Swift Combine Future测试中线程不一致问题求助

问题概述 #

测试中期望Future的sink闭包与Future完成闭包运行在同一非主线程，但该预期稳定性极差——每运行100次测试约有8次失败。此问题仅在使用Future时出现，PassthroughSubject无异常。已提交XCTest bug工单FB13358522，可在Xcode 14.2 (14C18)和Xcode 15 (15A240d)版本中复现。

复现代码 #

let expectation = expectation(description: "")

let future = Future<Void, Error> { future in
    DispatchQueue.global().async {
        future(.success(()))
    }
}

future.sink(receiveCompletion: { value in
}, receiveValue: { _ in
    XCTAssertNotEqual(Thread.current, Thread.main)
    expectation.fulfill()
})
.store(in: &cancellables)

waitForExpectations(timeout: 1.0)

临时解决方案 #

在DispatchQueue.global().async闭包内添加print语句，可临时规避问题：

DispatchQueue.global().async {
    print(Thread.current)
    future(.success(()))
}

更优规避方法 #

方法1：显式指定sink的接收队列 #

通过receive(on:)操作符强制sink闭包在全局队列执行，确保断言稳定通过：

let expectation = expectation(description: "")

let future = Future<Void, Error> { future in
    DispatchQueue.global().async {
        future(.success(()))
    }
}

future
    .receive(on: DispatchQueue.global()) // 显式指定接收队列
    .sink(receiveCompletion: { value in
    }, receiveValue: { _ in
        XCTAssertNotEqual(Thread.current, Thread.main)
        expectation.fulfill()
    })
.store(in: &cancellables)

waitForExpectations(timeout: 1.0)

方法2：固化Future完成回调的线程上下文 #

在调用future(.success(()))前，通过全局队列同步执行绑定线程，避免调度异常：

let future = Future<Void, Error> { future in
    DispatchQueue.global().async {
        DispatchQueue.global().sync {
            future(.success(()))
        }
    }
}

PassthroughSubject正常示例代码 #

对比之下，PassthroughSubject无此线程异常，代码示例：

let subject = PassthroughSubject<Void, Error>()

subject.sink(receiveCompletion: { value in
    XCTAssertNotEqual(Thread.current, Thread.main)
    expectation.fulfill()
})
.store(in: &cancellables)

DispatchQueue.global().async {
    subject.send(())
    subject.send(completion: .finished)
}

内容的提问来源于stack exchange，提问作者omarzl