Event Handlers

To handle return statements in class instantiation, the -> operator can be used to link a class instance with a handler expression (or block). In the handler, the internal _ret parameter holds the return value and is always defined using the right type (taken from the class declaration type).

If the class return type is void, _ret is not declared. Referring to it generates a compiler error.

You may have remarked that the instance line double of 7 = 14 is displayed after the Done with both! printout, even if it is before in the source code. This is expected. When the application has run its code, it does not end right after. It rather enters an event loop to retrieve and execute incoming events. The -> operator generates an event when return is called. This event is handled when the event loop starts, at the end of the script.

So with class return types used with class instantiation and ->, QED introduces the notion of an event when calling return. The return here really acts like a hypothetical {PostMessage(_ret); return;} code block that ends its execution right after posting the message, which will be handled by the event loop.

The event can be more complex than a basic predefined type; just define the class return type as another sophisticated class with many the fields (say ReturnTypeClass) you need and use return new ReturnTypeClass(field1, field2, ...). The _ret parameter will be of type ReturnTypeClass.

Also, we’ll see that return can be called multiple times so the object can emit multiple events!

It’s now time to reap the benefits that dual nature classes bring.