Get program control on a regular (timed) basis
Execute some code after a given number of seconds, with or without user interaction with the program.
When to use the ON TIMER trigger?
In some cases, an application needs to execute code at a regular interval, for example to process
a message arrived in a queue, refresh data on a dashboard, or display resources in time-based
graphs.
Important: Unlike the
ON IDLE
trigger which executes when there
is no user activity, the ON TIMER
trigger executes even when the user interacts
with the application. Therefore, the code executed in an ON TIMER
trigger must
perform quickly, otherwise the end user will experience poor performance. As a general rule, make
sure the time spent in the ON TIMER
code is less than the timer interval. For
example, if the processing time takes about 2 seconds, it doesn't make sense to have an ON
TIMER
that triggers every second.Implementing the ON TIMER trigger
To return control to the program on regular intervals, use the
ON TIMER
seconds
trigger in dialogs. This trigger is dialog specific. It is
typically defined in the main dialog of the program, but it can be defined in every dialog.Important:
- Consider using the
ON TIMER
only in dialogs that do not handle field input, such asDISPLAY ARRAY
andMENU
. If used in input dialogs, this trigger may execute in the middle of a field input, which can produce field value validation and raise an input error. However,ON TIMER
can be used in input dialogs where the user cannot enter invalid values (for example when usingCHECKBOX
,RADIOGROUP
,COMBOBOX
, and character-type fields likeTEXTEDIT
) - When implementing multiple or parallel dialogs with
DIALOG
block, do not mix severalON TIMER
clauses in different sub-dialog blocks: Specify a uniqueON TIMER
clause at theDIALOG
block level. Obviously, it does not make much sense to mixON TIMER
andON IDLE
clauses.
For
example:
DEFINE seconds SMALLINT
LET seconds = 120
DISPLAY ARRAY ...
...
ON TIMER seconds
MESSAGE "Check for messages in queue..."
-- Query the message server for new messages.
...
Note that the parameter of the ON TIMER
trigger can be an integer variable, but
it will only be read when the dialog is started. Changing the variable during dialog execution will
have no effect.
A value of zero or less than zero disables the timeout trigger.