[swift-users] Instantiate Swift class from string

[Matthew Davies]

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>
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On Thu, Dec 10, 2015 at 10:47 AM, Dan Stenmark <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> 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 <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 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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>>>
>>>
>>>
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