FWIW, even in Swift2, factory initializer is possible.
I've post a gist here:
It's pretty hackish though :)
···
2016-03-22 15:16 GMT+09:00 Riley Testut via swift-evolution < swift-evolution@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@charlessoft.com> > wrote:
>> On Dec 17, 2015, at 3:41 PM, Riley Testut via swift-evolution < > swift-evolution@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@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@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
>> _______________________________________________
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>> swift-evolution@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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