Native support for callback arguments

[danielr]

Mocking functions with callbacks usually involves calling the callback with configurable arguments. It would be nice, if Mokka could directly support this.

Something like this could work:

public class CallbackFunctionMock<Args, CallbackArgs, CallbackReturnValue>: FunctionMock<Args> {

    var result: CallbackArgs?
    
    public func recordCall(_ args: Args, _ callback: (CallbackArgs) -> CallbackReturnValue) {
        super.recordCall(args)
        
        if let callbackArgs = result {
            _ = callback(callbackArgs)
        }
    }
}

This would then be used like this:

let doSomethingAsyncFunc = CallbackFunctionMock<String, Int, Void>(name: "doSomethingAsync(arg:completion:)")
func doSomethingAsync(arg: String, completion: (Int) -> Void) {
    doSomethingAsyncFunc.recordCall(arg, completion)
}

There are some open questions, though:

• How to support callbacks with multiple arguments? (Is tuple splat still a thing, or has it been removed in some Swift version?)
• How to support multiple callbacks?
• What about functions that have a callback and a return value?
• Can we somehow get rid of the requirement to separately specify the generic type parameters for the callback arguments and the callback return value? This is not very intuitive.