Обобщенные интерфейсы в Delphi

Generic Interfaces in Delphi

Most of the examples I’ve seen of Generics in Delphi use classes containing a generic type. However, while working on a personal project, I decided I wanted an Interface containing a generic type.

The project uses an in-process publish/subscribe mechanism, and I wanted a subscriber to have a separate Receive method for each event type, rather than a single method which contained a big case statement with branches for each event type. Equally, however, I didn’t want to have to define an interface type for each event type. What I wanted was to have a generic Subscriber interface, that took the event type as a parameter.

However, I had no idea if I could define a generic Interface, let alone implement one. Even assuming I could do both of those things, would Delphi be able to resolve the correct Receive method to invoke? Only one way to find out….

NB: In this example, I’ve stripped out most of the pub/sub stuff, just leaving the pieces needed to show the generic interfaces. I’ll write about the other pieces over the next few posts.

First, I implemented a few sample events. The contents of these are not that interesting:

TSomeEvent = class

// other stuff

end;

TSomeOtherEvent = class

// other stuff

end;

Then, I defined a generic interface

ISubscriber<T> = interface

procedure Receive(Event : T);

end;

This is the interface that will need to be implemented by my subscribers in order to receive events of a particular type. Note, the type of event is set up as a generic type T.

Then, my subscribers need to implement an interface for each Event type they want to receive, however because it is a generic interface, it’s pretty straight-forward:

TMySubscribingObject = class(TInterfacedObject, ISubscriber<TSomeEvent>, ISubscriber<TSomeOtherEvent>)

protected

procedure Receive(Event : TSomeEvent); overload;

procedure Receive(Event : TSomeOtherEvent); overload;

end;

Note there’s no definition of a ISomeEventSubscriber and a ISomeOtherEventSubscriber interface, we can just use ISubscriber<T> and pass the type in in-place. We just need to implement the necessary overloaded Receive events.