User's Guide


Designing the part

When you create a VisualAge part, you are actually creating a Smalltalk class. You can think of a class as a template for creating new objects. When you design the class, you decide what instance variables and methods the class will have, and you decide which of these instance variables and methods are made public to other parts. The instance variables that are visible to other parts are called attributes; the methods that are made visible to other parts are called actions.

In this example, you will build a class that represents a timer object. The timer will run for a certain length of time, and then it will notify other parts that it has expired. You could use the timer in applications to remind a user to enter data, or to close a window after it has displayed for a few seconds. The timer has the following instance variables:

length
The number of milliseconds that will elapse before the timer expires

repeat
A flag that indicates whether or not the timer starts again after it expires

timer
A private variable that holds the identity of the timer

The timer has the following methods:

start
Starts the timer

stop
Stops the timer

eventTimerFired:
A private method that is called whenever the timer's length of time elapses

length
Gets the length instance variable

length:
Sets the length instance variable

repeat
Gets the repeat instance variable

repeat:
Sets the repeat instance variable


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