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ZZT provides a special type of creature, the Object, that can be made to interact with the player and the environment by using a small object-oriented command language. Objects are very versatile, and can move, talk and bargain with the player, and communicate with other Objects.
To put an Object on the playing field, type F2 O. Next, you can choose a character to represent the Object. Finally, you can enter a program to control it.
To change an Object's program, move the cursor to the Object and press Enter.
How they work.
During game play, the Objects are actively controlled by their associated programs. Commands are executed in tempo with the game. Usually, an Object is updated 4 or 5 times per second, and several commands are executed on each update.
Objects communicate with each other and with the environment by sending and receiving messages. Understanding ZZT-OOP is often difficult because it is possible to have many objects operating at the same time, and each object can interact with any other object at any time.
A programmable creature can put flashing one-line messages at the bottom of the screen. It can also put multi-line scrolling messages on the screen. These messages can be interactive, so that the player can pick choices from a menu.
Objects can also move, shoot, and interact with other objects.
Messages - What are they?
ZZT's programming language is object-oriented, meaning that a program's flow of execution is controlled by sending and receiving messages. Whenever an important event occurs, a message is sent to an object. This causes the program to execute specific instructions to handle the event. Messages can come from two sources: from other programmable creatures and from the game itself.
For example, whenever the player touches an object, that objects receives the message, "TOUCH". The object can then act on the message, for example by greeting the player or attacking. Or, the object can ignore the message.
Similarly, objects can communicate with each other by exchanging messages. For example, a board contains a "BUTTON" object and a "GATE" object. The player touches the button. Upon being touches the button sends the message "OPEN" to the gate object. The gate, upon receiving this message, moves aside.
The first step to learning ZZT-OOP is seeing actual programs with explainations.