ITT Rule IDL Version 7.0 User Manual download pdf (Page 156)

156 Chapter 7: Creating an Operation

Creating a New Data-Centric Operation iTool Developer’s Guide

Note

Your operation class may have multiple superclasses. In general, each superclass’

Init method should be invoked by your class’ Init method.

Error Checking

Rather than simply calling the superclass Init method, it is a good idea to check

whether the call to the superclass Init method succeeded. The following statement

checks the value returned by the superclass Init method; if the returned value is 0

(indicating failure), the current Init method also immediately returns with a value of

IF ( self->

SomeOperationClass

::Init(_EXTRA = _extra) EQ 0) THEN $

RETURN, 0

This convention is used in all operation classes included with IDL. We strongly

suggest that you include similar checks in your own class definition files.

Keywords to the Init Method

Properties of the operation class can be set in the Init method by specifying the

property names and values as IDL keyword-value pairs. In addition to any keywords

implemented directly in the Init method of the superclass on which you base your

class, the properties of the IDLitOperation class and the IDLitComponent class are

available to any operation class. See “IDLitOperation Properties” and

“IDLitComponent Properties” (IDL Reference Guide).

Note

Always use keyword inheritance (the _EXTRA keyword) to pass keyword

parameters through to the superclass. (See “Keyword Inheritance” (Chapter 5,

Application Programming) for details on IDL’s keyword inheritance mechanism.)

Standard Base Class

While you can create your new operation class from any existing operation class, in

many cases, data-centric operation classes you create will be subclassed directly from

the base class IDLitDataOperation:

IF (self->IDLitDataOperation::Init(_EXTRA = _extra) EQ 0) $

THEN RETURN, 0

The IDLitDataOperation class provides the base iTool functionality used in the data-

centric operation classes created by ITT Visual Information Solutions. See

“Subclassing from the IDLitDataOperation Class” on page 153 for details.

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ITT Rule IDL Version 7.0 User Manual Page 156