Sample Bouncing Ball

ball.vala

public class Ball : Darkcore.Sprite {
    public double velocity_x;
    public double velocity_y;
    public double acceleration {
        get; set; default = 0.010;
    }
    
    public Ball (ref Darkcore.Engine engine) {
        base.from_file (engine, "resources/panda-ball.png");
        
        this.width = 64.00;
        this.height = 64.00;
        this.x = 100.00;
        this.y = 100.00;
        this.velocity_x = 2.00;
        this.velocity_y = 2.00;
        
        this.on_render = (engine, ball) => {
            var half_height = height / 2.00;
            var half_width = width / 2.00;
            if (y + half_height + velocity_y >= engine.height) {
                velocity_y = (Math.fabs(velocity_y) + acceleration) * -1.00;
            }
            if (y - half_height - velocity_y <= 0) {
                velocity_y = (Math.fabs(velocity_y) + acceleration);
            }
            if (x + (width / 2) + velocity_x >= engine.width) {
                velocity_x = (Math.fabs(velocity_x) + acceleration) * -1.00;
            }
            if (x - (width / 2) - velocity_x <= 0) {
                velocity_x = (Math.fabs(velocity_x) + acceleration);
            }
             b
            x += velocity_x;
            y += velocity_y;
        };
    }
}