Flatmap的使用与匹配错误类型
使用flatmap从上行数据流发布者中获取数据并打平至一个数据流中，在本例中，将演示使用mapError将错误类型进行映射
步骤一：自定义错误类型
import Foundation
import UIKit
import Combine
enum RequestError: Error {
case sessionError(error: Error)
}步骤二：定义发布者和订阅者
//: we will send URLs through this publisher to trigger requests
let URLPublisher = PassthroughSubject<URL, RequestError>()
//: use `flatMap` to turn a URL into a requested data publisher
let subscription = URLPublisher.flatMap { requestURL in
URLSession.shared
.dataTaskPublisher(for: requestURL)
.mapError { error -> RequestError in
RequestError.sessionError(error: error)
}
}
.assertNoFailure()
.sink { result in
print("Request completed!")
_ = UIImage(data: result.data)
}步骤三：发射数据
URLPublisher.send(URL(string: "https://httpbin.org/image/jpeg")!)