Adding a Gadget Callback

To perform intelligent actions a form or gadget must invoke a callback We will now add a simple gadget callback to the hello form.

We will add a second button, change message, which will execute a callback to modify the Message paragraph gadget.

setup form !!hello

 paragraph .Message text 'Hello world'

 button .change ‘Change message’ callback |!this.message.val = ‘Modified’|

 button .bye 'Goodbye' OK

exit

Figure 13:2. A Form with a Gadget Callback

A gadget callback is defined by the callback command followed by a command or PML Function enclosed in text delimiters. As we are giving a text string as part of the command which is itself supplied as a text string, we have had to use two kinds of delimiter: the apostrophe, (‘) and the (|) vertical bar.

Note: The use of This to mean the current form. When the callback is executed,

!this.message.val = ‘Modified’

will set the value member of the gadget Message on this form to read Modified rather than Hello world.

Note: The gadget callback could have been

|!!OtherForm.Para.val = ‘Modified’|

to change the value of a paragraph gadget on another form named !!OtherForm.

Typically a callback will involve many commands and could be a complex piece of code in its own right. In practice, the recommended way of defining a complicated callback is to use a form method.

First though, we need to store our form definition in a file so that we can edit it at will.