Small World: A Little (More) Smalltalk

Lesson 3: Using class Application

You may have noticed that the first class listed by the Smalltalk browser is named Application. This class can be used to help create simple applications. In this lesson we will explore this class, and use it to create a new application. As a first step, you should explore this class by creating a class browser. You will see that it defines four methods. If you press the instance variable button you can also see that the class defines one instance variable, named win.

An instance variable is a variable that is created when an instance of the class is created, and exists as long as the instance exists. This is unlike a temporary variable, which exists only as long as the method in which it is defined is executing. So instance variables are used for sharing information between different methods. As you might expect from the name, the instance variable defined here will hold the window for the application.

Part of the behavior for class Application is created by redefining the method new in the meta-class, the class for Application itself. You can see this by first pressing the examine metaclass button. This will create a class browser for the meta-class. You will see that there is just one method. If you select the method you will see the definition, which is as follows:

When an instance of Application is created (by method new), the message is first passed to the parent class to do the actual creation of the object. Then, having created the new object, it is given two messages. The first is the message start, and the second is the message run. Typically start is used for purposes of initialization, and run is used to run the application. Now go back to the class browser for Application, and select the method start.

The comment tells you that start should be overridden by subclasses in order to perform any application specific initialization. There is a similar comment in the method run. Initialization should include invoking the method title:size:pane:. You can examine this method, which has the following definition:

The method title:size:pane: creates a new window. You have seen similar commands in the previous lesson. It adds a title to the window, and sets the size and the pane. In addition, it does something we did not do in the previous chapter. This method creates a menu item named run, and within that menu item it defines two commands. The first, quit, will halt the current execution, using the message Object halt. The second menu item, close, closes the application window.

Rewriting the Hello Application

As a first example of the use of the class Application, let us rewrite the Hello World application developed in the previous lesson. Proceed as before, only this time create the class by passing the message subclass to the class Application:

Name the new application HelloApp. There are no new instance variables or class variables. Once you have successfully created the class, select the class name and open a class editor. Once there, define the method start as follows:

Compare this to the method run in the previous lesson. We are still creating an instance of Pane and an Image, but we're letting the method in class Application create the window. After you have successfully compiled this method, go back to the class browser and enter a command to create an instance of your new class:

Notice that we only need to use the message new, whereas in the previous lesson we first created an object using new and then gave it the message run. That is because the overridden method for new in the meta class for Application will itself invoke the methods start and run. If all works successfully you should not see an application window that is similar to the one in the previous lesson:

Notice there are slight differences. Now we have a menu bar, which includes a menu item Run and two options, close and quit. If you hit the close button you will notice that the application window disappears.

A More Interesting Application

The Hello World application is moderately interesting, but it gets quickly boring. Lets create a more exciting application that uses both the start and the run methods, and introduces new ideas, such as the use of instance variables to share information across methods. This application will display a ball bouncing around the window pane. First, create a new application named Bounce, subclassing from Application, but this time include three instance variables, named img, ballLoc, and change.

The first, as you might expect, will be the image. The second will be the location of the ball, and the third will be the amount that the ball will change each time it moves. These are initialized in the method start as follows:

Notice that the method invokes title:size:pane: to set the size. The method run will be used to run the application. This method will create a loop that will run for 1000 steps. On each step we will erase the ball, move it a little, and redraw it. Because the Smalltalk system runs so quickly, it will be useful to slow the system down to get the animation effect. The message sleep halts the application for a given number of milleseconds.

The run method uses two new methods. The first, drawBallInColor: sets the image color and draws the ball at the current location. The ball is represented by an oval filled with the current color. The method at:fillOval: takes as arguments the upper left and lower right corner of the oval. The latter is determined by adding 10 to the starting location. This method is defined as follows:

The second new method is moveBall. This method moves the ball a small amount (the amount given by the variable change), and if the ball reaches the edge of the window it changes the direction of the ball.

If you have entered all of this successfully you should now be able to create an instance of Bounce and see the ball bouncing around the screen.

You can now try experimenting with the menu items under the run menu. If you hit quit the ball should stop moving. If you hit close the window should close. (If you hit close before hitting quit the ball will keep moving, only you won't be able to see it!).

Variations

Based on the previous lesson, you should be able to figure out how to make the ball change color as it moves around the screen. Another variation would be to trap a mouse press, and move the ball to the given location.

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