Allow for strategies to be objects or callables.

[Nyholm]

This is just an idea. Im not sure if we want to do this or not but I would like to hear your opinions.

Do we want to allow that strategies are objects or callables? This might be overkill when we allow candidates to be callables.

Also, the DiscoveryStrategy interface says that getCandidates should be static. But calling it in a non static way is okey. https://3v4l.org/IbRNo

[sagikazarmark]

Shouldn't we check an instance of DiscoveryStrategy too?

[dbu]

i personally think we should not invest much into discovery and promote dependency injection with explicit instantiation instead. otherwise this ends up being a global service container thing. that said, if there is a use case for this it looks like the right way.

[Nyholm]

The reason why I've been back and forth with this is because Im not 100% happy with this: php-http/HttplugBundle#85 (comment)

But Im not sure this PR will create a possibility to make php-http/HttplugBundle#85 better. I know this is very vague..

[Nyholm]

No, changed my mind with this for good.

Since we allow closures as candidates once could easy create something like this:

class Strategy implements DiscoveryStrategy {

  private static $object; 
  public function __construct($object) { 
      self::$object = $object;
  }
  static function getCandidates($type) {
     $obj = self::$object;
     return [[
          'class'=>function() use ($obj) {
              return $obj->getClass($type);
          }, 
          'condition'=>function() use ($obj) {
              return $obj->evaluateCondition($type);
          }, 
       ]];
  }
}