Read-only properties of protocols in Swift

Question

From the "Learn the Essentials of Swift" playground, there's an example protocol:

protocol ExampleProtocol {
    var simpleDescription: String { get }
    func adjust()
}

There's a short passage after this example that reads:

Note: The { get } following the simpleDescription property indicates that it is read-only, meaning that the value of the property can be viewed, but never changed.

Additionally an example is given of a class conforming to this protocol:

class SimpleClass: ExampleProtocol {
    var simpleDescription: String = "A very simple class."
    var anotherProperty: Int = 69105
    func adjust() {
        simpleDescription += "  Now 100% adjusted."
    }
}

var a = SimpleClass()
a.adjust()
let aDescription = a.simpleDescription

However how does this class conform to the protocol? What's stopping me from mutating simpleDescription? What don't I understand?

Answer 1

There's no way to specify in a protocol that you must have a read-only property. Your protocol asks for a simpleDescription property, and allows but does not require a setter.

Note also that the only reason you may mutate simpleDescription is because you know your a is of type SimpleClass. If we have a variable of type ExampleProtocol instead...

var a: ExampleProtocol = SimpleClass()
a.simpleDescription = "newValue" //Not allowed!

Answer 2

Protocols place requirements on object's interface, but do not restrict implementation from providing more operations than that.

In this example, the protocol requires a readable simpleDescription property and adjust() method. The class provides that, so it conforms to the protocol. In other words, this protocol says implementation must have get operation, but it does not prohibit them from also having set.

You will not be able to mutate simpleDescription via that protocol interface, because it does not provide such operation, but nothing prevents you from mutating it through different interface — in this example, the interface of the implementing class.