求助 Alamofire 的 publisher 用法

在学习 macOS 开发想捣鼓个小东西自己用，零基础。这两天研究了研究 Publisher ，感觉也算是大概懂了，但是在用 Alamofire 的 publisher 的时候，怎么也搞不定了。比如如下这个只有一个函数的 APP ，为什么请求不成功呢：

import SwiftUI

import Alamofire

struct ContentView: View {
    var body: some View {
        Text("Hello, world!")
            .padding()
            .onAppear(perform: load)
    }
    
    struct TestResponse: Decodable {
        let userId: Int
        let id: Int
        let title: String
        let body: String
    }
    
    func load(){
        AF.request("https://jsonplaceholder.typicode.com/posts", method: .get, parameters: nil)
            .validate()
            .publishDecodable(type: [TestResponse].self)
            .print()
            .sink { print($0) }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

打印出来的是：

receive subscription: (Alamofire.DataResponsePublisher<Swift.Array<Test.ContentView.TestResponse>>.(unknown context at $1080f8314).Inner<Combine.Publishers.Print<Alamofire.DataResponsePublisher<Swift.Array<Test.ContentView.TestResponse>>>.(unknown context at $7ff81332d748).Inner<Combine.Subscribers.Sink<Alamofire.DataResponse<Swift.Array<Test.ContentView.TestResponse>, Alamofire.AFError>, Swift.Never>>>)
request unlimited
receive cancel
receive value: (failure(Alamofire.AFError.explicitlyCancelled))
receive finished

为什么会出现 explicitlyCancelled 呢，谢谢大家。