在Swift中创建没有外部属性的struct / protocol / class

Question

I have a class that wants to return a token which it alone can create and read, but can be handed to an client class which cannot do anything with it except give it back. The token contains a value. How can I design this so it only has private properties/methods but looks completely opaque to public. I don't want to do any casting, it needs to be type safe internal to the owning class. Swift 4.x.

Answer 1

Here's my feeble attempt. I don't think there is a way to make a class that is completely opaque in the sense that you hide the names of all its members, but you can make them inaccessible.

struct Token {
    //everyone can see this property exists, but only 
    //classes defined in this file can see its value
    fileprivate let value: String
    init(value: String) {
        self.value = value
    }
}

I'm assuming you'll need a class for issuing and validating the tokens. It will need to be able to see inside the token.

class TokenManager {
    func requestToken() -> Token {
        return Token(value: "a magic token value")
    }
    func isValid(token: Token) -> Bool {
        //to inspect the token's value, this class needs to be defined  
        //in the same file as the Token
        return token.value == "a magic token value"
    }
}

The consumer of the token would request a token like this:

    let myToken = TokenManager().requestToken()

It would know that the token has a value property, but would be unable to access it:

    //print(myToken.value) //'value' is inaccessible due to 'fileprivate' protection level

But the TokenManager could inspect the token's value:

    let isValid = TokenManager().isValid(token: myToken)
    print(isValid) //true

You would undoubtedly want to pass some data into the requestToken method that you would validate in the isValid method, but that is, as they say, an exercise left for the reader.

Note you can't make Token a fileprivate struct because then nobody could get a token.