The Ivory Timer Module

Timers are used either to evaluate an expression periodically or after a given delay.

Types
- Timer
  
  Timers are basic property set based objects with type definition, data constructor and instance:
```
object Timer ::= Timer [Property];

Timer properties = PropertySet properties;

instance PropertySetBasedObject Timer where
{
};
```
  They are periodic if there is a period property and one-shot if there is an interval property; in both cases the units are milliseconds. When the interval or period expires, if there is a proc property (also known as a callback) it is invoked, otherwise a TimerEvent is raised for the store containing the timer. The type signature for the optional proc property is:
```
(Timer -> Void)
```
  The purpose of passing a reference to the timer is to allow access to other timer properties within callback.
  
  Finally, one-shot timers are destroyed immediately after the timer event or callback.
Built-in Function
- monitorTimer
  
  monitorTimer is a built-in method designed for a standard rule to monitor a data store containing timer objects. Its condition includes patterns for
```
StartEvent
CreateObjectEvent
DestroyObjectEvent
UpdatePropertyEvent
StopEvent
```
Example

The clock example illustrates the use of a periodic timer.

Last update: 11 October, 2005