[swift-users] Instantiate Swift class from string
Daniel Dunbar
daniel_dunbar at apple.com
Thu Dec 10 17:47:38 CST 2015
> On Dec 10, 2015, at 1:01 PM, Matthew Davies via swift-users <swift-users at swift.org> wrote:
>
> Yes I have the protocol, but the problem is that I would want to be able to call a method on the class that isn't necessarily defined in the protocol.
You should only call methods that are defined in the protocol, and you should extend the protocol as necessary. You can always have instances conform to additional protocols and use conditional casts, if necessary.
The value of this approach is that it is statically type safe, and users of your framework can easily tell what methods they need to implement to work with it.
- Daniel
>
> I.e., I would like to be able to do something like this:
>
>
> ---
>
> protocol Controller {
> init()
> }
>
> class MainController : Controller {
>
> required init() {}
>
> func index() -> String {
> return "This is the index"
> }
> }
>
> class Router {
> func get(url: String, ctrl: Controller.Type, method: String) {
> let inst = ctrl.init()
> // Run the method that is passed in here
> }
> }
>
> let router = Router()
> router.get("/", ctrl: MainController.self, method: "index")
> ---
>
> Does that make sense as to what I'm trying to accomplish? As I said, I'm open to suggestions. I'm relatively new to Swift's design patterns, so I may be thinking about this in completely the wrong way…
>
>
> On Thu, Dec 10, 2015 at 12:48 David Owens II <david at owensd.io <mailto:david at owensd.io>> wrote:
> You have the protocol conformance, why can’t you simply call the method directly?
>
>> On Dec 10, 2015, at 12:22 PM, Matthew Davies via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>
>> I'm building a URL router in which I'd like to pass a controller and a method. I don't want to instantiate all the controllers up front and pass the methods in as closures, nor do I want old controller instances still kept around. If there's a better way, I'm definitely open to any suggestions. I'm still learning the "Swift" way to do things.
>>
>>
>> Matthew Davies
>> Junior Developer, GeoStrategies <http://geostrategies.com/>
>> Director of Photography, OffBlock Films <http://offblockfilms.com/>
>> 209-225-3246 <tel:209-225.3246> | 209-202-3284 <tel:209-202-3284> | daviesgeek at gmail.com <mailto:daviesgeek at gmail.com> | daviesgeek.com <https://daviesgeek.com/>
>> <http://facebook.com/daviesgeek> <http://us.linkedin.com/in/daviesgeek> <http://twitter.com/daviesgeek> <http://daviesgeek.com/feed.xml> <http://github.com/daviesgeek>
>>
>> On Thu, Dec 10, 2015 at 10:47 AM, Dan Stenmark <daniel.j.stenmark at gmail.com <mailto:daniel.j.stenmark at gmail.com>> wrote:
>> NSSelectorFromString() is still available in Swift, and you should be able to use the result of that in performSelector, though I’m hesitant to support this approach as it flies in the face of the safety Swift tries to enforce. I’m curious about your use case here; are you trying to create some kind of dynamic proxy for a remote object ala NSXPCConnection?
>>
>> Dan
>>
>>> On Dec 10, 2015, at 10:33 AM, Matthew Davies via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>
>>> Ooh okay. I think that should work for my purposes. Thanks.
>>>
>>> Somewhat related to this, how would I then call a method dynamically on an instance of the class, after instantiating it?
>>>
>>> ---
>>> class Graph {
>>> func call(method: String) {
>>> // Something goes here
>>> }
>>>
>>> func redraw() -> String {
>>> return "Redraws"
>>> }
>>> }
>>>
>>> let inst = Graph()
>>> inst.call("redraw")
>>> ---
>>>
>>>
>>> Matthew Davies
>>> Junior Developer, GeoStrategies <http://geostrategies.com/>
>>> Director of Photography, OffBlock Films <http://offblockfilms.com/>
>>> 209-225-3246 <tel:209-225.3246> | 209-202-3284 <tel:209-202-3284> | daviesgeek at gmail.com <mailto:daviesgeek at gmail.com> | daviesgeek.com <https://daviesgeek.com/>
>>> <http://facebook.com/daviesgeek> <http://us.linkedin.com/in/daviesgeek> <http://twitter.com/daviesgeek> <http://daviesgeek.com/feed.xml> <http://github.com/daviesgeek>
>>>
>>> On Thu, Dec 10, 2015 at 10:18 AM, Daniel Dunbar <daniel_dunbar at apple.com <mailto:daniel_dunbar at apple.com>> wrote:
>>> Note that you can define a protocol which will allow your framework to instantiate the type, and to call methods on instances of that type. If you can structure your code in this fashion, it can be very elegant in that it doesn't require factory functions and it is type safe.
