任务调度操作符

任务调度操作符在 DispatchQueue、RunLoop 中采用，可确定订阅任务执行顺序

import Foundation
import Combine

let firstStepDone = DispatchSemaphore(value: 0)

/*:
## `receive(on:)`
- determines on which scheduler values will be received by the next operator and then on
- used with a `DispatchQueue`, lets you control on which queue values are being delivered
*/
print("Demonstrating receive(on:)")

let publisher = PassthroughSubject<String, Never>()
let receivingQueue = DispatchQueue(label: "receiving-queue")
let subscription = publisher
    .receive(on: receivingQueue)
    .sink { value in
        print("Received value: \(value) on thread \(Thread.current)")
        if value == "Four" {
            //firstStepDone.signal()
        }
}

for string in ["One","Two","Three","Four"] {
    DispatchQueue.global().async {
        publisher.send(string)
    }
}

firstStepDone.wait()

如上例，将firstStepDone.signal()注释掉后，或许如果还有其他的任务将无法继续，因为firstStepDone.wait()将会阻塞。
输出：

Demonstrating receive(on:)
Received value: One on thread <NSThread: 0x60000171cec0>{number = 6, name = (null)}
Received value: Two on thread <NSThread: 0x60000171cec0>{number = 6, name = (null)}
Received value: Three on thread <NSThread: 0x60000171cec0>{number = 6, name = (null)}
Received value: Four on thread <NSThread: 0x60000171cec0>{number = 6, name = (null)}

去掉firstStepDone.signal()的注释后再添加其他任务：

/*:
## `subscribe(on:)`
- determines on which scheduler the subscription occurs
- useful to control on which scheduler the work _starts_
- may or may not impact the queue on which values are delivered
*/
print("Demonstrating subscribe(on:)")
let subscription2 = [1,2,3,4,5].publisher
    .subscribe(on: DispatchQueue.global())
    .handleEvents(receiveOutput: { value in
        print("Value \(value) emitted on thread \(Thread.current)")
    })
    .receive(on: receivingQueue)
    .sink { value in
        print("Received value: \(value) on thread \(Thread.current)")
}

输出：

Demonstrating receive(on:)
Received value: Three on thread <NSThread: 0x600001b224c0>{number = 4, name = (null)}
Received value: Two on thread <NSThread: 0x600001b224c0>{number = 4, name = (null)}
Received value: One on thread <NSThread: 0x600001b224c0>{number = 4, name = (null)}
Received value: Four on thread <NSThread: 0x600001b224c0>{number = 4, name = (null)}

Demonstrating subscribe(on:)
Value 1 emitted on thread <NSThread: 0x600001b224c0>{number = 4, name = (null)}
Value 2 emitted on thread <NSThread: 0x600001b224c0>{number = 4, name = (null)}
Received value: 1 on thread <NSThread: 0x600001b2ed00>{number = 5, name = (null)}
Value 3 emitted on thread <NSThread: 0x600001b224c0>{number = 4, name = (null)}
Received value: 2 on thread <NSThread: 0x600001b2ed00>{number = 5, name = (null)}
Value 4 emitted on thread <NSThread: 0x600001b224c0>{number = 4, name = (null)}

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