Delegate classifier overview

The capabilities of the Delegate classifier are made available when you code a Delegate part, which is the basis of a variable that references an EGL function. A primary use of Delegate parts and the related variable is in the EGL system libraries.

For example, consider the following statement, which is in a Rich UI handler that you might code:

call myService.myFunction()
   returning to myCallbackFunction 
             onException ServiceLib.serviceExceptionHandler;

The statement calls the function myFunction in a remote service and ensures that an exception handler (a second function) is invoked if the call returns an error. However, ServiceLib.serviceExceptionHandler is not the name of a function. Rather, ServiceLib.serviceExceptionHandler is the fully qualified name of a variable declared in the ServiceLib system library. The variable is based on a Delegate part that is essentially as follows:

Delegate SystemDelegatePart
    (exp AnyException in) 
end

In short, a declaration similar to the following one is found in the ServiceLib system library:

   serviceExceptionHandler SystemDelegatePart;

The mechanism provides flexibility:

EGL system code assigns a value to the serviceExceptionHandler variable. The value refers to an exception handler that is invoked by default if your call to a service returns an error.
You can write code to assign your own function to the serviceExceptionHandler variable and in this way cause your function to be invoked instead. Here is an example assignment:
```
ServiceLib.serviceExceptionHandler = myExceptionHandler;
```

The function you might have written has the following outline:

Function myExceptionHandler(exp AnyException in)
   ;
end

Please note that the characteristics of the myExceptionHandler function are the same as the characteristics of the SystemDelegatePart part. That is, the SystemDelegatePart part describes a function that accepts a single parameter of type AnyException and does not return a value. That sameness is what enables you to substitute your function for the one that is used by default.

A Delegate part does not include a stereotype.