[swift-users] Instantiate Swift class from string

[Daniel Dunbar]

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 <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/>
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> 
> 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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