Object Framework for LOGO

Using Getters and Setters:

When you define a property for a class, a general purpose getter and setter are created for you. However, you can create your own getter and setter for a property. A Getter for a property is used to retrieve the value of a property. A Setter for a property is used to set the value of the property.

Getting and Setting a Property:

Before you define your own getters and setters, you should become familiar with the helper procedures: GetProperty and SetProperty.

GetProperty:

The GetProperty procedure is used to get the value of a property for a specific object. Note using the GetProperty procedure does not run the "Getter" procedure for the property. It is designed to be used within a user defined Getter, and should not be used in other circumstances. The GetProperty procedure takes three arguments: The Class of the Object, The Name of the Object, and the Property.

For instance, in our Rectangle Class, we could retrieve the Width of R1 by entering.

SetProperty:

The SetProperty procedure is used to set the value of a property for a specific object. Note using the SetProperty procedure does not run the "Setter" procedure for the property. It is designed to be used within a user defined Setter, and should not be used in other circumstances. The SetProperty procedure takes four arguments: The Class of the Object, The Name of the Object, the Property to set, and the value to set the property to.

So we could set the Width of R1 be entering:

Creating your own Getter and Setters:

OK, now we have the tools necessary to create our own Getter and Setter. Let's create a small class for demostration purposes called Circle. The Circle will have one property called radius. We will create our own getter and setter for the radius so we can ensure whatever radius is set to is a number.

The DefineProperty procedure we have used throughout our exercises, actually has two default arguments: the Getter and Setter for the property. So far, we haven't used these arguments so the getter and setter for the property was created for us.

Getter:

The Getter for a property should always output a value. If you notice in the example below we use the :this variable to Get the property of the current object.

Setter:

The Setter for a property has an automatic argument called :value. This is the value that the property should be set to. Again in our example below we use the :this variable to Set the property of the current object.

Now, we will create our own Getter and Setter for a property.