Because none of the methods exist yet, select Generate All. After VA Smalltalk generates the code for the methods, switch to the Script Editor to see the code.

First, notice the class definition for the part is as follows:

Two instance variables have been added for you: repeat and length. The MyTimer class is a subclass of AbtAppBldrPart. AbtAppBldrPart provides all of the general properties and behaviors that a timer part needs.

Select the Not categorized category. In the method list, you see length, length:, repeat, repeat:, start, and stop. Select length to see the code that was generated for you.

Before an instance variable has been set, its value is nil. A popular technique for initializing instance variables is lazy initialization, where the value is set the first time the instance variable is referenced. To use lazy initialization, add one line to the length method so it looks like the following:

The preceding initialization technique is a convenient way to prevent errors, and is generally easier than checking for nil values each time you reference the attribute. Follow the same steps for the repeat method, using false as the initial value, as follows:

The set methods do not need to be changed. To understand what they do, let's examine the code that was generated for the length: method:

The preceding method simply stores a new value into the variable, and tells VA Smalltalk that the value has been changed. Then VA Smalltalk can notify other parts of the new value.

For the timer's length and repeat attributes, you entered both get and set selector names in the Public Interface Editor. Usually, get and set selectors come in pairs.

The exception is when an attribute is read-only. If you do not want other parts to be able to set the value of an attribute, you should enter only the get selector. When the set selector is not defined, VA Smalltalk cannot set the attribute's value.