>>>
>>> For example:
>>> --
>>> struct GraphableDescription { }
>>>
>>> protocol Graphable {
>>> /// Construct a graphable item from a description.
>>> init(description: GraphableDescription)
>>>
>>> func graph()
>>> }
>>>
>>> // Example framework method.
>>> func graphItem(description: GraphableDescription, graphable: Graphable.Type) {
>>> // Instantiate the graphable.
>>> let item = graphable.init(description: description)
>>>
>>> // Graph it.
>>> item.graph()
>>> }
>>>
>>> // Example Graphable client.
>>> struct Circle: Graphable {
>>> init(description: GraphableDescription) { }
>>>
>>> func graph() { }
>>> }
>>>
>>> // Example framework client.
>>> func foo() {
>>> graphItem(GraphableDescription(), graphable: Circle.self)
>>> }
>>> --
>>>
>>> - Daniel
>>>
>>>> On Dec 10, 2015, at 9:59 AM, Matthew Davies via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>>
>>>> I don't really like the idea of a factory function, but unfortunately that might be the only way to do it :( However, due to my specific use case, I don't think a factory function will work. I'm working on a framework that will need to both instantiate the class from a string (or class type) and call methods dynamically on it. Which, I'm not sure I can do in the build tools that are provided in the open source package. Foundation hasn't been fully implemented and is missing a lot of the methods that would allow this to work.
>>>>
>>>> @Jens thanks for that blog post. I'll have to make sure I check back to see what his solution is for it.
>>>>
>>>>
>>>>
>>>>
>>>> Matthew Davies
>>>> Junior Developer, GeoStrategies <http://geostrategies.com/>
>>>> Director of Photography, OffBlock Films <http://offblockfilms.com/>
>>>> 209-225-3246 <tel:209-225.3246> | 209-202-3284 <tel:209-202-3284> | daviesgeek at gmail.com <mailto:daviesgeek at gmail.com> | daviesgeek.com <https://daviesgeek.com/>
>>>> <http://facebook.com/daviesgeek> <http://us.linkedin.com/in/daviesgeek> <http://twitter.com/daviesgeek> <http://daviesgeek.com/feed.xml> <http://github.com/daviesgeek>
>>>>
>>>> On Thu, Dec 10, 2015 at 9:30 AM, Jan Neumüller <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>> Please no factory madness in Swift. This stuff is bad enough in Java - don’t infect Swift with it.
>>>>
>>>> Jan
>>>>
>>>>> On 10.12.2015, at 18:23, Jens Alfke via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>>>
>>>>>
>>>>>> On Dec 10, 2015, at 7:26 AM, Harlan Haskins via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>>>>
>>>>>> IIRC this isn’t possible because there’s no Runtime to query for classnames (it’s inherently unsafe anyway).
>>>>>
>>>>> It’s not unsafe if you specify a base class/protocol that the loaded class must conform to.
>>>>>
>>>>>> You might want to look into a better way of doing that you’re trying to do.
>>>>>
>>>>> I disagree with “a better way” — “a workaround” is how I’d rephrase it. This kind of dynamism is often the best tool for the job, and a lot of Cocoa developers are frustrated by its absence in Swift. For example, there’s a series of blog posts from earlier this year by the highly respected Brent Simmons [NetNewsWire, MarsEdit, Glassboard, etc., currently at Omni]:
>>>>> http://inessential.com/swiftdiary <http://inessential.com/swiftdiary>
>>>>> http://inessential.com/2015/07/20/swift_diary_1_class_or_struct_from_str <http://inessential.com/2015/07/20/swift_diary_1_class_or_struct_from_str>
>>>>>
>>>>> The workaround I’d suggest is a factory function that contains a switch statement that matches class names and returns newly initialized instances.
>>>>>
>>>>> —Jens
>>>>>
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>>>>
>>>>
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