Lesson 4: Programming the Shooter Game in Squeak/Smalltalk

Lesson 4: Using Cascades in Code

This lesson will be the last introductory lesson (lesson practices) before we begin the actual game creation.

This lesson is simple and quick, but it introduces cascades, which are commonly used in Smalltalk code. Understanding and becoming familiar with them will help you utilize them effectively, as well as recognize them when you encounter other Smalltalk code. By using cascades, you can reduce code clutter and minimize the amount of typing needed.

The following code is an example from Lesson 3. This code completes messages by using a period which in Smalltalk is the statement separator. Placing a period between two expressions turns them into independent statements.

s := SimpleButtonMorph new.
s label: ‘Move right’.
s target: moveRight.
s actionSelector: #value.
s openInWorld.

The following code is the example from Lesson 4 using cascades.

s := SimpleButtonMorph new
label: ‘Move right’;
target: moveRight;
actionSelector: #value;
openInWorld.

Even though the code looks simpler, it is important to understand what is happening. When the same object is the receiver of a sequence of consecutive messages, you do not need to repeat it. You can remove the repeated receiving object and replace the ending period with a semicolon (;). So, the basic rules for cascades are:

• Cascaded expressions are separated by semicolons
• All cascaded messages share the same receiver as the last message executed before the first semicolon

So, if it is as simple as replacing periods with semicolons, why does this lesson’s example not work if a semicolon were placed after “SimpleButtonMorph new”?

SimpleButtonMorph is a class, not an instance of that class. This means that you cannot send instance messages directly to the class itself. A class can receive class messages, but in this example, we are sending instance messages, which can only be sent to instances of the class, not the class itself. Therefore, to interact with the instance, you must first create an instance by sending the message “new” to SimpleButtonMorph. It is this instance of SimpleButtonMorph that can receive instance messages. In other words, all the cascaded messages are sent to the object created by “SimpleButtonMorph new”, demonstrating how a sequence of cascaded messages can be sent to the result of an expression.

In conclusion, understanding and using cascades in Smalltalk can greatly simplify your code, making it more readable and reducing redundancy. By chaining multiple messages to the same object in a single expression, you can streamline your code and focus on the logic rather than repetitive syntax. With this knowledge, you can write more efficient, elegant Smalltalk code and recognize cascading patterns in other code you encounter.

For Lesson 4 and the complete series of lessons on creating the Shooter Game, including all the necessary resources such as PDFs, source code, images, sound file, and more, visit https://scottgibson.site/ShooterGame/. This site not only provides everything you need to progress but also allows you to play the game directly in your browser using SqueakJS. Whether you are just getting started with Squeak/Smalltalk or advancing your skills, these resources will help guide you as you move forward through the game development process.