Could not cast value of type #MyClass to NSObject about promises HOT 4 CLOSED

Thank you for reporting that @ingun37 !

The root of the issue goes deeply in Swfit-ObjC interoperability, which has special handling for hashable containers (Set and Dictionary). Feel free to play with the attached example: SwiftObjCCasting.zip

Briefly, if we have the following code in ObjC:

@implementation ObjC
- (nullable id)cast:(nullable id)object {
  return object;
}
@end

When we pass a Swift object in such a method and try to restore the type for the returned value, the Swift dynamic cast will crash in case the object is a hashable container with object of custom Hashable (non-builtin and non-NSObject) types in it:

class TestClass: Hashable {
  init(_ string: String) {
    self.string = string
  }
  var hashValue: Int { return string.hashValue }
  static func == (lhs: TestClass, rhs: TestClass) -> Bool {
    return lhs.string == rhs.string
  }
  private let string: String
}

let input = Set([TestClass("a")])
let output = ObjC().cast(input)
let value = output as? Set<TestClass>  // Run-time crash!

Or the same in case of a Dictionary:

let input = [TestClass("a"): "a"]
let output = ObjC().cast(input)
let value = output as? Dictionary<TestClass, String>  // Run-time crash!

Since Promises for Swift rely on ObjC implementation for compatibility reasons, any value a promise gets resolved with goes through conversion to nullable id and then back to its original Swift type. That's what you see in your original crash stack trace: some promise got resolved with Swift value of hashable container type and tries to notify subscribers about that. The control leaves ObjC core logic and enters Swift wrapper, where the original Swift value is initially passed as Any?, which we try to cast back to the original type in asValue helper func.

Generally, the issue may be perceived as a flaw in Swift-ObjC interoperability and unfortunately, we don't currently have a proper fix or a good workaround for it, other than not using Promises with hashable containers holding custom types (any NSObjects are fine, though), which is obviously less than ideal.

A direction to explore would be something like:

let output = ObjC().cast(input) as AnyObject
if let keys = output.allKeys as? [AnyHashable], let values = output.allObjects {
  let value =  Dictionary(uniqueKeysWithValues: zip(keys, values)) as? Type
} else if let values = output.allObjects as? [AnyHashable] {
  let value = Set(values) as? Type
} else {
  let value = output as? Type
}

Where Type is the original type of input. But that approach is quite suboptimal due to the copy overhead.

Anyhow, I hope the above sheds some light on the root of the problem. We're open for any suggestions and continue searching for a real fix w/o modifying the compiler, of course.

from promises.

I tested inheriting NSObject and it worked fine. I hope theres other solution because I wouldn't like the idea of inheriting unnecessary class.
Thank you

from promises.

Hi @ingun37, could you provide more context, please?
A small code snippet which reproduces the issue for you would be much appreciated!
Also, Xcode and Swift version may be useful.
Thanks!

from promises.

I found a solution
I changed Promise<Set<MyClass>> to Promise<[MyClass]> and it works fine now
Im satisfied with the solution but I hv made this reproducing unit test code anyway because I would like to contribute as much as I love this library.

Xcode version: 9.4.1
Swift version: 4.1

class MyClass: Hashable, Decodable {
    let s:String
    init(_ s:String) {
        self.s = s
    }
    var hashValue: Int {return s.hashValue}
    static func == (lhs: apiTests.MyClass, rhs: apiTests.MyClass) -> Bool {
        return lhs.s == rhs.s
    }
}
    
func testFail() {
    let ex = XCTestExpectation(description: "test")
    Promise { Set([MyClass("a"), MyClass("b")]) }.then { (a)-> Promise<Set<MyClass>> in
        Promise {a}
    }.then {
        print($0)
        ex.fulfill()
    }
    wait(for: [ex], timeout: 10.0)
}
func testSuccess() {
    let ex = XCTestExpectation(description: "test")
    Promise { Set([MyClass("a"), MyClass("b")]) }.then {
        print($0)
        ex.fulfill()
    }
    wait(for: [ex], timeout: 10.0)
}

It doesnt break at the exactly same point but I think its the same bug
Thank you

from promises.