[swift-evolution] [Proposal] Factory Initializers

[rintaro ishizaki]

FWIW, even in Swift2, factory initializer is possible.

I've post a gist here:
https://gist.github.com/rintaro/9eadc2720ac8be6a7898
It's pretty hackish though :)


2016-03-22 15:16 GMT+09:00 Riley Testut via swift-evolution <
swift-evolution at swift.org>:

> Hey all!
>
> Very sorry, restored my MacBook at the beginning of the calendar year, and
> forgot to re-subscribe to Swift-Evolution 😄. Once I realized this, I
> decided to hold off on pushing this forward till after Swift 2.2, and now
> that it's been released, I'd love to make moves on this!
>
> So, is there still an interest in the proposal? If so, I'll write up a new
> proposal with everyone's feedback, and then post it here for more
> discussion. I think this would very valuable (and would certainly help a
> bunch in my current app), but want to see where everyone stands!
>
> Riley Testut
>
> On Feb 8, 2016, at 11:26 AM, Charles Srstka <cocoadev at charlessoft.com>
> wrote:
>
> >> On Dec 17, 2015, at 3:41 PM, Riley Testut via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>
> >> Recently, I proposed the idea of adding the ability to implement the
> "class cluster" pattern from Cocoa (Touch) in Swift. However, as we
> discussed it and came up with different approaches, it evolved into a
> functionality that I believe is far more beneficial to Swift, and
> subsequently should be the focus of its own proposal. So here is the
> improved (pre-)proposal:
> >>
> >> # Factory Initializers
> >>
> >> The "factory" pattern is common in many languages, including
> Objective-C. Essentially, instead of initializing a type directly, a method
> is called that returns an instance of the appropriate type determined by
> the input parameters. Functionally this works well, but ultimately it
> forces the client of the API to remember to call the factory method
> instead, rather than the type's initializer. This might seem like a minor
> gripe, but given that we want Swift to be as approachable as possible to
> new developers, I think we can do better in this regard.
> >>
> >> Rather than have a separate factory method, I propose we build the
> factory pattern right into Swift, by way of specialized “factory
> initializers”. The exact syntax was proposed by Philippe Hausler from the
> previous thread, and I think it is an excellent solution:
> >>
> >> class AbstractBase {
> >>   public factory init(type: InformationToSwitchOn) {
> >>       return ConcreteImplementation(type)
> >>   }
> >> }
> >>
> >> class ConcreteImplementation : AbstractBase {
> >>
> >> }
> >>
> >> Why exactly would this be useful in practice? In my own development,
> I’ve come across a few places where this would especially be relevant:
> >>
> >> ## Class Cluster/Abstract Classes
> >> This was the reasoning behind the original proposal, and I still think
> it would be a very valid use case. The public superclass would declare all
> the public methods, and could delegate off the specific implementations to
> the private subclasses. Alternatively, this method could be used as an easy
> way to handle backwards-compatibility: rather than litter the code with
> branches depending on the OS version, simply return the OS-appropriate
> subclass from the factory initializer. Very useful.
> >>
> >> ## Protocol Initializers
> >> Proposed by Brent Royal-Gordon, we could use factory initializers with
> protocol extensions to return the appropriate instance conforming to a
> protocol for the given needs. Similar to the class cluster/abstract class
> method, but can work with structs too. This would be closer to the factory
> method pattern, since you don’t need to know exactly what type is returned,
> just the protocol it conforms to.
> >>
> >> ## Initializing Storyboard-backed View Controller
> >> This is more specific to Apple Frameworks, but having factory
> initializers could definitely help here. Currently, view controllers
> associated with a storyboard must be initialized from the client through a
> factory method on the storyboard instance (storyboard.
> instantiateViewControllerWithIdentifier()). This works when the entire flow
> of the app is storyboard based, but when a single storyboard is used to
> configure a one-off view controller, having to initialize through the
> storyboard is essentially use of private implementation details; it
> shouldn’t matter whether the VC was designed in code or storyboards,
> ultimately a single initializer should “do the right thing” (just as it
> does when using XIBs directly). A factory initializer for a View Controller
> subclass could handle the loading of the storyboard and returning the
> appropriate view controller.
> >>
> >> Here are some comments from the previous thread that I believe are
> still relevant:
> >>
> >>
> >>> On Dec 9, 2015, at 1:06 PM, Philippe Hausler <phausler at apple.com>
> wrote:
> >>>
> >>> I can definitely attest that in implementing Foundation we could have
> much more idiomatic swift and much more similar behavior to the way
> Foundation on Darwin actually works if we had factory initializers.
> >>
> >>
> >>> On Dec 7, 2015, at 5:24 PM, Brent Royal-Gordon <brent at architechies.com>
> wrote:
> >>>
> >>> A `protocol init` in a protocol extension creates an initializer which
> is *not* applied to types conforming to the protocol. Instead, it is
> actually an initializer on the protocol itself. `self` is the protocol
> metatype, not an instance of anything. The provided implementation should
> `return` an instance conforming to (and implicitly casted to) the protocol.
> Just like any other initializer, a `protocol init` can be failable or
> throwing.
> >>>
> >>> Unlike other initializers, Swift usually won’t be able to tell at
> compile time which concrete type will be returned by a protocol init(),
> reducing opportunities to statically bind methods and perform other
> optimization tricks. Frankly, though, that’s just the cost of doing
> business. If you want to select a type dynamically, you’re going to lose
> the ability to aggressively optimize calls to the resulting instance.
> >>
> >>
> >> I’d love to hear everyone’s thoughts on this!
> >>
> >> Best,
> >> Riley Testut
> >> _______________________________________________
> >> swift-evolution mailing list
> >> swift-evolution at swift.org
> >> https://lists.swift.org/mailman/listinfo/swift-evolution
> >
> > Was any proposal for this ever written up? It would be really useful to
> have, and it appeared to have the support of several Apple staff members.
> >
> > Charles
> >
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