ITT Rule IDL Version 7.0 User Manual download pdf (Page 249)

Chapter 9: Creating a File Reader 249

iTool Developer’s Guide Example: TIFF File Reader

Init Method

FUNCTION example1_readtiff::Init, _REF_EXTRA = _extra

; Call the superclass Init method

IF (self->IDLitReader::Init(["tiff", "tif"],$

NAME="Tiff Files", $

DESCRIPTION="TIFF File format", $

_EXTRA = _extra) NE 1) THEN $

RETURN, 0

; Initialize the instance data field

self._index = 0

; Register the index property

self->RegisterProperty, 'IMAGE_INDEX', /INTEGER, $

Description='Index of the image to read from the TIFF file.'

RETURN,1

END

Discussion

The first item in our class definition file is the Init method. The Init method’s

function signature is defined first, using the class name example1_readtiff. The

_REF_EXTRA keyword inheritance mechanism allows any keywords specified in a

call to the Init method to be passed through to routines that are called within the Init

method even if we do not know the names of those keywords in advance.

Next, we call the Init method of the superclass. In this case, we are creating a subclass

of the IDLitReader class; this provides us with all of the standard iTool file reader

functionality automatically. Any “extra” keywords specified in the call to our Init

method are passed to the IDLitReader::Init method via the keyword inheritance

mechanism.

We specify a list of accepted filename extensions (

tiff and tif, in this case) via the

Extensions argument. We specify a value for the NAME property of the reader object

(this is displayed in the system preferences dialog) and include a description of the

reader via the DESCRIPTION keyword. Finally, we use the _EXTRA keyword

inheritance mechanism to pass through any keywords provided when the Init method

is called.

Our TIFF reader object has a single instance data field: _index, which is used to store

the index number of the image to read from a multi-image TIFF file. We initialize

this instance data field to 0, and register the IMAGE_INDEX property to provide

access to this field via the property sheet interface.

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ITT Rule IDL Version 7.0 User Manual Page 249