Some convenient methods for Dispatch group to synchronize async function calls - very useful for testing

DispatchGroup-await.swift

extension DispatchGroup {
    
    typealias Callback<T> = (T) -> Void
    typealias ClosureCallback<T> = Callback<Callback<T>>
    
    enum AwaitError: Error {
        case timeout
        case nilValue
    }
    
    /// You should call this method inside a custom queue
    func awaitResult<ElementType, ErrorType: Error>(timeout: DispatchWallTime = .now() + .seconds(10), block: @escaping ClosureCallback<Result<ElementType, ErrorType>> ) throws -> Result<ElementType, ErrorType> {
        
        self.enter()
        
        var result: Result<ElementType, ErrorType>!
        let resultF: (Result<ElementType, ErrorType>) -> Void = { r in
            result = r
            self.leave()
        }

        block(resultF)
        
        if self.wait(wallTimeout: timeout) != .success {
            throw AwaitError.timeout
        }
        
        if let result = result {
            return result
        } else {
            throw AwaitError.nilValue
        }
    }
    
    /// You should call this method inside a custom queue
    func awaitResultValue<ElementType, ErrorType: Error>(timeout: DispatchWallTime = .now() + .seconds(10), block: @escaping ClosureCallback<Result<ElementType, ErrorType>> ) throws -> ElementType {
        return try awaitResult(timeout: timeout, block: block).get()
    }
    
    /// You should call this method inside a custom queue
    func awaitValue<ElementType>(timeout: DispatchWallTime = .now() + .seconds(10), block: @escaping ClosureCallback<ElementType> ) throws -> ElementType {
        
        self.enter()
        
        var result: ElementType!
        let resultF: (ElementType) -> Void = { r in
            result = r
            self.leave()
        }

        block(resultF)
        
        if self.wait(wallTimeout: timeout) != .success {
            throw AwaitError.timeout
        }
        
        if let result = result {
            return result
        } else {
            throw AwaitError.nilValue
        }
    }
}