這個問題建立在我之前的問題之上。基本上，當按下某個按鈕時，我會對 Google Books Api 進行異步呼叫。雖然當它是 View 的一種方法時我得到了呼叫，但是我想在它加載時覆寫一個活動指示器。因此，我嘗試制作一個 ObservableObject 來進行呼叫，但我不知道該怎么做。
這是我到目前為止所擁有的：
class GoogleBooksApi: ObservableObject {
enum LoadingState<Value> {
case loading(Double)
case loaded(Value)
}
@Published var state: LoadingState<GoogleBook> = .loading(0.0)
enum URLError : Error {
case badURL
}
func fetchBook(id identifier: String) async throws {
var components = URLComponents(string: "https://www.googleapis.com/books/v1/volumes")
components?.queryItems = [URLQueryItem(name: "q", value: "isbn=\(identifier)")]
guard let url = components?.url else { throw URLError.badURL }
self.state = .loading(0.25)
let (data, _) = try await URLSession.shared.data(from: url)
self.state = .loading(0.75)
self.state = .loaded(try JSONDecoder().decode(GoogleBook.self, from: data))
}
}
struct ContentView: View {
@State var name: String = ""
@State var author: String = ""
@State var total: String = ""
@State var code = "ISBN"
@ObservedObject var api: GoogleBooksApi
var body: some View {
VStack {
Text("Name: \(name)")
Text("Author: \(author)")
Text("total: \(total)")
Button(action: {
code = "978-0441013593"
Task {
do {
try await api.fetchBook(id: code)
let fetchedBooks = api.state
let book = fetchedBooks.items[0].volumeInfo
name = book.title
author = book.authors?[0] ?? ""
total = String(book.pageCount!)
} catch {
print(error)
}
}
}, label: {
Rectangle()
.frame(width: 200, height: 100)
.foregroundColor(.blue)
})
}
}
}
// MARK: - GoogleBook
struct GoogleBook: Codable {
let kind: String
let totalItems: Int
let items: [Item]
}
// MARK: - Item
struct Item: Codable {
let id, etag: String
let selfLink: String
let volumeInfo: VolumeInfo
}
// MARK: - VolumeInfo
struct VolumeInfo: Codable {
let title: String
let authors: [String]?
let pageCount: Int?
let categories: [String]?
enum CodingKeys: String, CodingKey {
case title, authors
case pageCount, categories
}
}
這就是沒有加載狀態的作業：
struct ContentView: View {
@State var name: String = ""
@State var author: String = ""
@State var total: String = ""
@State var code = "ISBN"
enum URLError : Error {
case badURL
}
private func fetchBook(id identifier: String) async throws -> GoogleBook {
guard let encodedString = "https://www.googleapis.com/books/v1/volumes?q={\(identifier)}"
.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed),
let url = URL(string: encodedString) else { throw URLError.badURL}
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(GoogleBook.self, from: data)
}
var body: some View {
VStack {
Text("Name: \(name)")
Text("Author: \(author)")
Text("total: \(total)")
Button(action: {
code = "978-0441013593"
Task {
do {
let fetchedBooks = try await fetchBook(id: code)
let book = fetchedBooks.items[0].volumeInfo
name = book.title
author = book.authors?[0] ?? ""
total = String(book.pageCount!)
} catch {
print(error)
}
}
}, label: {
Rectangle()
.frame(width: 200, height: 100)
.foregroundColor(.blue)
})
}
}
}
// MARK: - GoogleBook
struct GoogleBook: Codable {
let kind: String
let totalItems: Int
let items: [Item]
}
// MARK: - Item
struct Item: Codable {
let id, etag: String
let selfLink: String
let volumeInfo: VolumeInfo
}
// MARK: - VolumeInfo
struct VolumeInfo: Codable {
let title: String
let authors: [String]?
let pageCount: Int?
let categories: [String]?
enum CodingKeys: String, CodingKey {
case title, authors
case pageCount, categories
}
}
uj5u.com熱心網友回復：
我會更進一步并添加idle和failed狀態。
然后不要拋出錯誤，而是將狀態更改為failed并傳遞錯誤描述。我Double從狀態中洗掉了值loading以顯示旋轉ProgressView
@MainActor
class GoogleBooksApi: ObservableObject {
enum LoadingState {
case idle
case loading
case loaded(GoogleBook)
case failed(Error)
}
@Published var state: LoadingState = .idle
func fetchBook(id identifier: String) async {
var components = URLComponents(string: "https://www.googleapis.com/books/v1/volumes")
components?.queryItems = [URLQueryItem(name: "q", value: "isbn=\(identifier)")]
guard let url = components?.url else { state = .failed(URLError(.badURL)); return }
self.state = .loading
do {
let (data, _) = try await URLSession.shared.data(from: url)
let response = try JSONDecoder().decode(GoogleBook.self, from: data)
self.state = .loaded(response)
} catch {
state = .failed(error)
}
}
}
在視圖中你必須switch對state和顯示不同的視圖。而且——非常重要——你必須將 observable 物件宣告為@StateObject. 這是一個非常簡單的實作
struct ContentView: View {
@State var code = "ISBN"
@StateObject var api = GoogleBooksApi()
var body: some View {
VStack {
switch api.state {
case .idle: EmptyView()
case .loading: ProgressView()
case .loaded(let books):
if let info = books.items.first?.volumeInfo {
Text("Name: \(info.title)")
Text("Author: \(info.authors?.joined(separator: ", ") ?? "")")
Text("total: \(books.totalItems)")
}
case .failed(let error):
if error is DecodingError {
Text(error.description)
} else {
Text(error.localizedDescription)
}
}
Button(action: {
code = "978-0441013593"
Task {
await api.fetchBook(id: code)
}
}, label: {
Rectangle()
.frame(width: 200, height: 100)
.foregroundColor(.blue)
})
}
}
}
uj5u.com熱心網友回復：
好像你沒有初始化GoogleBooksApi.
@ObservedObject var api: GoogleBooksApi
沒有任何可以修改的init。
除此之外 - 我建議使用@StateObject（前提是您的部署目標是最低 iOS 14.0）。使用ObservableObject可能會導致多次初始化GoogleBooksApi（而您只需要一次）
您應該使用
@StateObject在使用它的視圖中初始化的任何可觀察屬性。如果ObservableObject實體是在外部創建并傳遞給使用它的視圖，則使用 . 標記您的屬性@ObservedObject。
轉載請註明出處，本文鏈接：https://www.uj5u.com/gongcheng/410739.html
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