package require Observer
Observer name
$name addObserver script
$name removeObserver script
$name invoke ?args?
This package provides an implementation of the observer pattern described in http://en.wikipedia.org/wiki/Observer_pattern.
The observer pattern provides a mechanism for decoupling action from stimulus. The idea is that an object using the observer pattern will maintain an ordered list of observer objects which. When the appropriate stimulus occurs, these objects are invoked in order.
A trivial use case is statistics matinenance. Statistics objects can be registered as an Observer which increments appropriate counters when invoked. Observers can also be used to implement a chain of responsibility.
See EXAMPLES below for ideas of how to incorporate observers into snit class like objects (snit::type e.g.).
Observer name
Create a new observer object named name
If the name is the special word
%AUTO%, a unique object name is assigned.
This constructor also returns the object name so constructs
like
$name addObserver script
Adds an observer script to the end
of the list of observer scripts maintained by the
object name. If the observer's
invoke method is called, this script
will be invoked.
Several scripts can be registered to observe the event. Scripts are called in the order in which they were registered.
$name removeObserver script
Removes script from the observer
list for the observer name.
It is an error to remove a script that is not registered
as an observer.
$name invoke ?args?
Invokes the observers that have been registered on
name. The observers are invoked
in the order in which they were registered. If additional
command line parameters,
args, are supplied, they are
passed to each observer script.
Example 1. Supporting an observer in a snit type or widget
Snit's delegation capability allows you to integrate observers into snit class like objects (e.g. snit::widgets) in a very natural way. This example shows how to implement a single observer within a snit::type
snit::type ObservedType {
component observers
delegate addObserver to observers
delegate removeObserver to observers
...
constructor args {
...
install observers using Observer %AUTO%
...
}
}
This example installs an Observer
object and exposes its addObserver
and removeObserver methods as
methods of observedType.
That is the function of delegation in snit.
Example 2. Supporting multiple observers in a snit type or widget
In some cases you may want to build an object that supports more than one set of observers. Suppose, for exmample you want to build a snit::widget that consists of two buttons and you want to support a list of scripts to be invoked when each button is clicked. Naturally the set of scripts will differ between the buttons.
This example shows how to install two observers. In this case the as form of the delegate command is used to provide methods to add scripts to each observer.
snit::type MultipleObservers {
component observer1
component observer2
delegate addObserver to observer1 as addSomeTypeOfObserver
delegate removeObserver to observer1 as removeSomeTypeOfObserver
delegate addObserver to observer2 as addSomeOtherObserver
delegate removeObserver to observer2 as removeSomeOtherObserver
...
constructor args {
...
install observer1 using Observer %AUTO%
install observer2 using Obserer %AUTO%
...
}
}