I'm trying to write a wrapper around URLSessionTask in Swift. According to the documentation
All task properties support key-value observing.
So I want to keep this behavior and make all the properties on my wrapper also KVO-compliant (usually delegating to the wrapped task) and fully accessible to Objective-C. I'll describe what I'm doing with one property, but I basically want to do the same thing for all properties.
Let's take the property state of URLSessionTask. I create my wrapper like this:
@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
}
}
}
I added @objc to the properties so they are available to be called from Objective-C. And I added dynamic to the properties so they will be called via message-passing/the runtime even from Swift, to make sure the correct KVO-Notifications can be generated by NSObject. This is supposed to be enough according to Apple's KVO chapter in the "Using Swift with Cocoa and Objective-C" book.
I then implemented the static class methods 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
}
}
Then I wrote a unit test to check whether the notifications are called correctly:
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)
}
When I run it, the test crashes with an 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.'
But by making the underlyingTask-property @objc and dynamic, the Objective-C runtime should ensure that this notification is sent, even when the task is changed from Swift, right?
I can make the test work correctly by sending the KVO-notifications for the underlyingTask manually like this:
@objc public internal(set) var underlyingTask: URLSessionTask? {
willSet {
willChangeValue(for: \.underlyingTask)
}
didSet {
didChangeValue(for: \.underlyingTask)
}
}
But I'd much rather avoid having to implement this for every property and would prefer to use the existing keyPathsForValuesAffecting<Key> methods. Am I missing something to make this work? Or should it work and this is a bug?
