Combine中NotificationCenter sink调用异步函数报错，求解决方法

问题描述 #

我是Combine和SwiftUI的新手，编写了如下发布者/订阅者代码：

NotificationCenter.default
            .publisher(for: UIApplication.willEnterForegroundNotification)
            .receive(on: DispatchQueue.main)
            .sink { [weak self] _ in
               print("update server side")
                self?.uploadContent()
            }
            .store(in: &subscriptions)

func uploadContent async () {
print("uploading...")
}

运行后出现以下错误：

'async' call in a function that does not support concurrency

Call to main actor-isolated instance method 'uploadContent()' in a synchronous nonisolated context

请问是否存在调用该异步函数的可行方式？

解决方案 #

方法1：用Task包裹异步调用 #

Combine的sink闭包是同步上下文，无法直接调用异步函数，通过Task可以开启并发上下文执行异步代码：

NotificationCenter.default
    .publisher(for: UIApplication.willEnterForegroundNotification)
    .receive(on: DispatchQueue.main)
    .sink { [weak self] _ in
        print("update server side")
        // 用Task包裹异步调用
        Task {
            await self?.uploadContent()
        }
    }
    .store(in: &subscriptions)

// 修正函数语法（async关键字位置错误）
func uploadContent() async {
    print("uploading...")
}

如果uploadContent涉及UI操作，需要在主Actor执行，可给函数标记@MainActor，或者在Task里指定主Actor：

@MainActor
func uploadContent() async {
    print("uploading...")
}

// 或者在Task中显式指定主Actor
Task { @MainActor in
    await self?.uploadContent()
}

方法2：将异步操作转为Combine Publisher #

如果想保持链式调用风格，可以用Future把异步函数包装成Publisher：

NotificationCenter.default
    .publisher(for: UIApplication.willEnterForegroundNotification)
    .receive(on: DispatchQueue.main)
    .flatMap { _ in
        Future { promise in
            Task {
                await uploadContent()
                promise(.success(()))
            }
        }
    }
    .sink(receiveCompletion: { completion in
        // 可在这里处理上传失败的逻辑
        if case .failure(let error) = completion {
            print("上传失败：\(error)")
        }
    }, receiveValue: {
        print("上传完成")
    })
    .store(in: &subscriptions)

func uploadContent() async {
    print("uploading...")
}

方法3：自定义asyncMap扩展（iOS 15+） #

针对iOS 15及以上版本，可以扩展Combine的Publisher，添加asyncMap方法直接处理异步逻辑：

import Combine

extension Publisher {
    func asyncMap<T>(_ transform: @escaping (Output) async -> T) -> AnyPublisher<T, Failure> {
        flatMap { value in
            Future { promise in
                Task {
                    let result = await transform(value)
                    promise(.success(result))
                }
            }
        }
        .eraseToAnyPublisher()
    }
}

// 使用示例
NotificationCenter.default
    .publisher(for: UIApplication.willEnterForegroundNotification)
    .receive(on: DispatchQueue.main)
    .asyncMap { _ in
        await uploadContent()
    }
    .sink(receiveCompletion: { _ in }, receiveValue: {
        print("上传完成")
    })
    .store(in: &subscriptions)

func uploadContent() async {
    print("uploading...")
}

内容的提问来源于stack exchange，提问作者stefanosn