Dispatch execution to main thread from callback

DispatchOnMainAsync_Excerpt.swift

receiveItems { //apparantly the method's declaration is that: receiveItems(completion: ()-> Void)
    //do some processing in completion block
    let items = ...
    DispatchQueue.main.async { [weak self] in 
        // update ui 
        guard let self = self else { return }
        self.items = items
    }
}

Initial commit