为什么在协议中声明只读属性？

Question

Why declare readonly property in protocol if we can set value trough class or struct? I can not understand usage of this. In "The Swift Programming Book" version 2.0

“If the protocol only requires a property to be gettable, the requirement can be satisfied by any kind of property, and it is valid for the property to be also settable if this is useful for your own code.”

Answer 1

So that it's not settable from outside the class/struct. Imagine your API returned some instance of a protocol that has a get and set property (in your protocol), then anyone getting this instance would be able to set the value!

Also get and set properties can't be constants:

protocol RWProt {
    var value : Int { get set }
}

// Error: Type 'Value' does not conform to protocol 'RWProt'
struct Value : RWProt {
    let value = 0
}

This however works:

protocol Read {
    var value : Int { get }
}

struct Value : Read {
    var value = 0

    mutating func change() {
        value++
    }
}

The protocol only needs the value to be gettable, so get protocols properties are not get only but rather get or set

Okay, here is another example:

import Foundation

public protocol ExternalInterface {
    var value : Int { get }
}


private struct PrivateStuff : ExternalInterface {
    var value = 0

    mutating func doSomePrivateChangingStuff() {
        value = Int(arc4random())
    }
}



public func getInterfaceToPrivateStuff() -> ExternalInterface {
    var stuff = PrivateStuff()
    stuff.doSomePrivateChangingStuff()
    return stuff
}




// In another file:

let interfaceToSomethingICantChange = getInterfaceToPrivateStuff()

// error: cannot assign to property: 'value' is a get-only property
interfaceToSomethingICantChange.value = 0