DEC Text Processing Utility Program Development

5.2 Programming in DECwindows DECTPU

5.2.5 Using Closures

With DECwindows, you can specify a closure value for a widget. (DECwindows documentation refers to closures as tags.) DECwindows does not define what a closure value is; a closure is simply a value that DECwindows understands how to recognize and manipulate so that a DECwindows application programmer can use the value if needed in the application. For general information about using closures in DECwindows, see the OpenVMS documentation overview.

When a widget calls back to the DECwindows application, the callback parameters include the closure value assigned to the widget. DECwindows allows the application to define the significance and possible values of the closure.

DECTPU supports closure values of type string and integer. Closure values are optional for widgets used by applications layered on DECTPU. If you do not specify a closure value, the GET_INFO (WIDGET, "callback_parameters", array) built-in procedure returns unspecified in the "closure" array element. If you create a widget without using a UIL file, the GET_INFO (WIDGET, "callback_ parameters", array) built-in procedure returns the closure you specified as a parameter to CREATE_WIDGET. If you create a widget by using a UIL file, the GET_INFO (WIDGET, "callback_parameters", array) built-in procedure returns the closure value (if any) defined in the X Resource Manager. If none is defined, the built-in returns unspecified.

DECTPU leaves it to the layered application to use the closure in any way the application programmer wishes. DECTPU passes through to the application any closure value received as part of a callback.

DECwindows EVE provides an example of how an application can use closure values. DECwindows EVE assigns a unique closure value to every widget instance that can be created during an EVE editing session. Each closure value corresponds to something that EVE must do in response to the activation of that particular widget. When an event causes DECTPU to execute EVE’s main callback program, the GET_INFO (WIDGET, "callback_parameters", array) built- in procedure returns the widget activated, the reason code (the reason the widget is calling back), and the closure associated with the particular widget instance.

EVE’s main callback program contains an array that is indexed with values identical to the widget closure values. Each array element contains a pointer to the EVE code to be executed in response to the corresponding widget’s callback. EVE’s callback program uses the closure value to locate the appropriate array index so the correct EVE routine can be executed in response to the callback.

If your layered application does not use EVE’s callback program, then its callback program or learn sequence must have a mechanism for determining which widget is calling back and which application code should be executed as a result.

5.2.6 Specifying Values for Widget Resources in DECwindows DECTPU

This section discusses techniques for specifying values for widget resources.

5.2.6.1 DECTPU Data Types for Specifying Resource Values

DECTPU supports the following data types with which to specify values for widget resources:

•String

•Array of strings

•Integer

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Compaq AA-PWCBD-TE manual Specifying Values for Widget Resources in DECwindows Dectpu
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