Implement factory injection

[tschneidereit]

Instead of letting SwiftSuspenders directly handle the creation of injected values, it might sometimes be useful to delegate that to a factory.
sammyt's Dawn framework supports factory injections, which looks like a perfect match for this functionality.

Syntax proposal:

Injector#mapFactoryClass(whenAskedFor : Class, useFactoryClass : Class, named : String = '');

and

Injector#mapFactory(whenAskedFor : Class, useFactoryInstance : Object, named : String = '');

The second option will enabled configuring a specific instance of the factory. It might be more useful to use child injectors for configuring the factory itself.

[texastoland]

Should the first snippet read Injector#mapFactoryClass()? What is a factory? Did you like his Provider annotation? That could negate any inter-library interface dependency.

[tschneidereit]

Yes, that's what I meant, thanks. I've corrected the text.

I like the [Provider] annotation indeed: Much better than to prescribe an interface for the factory. Also, it enables multiple factory methods in one class, each annoted with [Provider] and selected by its return type.

I haven't found any standard annotations, but will look some more. If Guice or Spring support something similar, it might be worthwhile to use they annoation syntax.

[texastoland]

I hadn't considered multiple Providers brilliant!

[tschneidereit]

Instead of explicitly supporting factory mappings, Swiftsuspenders 2 will have a generic way of mapping to custom providers that only need to implement the method apply(targetClass:Class, usingInjector:Injector):Object. To make it easier to use the custom provider, Swiftsuspenders 2 comes bundled with a FactoryProvider that takes a Class implementing DependencyProvider as a ctor argument and construct an instance of that class for each application, enabling easy implementation of DepencyProviders with their own dependencies.