")
do {
if FileManager.default.fileExists(atPath: fileURL.path) {
try FileManager.default.removeItem(at: fileURL)
}
try FileManager.default.moveItem(at: downloadedFileURL, to: fileURL)
Logger.main.info("File saved successfully to: \(fileURL.absoluteString)")
} catch {
Logger.main.error("File copy failed with: \(error.localizedDescription)")
}
}
}
private func fileName(for article: Article) -> URL? {
let fileName = article.title
guard let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
return nil
}
return documentsDirectory.appendingPathComponent(fileName)
}
}
Task.detached(priority: .background) {
...
}
guard let (downloadedFileURL, response) = try? await URLSession.shared.download(from: url) else {
...
}
do {
if FileManager.default.fileExists(atPath: fileURL.path) {
try FileManager.default.removeItem(at: fileURL)
}
try FileManager.default.moveItem(at: downloadedFileURL, to: fileURL)
Logger.main.info("File saved successfully to: \(fileURL.absoluteString)")
} catch {
Logger.main.error("File copy failed with: \(error.localizedDescription)")
}
let persistence: Persistence
@Environment(\.openURL)
var openURL
HStack {
Text(article.publishedAt?.formatted() ?? "Date not available")
.font(.caption)
Spacer()
Button("", systemImage: "square.and.arrow.up") {
if let url = article.url {
openURL(url)
}
}
Button("", systemImage: "square.and.arrow.down") {
Task { await persistence.saveToDisk(article) }
}
}
.buttonStyle(BorderlessButtonStyle())
ArticleView(article: .sample, persistence: Persistence())
private let persistence = Persistence()
ForEach(newsViewModel.news, id: \.url) { article in
ArticleView(article: article, persistence: persistence)
.listRowSeparator(.hidden)
.onTapGesture {
if let url = article.url {
openURL(url)
}
}
}