[swift-users] Instantiate Swift class from string

[Matthew Davies]

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>
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On Thu, Dec 10, 2015 at 10:18 AM, Daniel Dunbar <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> 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 <209-225.3246> | 209-202-3284 | daviesgeek at gmail.com |
> 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>
> 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> wrote:
>>
>>
>> On Dec 10, 2015, at 7:26 AM, Harlan Haskins via swift-users <
>> 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/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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