Protocol private(set) properties

Using Swift
, ,
1

I would like to prevent write access from outside my class to a property defined by protocol conformance.

protocol MutableClientProtocol {
    associatedType Client

    var client: Client { get private(set) }
    mutating func setClient(_ newClient: Client)
}

Do you think this is possible in some form with protocols?

2

A protocol is a statement about the minimum capabilities of a type, and it cannot really restrict the type to having a private setter. From the point of view of someone using the protocol, this is no different than if the protocol declared a { get } requirement, which cannot prevent the conforming type from having a setter, private or public.

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3

I agree with you, but you are describing the current state of protocol in swift. Do you mind if there's any intention to provide such kind of feature in future?

4

Such features were discussed in topics about abstract classes if I remember correctly. Abstract classes were deferred though. There were also discussions about private protocol requirements.

5

Why can't the following minimum capability be valid:

This conforming type has a variable that you can get and but may be set privately.

If we ever get type(set) or something equivalent, then I think this could be a powerful feature.

6

How would usage differ between the protocol you’ve written and one where the private(set) is simply absent from the protocol requirements?

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7

Simply in a protocol extension mutating func doesn’t work if property is { get } only, this mean u can’t provide a default method your own protocols. @Jumhyn

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8

There can be multiple users of the protocol, or rather multiple users of the eventual implementation of the protocol. One class of user is a pure consumer outside the module. Another has the context and expertise to perform the set and is internal to a module. But users outside the module can at least implement the protocol on their classes. They just hand instances off to the module to perform the expert computation. For example, a grandTotal property. I could write a function that is the source of truth for computing and writing a grand total, so my function accepts any GrandTotalWritable. You build a CartModel for your UI. You need my function to update the values displayed to users. So you need to implement GrandTotalWritable. Now I need to make GrandTotalWritable public, and now any of my internal implementations of GrandTotalWritable can't have internal(set). And so I risk you not using the source of truth function for computing grand total.