1

I have a class that conforms to a protocol. But it always fail when I try to cast or verify if it conforms to the protocol.

typealias DeviceHandler = (any Device) -> Void

protocol Device {
    var name: String { get }
}

class DevicesViewModel<T: Device> {
    var devices = [T]()

    func findDevices() {
        DevicesManager.findDevices() { device in
            // casting is failing
            // even if I do device is T, it fails
            guard let device = device as? T else { return }
            self.devices.append(device)
        }
    }
}

class DevicesManager {

    static func findDevices(completion: @escaping DeviceHandler) {
        completion(Computer(name: "Macbook"))
    }
}

class Computer: NSObject, Device {

    var name: String

    init(name: String) {
        self.name = name
        
        super.init()
    }
}
2

What is the X Type here? There is no declaration of it. I mean is it a struct / class / protocol?

3

Sorry, it was supposed to be the T

4

The following code has no type casting failures:

let devicesViewModel = DevicesViewModel<Computer>()
devicesViewModel.findDevices()
print(devicesViewModel.devices.count) // prints 1
5

If we change DevicesManager's findDevices() implementation to perform completion with something different from Computer Type, then type casting inside DevicesViewModel<Computer> will fail, which is expected.
So assume either provided example is incomplete or there are some additional conditions in the real code.

6

There is no casting or checking for protocol conformance in provided code sample.
device instance is casted to generic T type constrained with Device protocol, not to a Device protocol itself.

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