[swift-evolution] [Proposal] Protected Access Level

[Leonardo Pessoa]

Vanderlei, my point in bringing such topics to this discussion is to make everyone here think if we're trying to really enhance the language within its intended purpose or if we're trying to change the language into something else were familiar with from other languages we work/ed with just because we're used to work like that. I just started thinking about this today and just cannot stop now. No intention to start a war here but I think everyone should ask themselves this for every proposed change to the language.

About the topic at-hand, we have to remember Swift is bridged to Objective-C, which has no protected (or abstract). How do you propose these protected members be bridged should the proposal pass?


-----Original Message-----
From: "Vanderlei Martinelli via swift-evolution" <swift-evolution at swift.org>
Sent: ‎29/‎05/‎2016 06:56 PM
To: "swift-evolution" <swift-evolution at swift.org>
Subject: Re: [swift-evolution] [Proposal] Protected Access Level

Thank you all for your comments. :-)


Well... My goal is to keep the thing really simple and do not start a new "OOP x POP" (or "something" x "other thing") war.


"Protected" access level is not a new concept at all (except for the Swift language), so I did not propose anything preposterous.



Of course in the Swift of my dreams we also have "abstract" access level modifier, "protected" access level, *real* "private" access level and "file" access level modifier (along with many, many other things, of course). But this proposal is not about this. It is only about include the "protected" access level.


There is, however, something that I need to get off my chest: I really would like to have the freedom to go to the depths with protocols as well with classes. I work in real apps everyday that uses Cocoa frameworks (based on classes) and these apps must be shipped and I like them well written. Maybe am I insane for proposing a better support for classes in Swift? If so, this explains why every time I suggest better support for classes in Swift there is an endless discussion and someone proclaims the death of OOP and is it. OK... Maybe someday we will not have more classes in Swift. Until there: the current language status is the best way to handle OOP in Swift? Or is there a better way? I think there is.




Regards,


Vanderlei Martinelli














On Sat, May 28, 2016 at 7:52 PM, Vanderlei Martinelli <vmartinelli at alecrim.com> wrote:

Hello.




This is the first draft. I'd like to know your opinion about it.


(I know that this subject could have been discussed before. If so, please indicate me the correct thread to follow and interact.)




Regards,


Vanderlei Martinelli




---




Introduction
Protected access level will enable entities to be used within the container type and by derived types only.
Motivation
Today Swift has three access levels (public, internal and private), but lacks a way to describe a member that can be only visible to its type or derived types.
A common case is the UIView from UIKit. Many developers are tempted to make this call:
view.layoutSubviews()The documentation says: "You should not call this method directly. If you want to force a layout update, call the setNeedsLayoutmethod instead to do so prior to the next drawing update. If you want to update the layout of your views immediately, call the layoutIfNeeded method."
But yes, you should call this method directly if you are subclassing the view and needs to perform additional layout to its subviews ("subclasses can override this method as needed"):
public override func layoutSubviews() {
    // We are calling the super method directly here.
    super.layoutSubviews()
    
    // Do more adjustments to this view's subviews...
}So, yes, we can call this method directly when subclassing, but the Swift compiler will not prevent you from do this when not subclassing or from any other foreign class. It will not even issue a warning.
In Objective-C problems like this are usually "solved" my adding a kind of "protected" header (.h) that is intended to be included only when the developer is subclassing. In Swift we do not have headers, but we have the new access level model. So, if the declaration of this method was...
protected func layoutSubviews()... no one outside the class or derived classes would be allowed to call this method directly.
Of course, there are other cases in the Cocoa frameworks and there are many other cases when we are developing software in Swift that the protected access level would be very usefull.
Proposed solution
Create the protected access level.
Detailed design
Reference Types (classes)
When declarated by a class the protected member will be visible to the class itself and all the derived classes.
// BaseClass.swift
public class BaseClass {
    public protected(set) var x = 20
    protected let y = 10
    
    protected func doSomething() {
        // ...
    }
}

// DerivedClass.swift
public class DerivedClass: BaseClass {
    protected override doSomething() {
        self.x = 10 * self.y
    }
}If the member is declared as final then it will be visible but not can be overrided by the derived classes. Just like it works with other access levels.
Value Types (structs, enums, etc.)
Value types cannot have derived types. In this case the protected access level does not make sense and will not be allowed in their members.
Protocols
Protocols do not declare access level for their members. So the protected�

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