Swift class 的依赖键路径的 KVO 不能正常工作

Question

我正在尝试在 Swift 中围绕 URLSessionTask 编写包装器。根据to the documentation

All task properties support key-value observing.

所以我想保持这种行为并使我的包装器上的所有属性也符合 KVO（通常委派给包装任务）并且完全可供 Objective-C 访问。我将描述我用一个属性做什么，但我基本上想对所有属性做同样的事情。

我们取 URLSessionTask 的属性 state。我这样创建包装器：

@objc(MyURLSessionTask)
public class TaskWrapper: NSObject {
    @objc public internal(set) var underlyingTask: URLSessionTask?
    @objc dynamic public var state: URLSessionTask.State {
        return underlyingTask?.state ?? backupState
    }
    // the state to be used when we don't have an underlyingTask
    @objc dynamic private var backupState: URLSessionTask.State = .suspended

    @objc public func resume() {
        if let task = underlyingTask {
            task.resume()
            return
        }
        dispatchOnBackgroundQueue {
            let task:URLSessionTask = constructTask()
            task.resume()
            self.underlyingTask = task
        }
    }
}

我将 @objc 添加到属性中，以便可以从 Objective-C 调用它们。我将 dynamic 添加到属性中，因此它们将通过 message-passing/the 运行time 甚至从 Swift 调用，以确保 [= 可以生成正确的 KVO-Notifications 21=]。根据 Apple's KVO chapter in the "Using Swift with Cocoa and Objective-C" book，这应该足够了。

然后我实现了静态 class 方法 necessary to tell KVO about dependent key paths:

// MARK: KVO Support
extension TaskWrapper {
    @objc static var keyPathsForValuesAffectingState:Set<String> {
        let keypaths:Set<String> = [
            #keyPath(TaskWrapper.backupState),
            #keyPath(TaskWrapper.underlyingTask.state)
        ]
        return keypaths
    }
}

然后我写了一个单元测试来检查通知是否被正确调用：

var swiftKVOObserver:NSKeyValueObservation?

func testStateObservation() {
    let taskWrapper = TaskWrapper()
    let objcKVOExpectation = keyValueObservingExpectation(for: taskWrapper, keyPath: #keyPath(TaskWrapper.state), handler: nil)
    let swiftKVOExpectation = expectation(description: "Expect Swift KVO call for `state`-change")
    swiftKVOObserver = taskWrapper.observe(\.state) { (_, _) in
        swiftKVOExpectation.fulfill()
    }
    // this should trigger both KVO versions
    taskWrapper.underlyingTask = URLSession(configuration: .default).dataTask(with: url)
    self.wait(for: [swiftKVOExpectation, objcKVOExpectation], timeout: 0.1)
}

当我运行它时，测试崩溃 NSInternalInconsistencyException:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Cannot remove an observer <_XCKVOExpectationImplementation 0x60000009d6a0> for the key path "underlyingTask.state" from < MyURLSessionTask 0x6000002a1440>, most likely because the value for the key "underlyingTask" has changed without an appropriate KVO notification being sent. Check the KVO-compliance of the MyURLSessionTask class.'

但是通过underlyingTask-属性@objc和dynamic，Objective-C运行时间应该确保这个通知是已发送，即使任务从 Swift 更改，对吗？

我可以通过像这样手动发送底层任务的 KVO 通知来使测试正常工作：

@objc public internal(set) var underlyingTask: URLSessionTask? {
    willSet {
        willChangeValue(for: \.underlyingTask)
    }
    didSet {
        didChangeValue(for: \.underlyingTask)
    }
}

但我更愿意避免为每个属性实现它，并且更愿意使用现有的 keyPathsForValuesAffecting<Key> 方法。我是否遗漏了一些东西来完成这项工作？或者它应该工作，这是一个错误？

Answer 1

属性 underlyingTask 不是 dynamic.

Swift class 的依赖键路径的 KVO 不能正常工作

KVO for dependent key paths does not work properly for Swift class

dynamic

key-value-observing

swift

objective-c-swift-bridge