将map与switchToLatest混淆

Question

我有一个源发布者，还有一个依赖源发布者的发布者。这是针对我的情况的游乐场测试代码:

import Foundation
import Combine
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
typealias Image = Int
enum NetError: Error {
case invalidImage
}
func convertImageToVideo(_ image: Image) -> AnyPublisher<Image, NetError> {
Future { promise in
DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
if image == 20 {
promise(.failure(.invalidImage))
} else {
promise(.success(image))
}
}
}
.eraseToAnyPublisher()
}
var image = PassthroughSubject<Image, NetError>()
let subscription = image
.map { image in
convertImageToVideo(image)
}
.switchToLatest()
.sink { completion in
if case let .failure(error) = completion {
print("Receive error: (error)")
}
} receiveValue: { video in
print("Receive new video: (video)")
}
image.send(0)
image.send(20)
image.send(40)
DispatchQueue.main.async {
image.send(20)
}
DispatchQueue.main.asyncAfter(deadline: .now() + 4) {
print("Send 50 into image.")
image.send(50)
}

但是我在控制台只收到一个错误:

Receive error: invalidImage

这是不理想的，我想继续接收值，即使convertImageToVideo方法发生错误。所以我修改代码:

import Foundation
import Combine
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
typealias Image = Int
enum NetError: Error {
case invalidImage
}
func convertImageToVideo(_ image: Image) -> AnyPublisher<Image, NetError> {
Future { promise in
DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
if image == 20 {
promise(.failure(.invalidImage))
} else {
promise(.success(image))
}
}
}
.eraseToAnyPublisher()
}
var image = PassthroughSubject<Image, NetError>()
let subscription = image
.map { image -> AnyPublisher<Result<Image, NetError>, Never> in
convertImageToVideo(image)
.map { video in
.success(video)
}
.catch({ error in
Just(.failure(error))
})
.eraseToAnyPublisher()
}
.switchToLatest()
.sink { completion in
if case let .failure(error) = completion {
print("Receive error: (error)")
}
} receiveValue: { video in
print("Receive new video: (video)")
}
image.send(0)
image.send(20)
image.send(40)
DispatchQueue.main.async {
image.send(20)
}
DispatchQueue.main.asyncAfter(deadline: .now() + 4) {
print("Send 50 into image.")
image.send(50)
}

这次输出是理想的:

Receive new video: failure(__lldb_expr_64.NetError.invalidImage)
Send 50 into image.
Receive new video: success(50)

但是错误不是来自completion闭包，而是来自new value闭包，我必须处理complete和new value闭包的错误。有人有好主意吗?谢谢!

Answer 1

您遇到的问题是，当您的image发布者遇到错误时，它会完成序列。你真正想要流经你的序列是一个Result说是否每个图像转换工作。要做到这一点，convertImageToVideo需要发出Result值，而不仅仅是图像值。这导致你的Future的一些相当时髦的结果，因为它的未来总是成功的，然后告诉你是否转换工作。

这是你的代码，重新设计，使结果通过:

进口的基础进口相结合进口PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true
typealias Image = Int
enum NetError: Error {
case invalidImage
}
func convertImageToVideo(_ image: Image) -> AnyPublisher<Result<Image,Error>, Never> {
Future { promise in
DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
if image == 20 {
promise(.success(.failure(NetError.invalidImage)))
} else {
promise(.success(.success(image)))
}
}
}
.eraseToAnyPublisher()
}
var image = PassthroughSubject<Image, Never>()
let subscription = image
.flatMap { image in
convertImageToVideo(image)
}
.sink { video in
switch video {
case .success(let video):
print("Receive new video: (video)")
case .failure(let error):
print("Receive error: (error)")
}
}
image.send(0)
image.send(20)
image.send(40)
DispatchQueue.main.async {
image.send(20)
}
DispatchQueue.main.asyncAfter(deadline: .now() + 4) {
print("Send 50 into image.")
image.send(50)
}

输出为:

Receive new video: 0
Receive error: invalidImage
Receive error: invalidImage
Receive new video: 40
Send 50 into image.
Receive new video: 50