sourceafFormBean::OptionsProvider.fan

** Implement to provide options to be rendered in '<select>' tags. 'OptionsProviders' are used by the default skin for '<select>' tags.
** 
** The 'FormBean' libraries caters for 'Enum' values but sometimes you want the user to choose from a list supplied from a database, or config file.
** In these scenarios you can supply your own 'OptionsProvider'.
** 
** Options providers may be either be set directly on a form field or contributed to the 'OptionsProviders' service.
** Options providers are selected based on the field's type.
** 
**   syntax: fantom
** 
**   @Contribute { serviceType=OptionsProviders# }
**   static Void contributeOptionsProviders(Configuration config) {
**       config[MyValue#] = MyValueOptionsProvider()
**   }
** 
const mixin OptionsProvider {
    
    ** Return 'true' if the '<select>' should show a blank value at the start of the options list.
    abstract Bool showBlank()
    
    ** The label to display in the blank option, should 'showBlank' return 'true'. 
    abstract Str? blankLabel()
    
    ** A map of option values to display, keyed on the display label. 
    ** The returned map should be ordered.
    abstract Str:Obj options(Field field)
}