如何初始化一个变量，但一旦初始化，它的值就是final

I have a model object variable in Swift View Controller. The thing I'd like to do is that at initialisation of the VC, i don't have a value for it. But after an async network call I have the parsed model object which is what this variable should hold, but from then on I don't want anything to change the value of the model variable. I have a model object variable in Swift View Co