Event Types Are Method-pointer Types
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Because an event is a pointer to an event handler, the type of the event property must be a method-pointer type. Similarly, any code to be used as an event handler must be an appropriately typed method of a class.
All event-handler methods are procedures. To be compatible with an event of a given type, an event-handler method must have the same number and type of parameters, in the same order, passed in the same way.
Delphi defines method types for all its standard events. When you create your own events, you can use an existing type if that is appropriate, or define one of your own.