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 }
            


        

Answer 1

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.

Answer 2

Found this and thought it may be of interest in addition to the excellent answer already provided:

If a protocol requires a property to be gettable and settable, that property requirement cannot be fulfilled by a constant stored property or a read-only computed property. 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.

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 2.2).” iBooks. https://itun.es/us/jEUH0.l

Answer 3

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!