Swift Combine — Notification、URLSession、Timer等Publisher的理解与使用

Notification Publisher

在Swift的Combine框架中，可以使用NotificationCenter.Publisher来创建一个能够订阅和接收通知的Publisher。

// 创建一个订阅通知的Publisher
let notificationPublisher = NotificationCenter.default.publisher(for: Notification.Name("CustomNotification"))

接下来，我们可以订阅这个Publisher，并处理接收到的通知。

// 订阅通知
let cancellable = notificationPublisher.sink { notification in// 处理接收到的通知print("Received notification: \(notification)")
}

发送通知

// 发送通知
NotificationCenter.default.post(name: Notification.Name("CustomNotification"), object: nil)

下面代码中就是一个完整的例子：

class NotificationViewModel: ObservableObject {private var cancellable = Set<AnyCancellable>()func setUpNotification() {let notificationPublisher = NotificationCenter.default.publisher(for: Notification.Name("CustomNotification"))notificationPublisher.sink { notification inprint("Received notification: \(notification)")}.store(in: &cancellable)}func sendNotification() {NotificationCenter.default.post(name: Notification.Name("CustomNotification"), object: nil)}
}struct NotificationDemo: View {@StateObject private var viewModel = NotificationViewModel()var body: some View {Button("Send Notification") {viewModel.sendNotification()}.buttonStyle(BorderedProminentButtonStyle()).onAppear {viewModel.setUpNotification()}}
}

上面代码中在ViewModel中定义并且订阅了Notification Publisher，在SwiftUI界面触发NotificationCenter发送通知，随后在sink方法中收到了该通知。
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除了这种用法外，有的时候也可以直接在SwiftUI界面通过onReceive方式使用。
现在SwiftUI界面定义一个Notification。

// app 进入前台前的通知
let willEnterForegroundPublisher = NotificationCenter.default.publisher(for: UIApplication.willEnterForegroundNotification)

然后设置onReceive方法：

.onReceive(willEnterForegroundPublisher, perform: { notification inprint("Received App will enter foreground notification")
})

这样在App从后台回前台的时候就触发了这个通知，onReceive的闭包中的打印就会输出。

完整代码如下：

struct NotificationDemo1: View {// app回前台的通知let willEnterForegroundPublisher = NotificationCenter.default.publisher(for: UIApplication.willEnterForegroundNotification)var body: some View {VStack {Text("Hello World")}.onReceive(willEnterForegroundPublisher, perform: { notification inprint("Received App will enter foreground notification")})}
}

如果想在这个界面添加多个通知，那是不是要加多个onReceive方法呢？也可以不是的，比如像这样：

.onReceive(Publishers.MergeMany(willEnterForegroundPublisher, didEnterBackgroundPublisher), perform: { notification inprint("Received App \(notification)")
})

可以通过Publishers.MergeMany方法将多个Publisher合并，然后在一个回调中处理收到通知事件。

struct NotificationDemo1: View {// app回前台的通知let willEnterForegroundPublisher = NotificationCenter.default.publisher(for: UIApplication.willEnterForegroundNotification)// app进入后台通知let didEnterBackgroundPublisher = NotificationCenter.default.publisher(for: UIApplication.didEnterBackgroundNotification)var body: some View {VStack {Text("Hello World")}.onReceive(Publishers.MergeMany(willEnterForegroundPublisher, didEnterBackgroundPublisher), perform: { notification inprint("Received App \(notification)")})}
}

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URLSession Publisher

在Swift的Combine框架中，URLSession.DataTaskPublisher提供了一种方便的方式来执行网络请求并处理返回的数据。

首先创建一个Publisher：

// 创建一个网络请求Publisher
let url = URL(string: "https://......")!
let request = URLRequest(url: url)
let dataTaskPublisher = URLSession.shared.dataTaskPublisher(for: request)

接下来，我们可以订阅这个Publisher，并处理接收到的数据和错误。

// 订阅网络请求
let cancellable = dataTaskPublisher.map(\.data) // 提取返回的数据.decode(type: MyResponse.self, decoder: JSONDecoder()) // 解码数据为自定义类型.receive(on: DispatchQueue.main) // 切换到主线程处理结果.sink(receiveCompletion: { completion inswitch completion {case .finished:print("Request completed successfully")case .failure(let error):print("Request failed with error: \(error)")}}, receiveValue: { response inprint("Received response: \(response)")})

在 dataTaskPublisher 发送一个新的事件值时，我们将其中的 Data 通过 map 的方式提取出来，并交给 decode 这个 Operator 进行处理。decode 要求上游 Publisher 的 Output 类型是 Data，它会使用参数中接受的 decoder (本例中是 MyResponse) 来对上游数据进行解析，生成对应类型的实例，并作为新的 Publisher 事件发布出去。然后切换到主线程处理结果，包括刷新UI等等。

把上面的代码优化一下，具体化一下，实现一个真实的网络请求示例：

import SwiftUI
import Combine
import Foundationstruct Photo: Identifiable, Decodable {let id: Intlet albumId: Intlet title: Stringlet url: Stringlet thumbnailUrl: String
}class URLSessionViewModel: ObservableObject {private var cancellable = Set<AnyCancellable>()@Published var photos: [Photo] = []@Published var isFetching: Bool = falsefunc fetchPhotoData() {guard let url = URL(string: "https://jsonplaceholder.typicode.com/photos") else {return}isFetching = truelet request = URLRequest(url: url)URLSession.shared.dataTaskPublisher(for: request).map(\.data).decode(type: [Photo].self, decoder: JSONDecoder()).receive(on: DispatchQueue.main).sink { completion inswitch completion {case .finished:print("Request completed successfully")case .failure(let error):print("Request failed with error: \(error)")}} receiveValue: { photos inprint("Received response: \(photos)")self.isFetching = falseself.photos = photos}.store(in: &cancellable)}
}struct URLSessionDemo: View {@StateObject private var viewModel = URLSessionViewModel()var body: some View {VStack {if viewModel.photos.isEmpty {if viewModel.isFetching {ProgressView()} else {Button("Fetch photos data") {viewModel.fetchPhotoData()}.buttonStyle(BorderedProminentButtonStyle())}} else {List(viewModel.photos) { photo inPhotoView(photo: photo)}.listStyle(PlainListStyle())}}}
}struct PhotoView: View {let photo: Photovar body: some View {HStack(spacing: 16) {AsyncImage(url: URL(string: photo.thumbnailUrl)) { image inimage.resizable().aspectRatio(contentMode: .fill)} placeholder: {Rectangle().fill(Color.gray.opacity(0.3))}.frame(width: 80, height: 80)VStack {Text(String(photo.id)).font(.title).frame(maxWidth: .infinity, alignment: .leading)Text(photo.title).font(.headline).frame(maxWidth: .infinity, alignment: .leading).multilineTextAlignment(.leading).lineLimit(2)}}}
}