StrategyInterface interface

Strategies have to implement this interface in order to be recognized by
Strategy validator.
Strategy Validator

Strategies will be executed by this validator. Then their validation
result as boolean value will return for indication or whether the input
was valid or not.
Strategy Exception

Used when there is a problem fetching strategies and whey they are not
really a StrategyInterface type.
Strategy factory

Used for instantiating Strategy validator object.
StrategyValidator Module class

Used by zendframework 2's Module manager.
Registering Strategy validator as a service

It is registered as a service in validator plugin manager.
Making Strategy validator available to Apigility

So that it could be selected and used via Apigility admin page.