Your first script

A script is a .bs file attached to a node in a scene. Start with a new project:

balaur new hello
balaur run hello

The project holds one scene with one node, and one script.

# scenes/main.toml
[[nodes]]
name = "Hello"
script = { source = "scripts/hello.bs" }

// scripts/hello.bs
pub class Hello {
    node: Node,
    elapsed: float,
}

impl Hello {
    fn init() {
        log::info("hello from {self.node.name()}");
    }

    fn update(dt: float) {
        self.elapsed += dt;
    }
}

What each part does

• pub class Hello declares the script. Its name follows the file: hello.bs holds Hello, and door_bell.bs would hold DoorBell.
• Each node the script is attached to gets one object of this class.
• node: Node is set to the node the object sits on.
• elapsed: float is state the object keeps between frames.
• impl Hello holds the class's methods, and self is the object.
• The engine calls init once and update once a frame. dt is the time since the last frame, in seconds.
• log::info writes a line to the log. "{self.node.name()}" puts a value into the text.

Change it while it runs

Keep the game running, and make the script report once a second:

// scripts/hello.bs
pub class Hello {
    node: Node,
    elapsed: float,
}

impl Hello {
    fn init() {
        log::info("hello from {self.node.name()}");
    }

    fn update(dt: float) {
        self.elapsed += dt;
        if self.elapsed > 1.0 {
            self.elapsed -= 1.0;
            log::info("another second");
        }
    }
}

Save the file. The game keeps running on the new code, and elapsed keeps its value, because a reload carries each field across by name.

A save that does not compile changes nothing. The error goes to the log with its file, line and column, and the old code keeps running.

Next

Trying the examples shows how to run the code on the other pages. Scripts in the engine goes further with hooks and nodes.