Dojo Defender Session 1: Coach notes

Goal of this session

After this session a ninja can:

• Create and manage a list of asteroids with Actor
• Spawn asteroids with a timer and remove them when they are off screen
• Detect collisions with colliderect
• Keep track of lives and force game over
• Show a simple explosion animation

Preparation (30 min before the session)

• Starter ZIPs ready on USB and on the site: dojo-defender-s2-starter.en.zip and dojo-defender-s2-solution.en.zip.
• Run the starter yourself once on the dojo room laptop.
• Explain that Dojo Defender comes back every session — encourage them to keep the code safe.

Schedule (3 hours)

This session is an integral part of the normal session 2 (Dodge Meteors). Use the time as follows:

Common questions

“My asteroids aren’t drawn” The for loop for asteroids is probably not in draw(), or it sits after a return statement.

"global gives an error" global timer has to be at the top of the function, before the first time timer is used.

“The ship is invisible after a collision” Check that ship.visible = True is set when the explosion is over. The reset has to come after frame 4 of the explosion.

“The explosion isn’t shown” Don’t forget to call explosion.draw() in draw(), and to set explosion_active to True on a collision.

Hints for Stretch and Expert

Stretch: collision Explain that colliderect checks whether two rectangles (hitboxes) overlap. This is the standard way in pgzero.

Expert: explosion The explosion mechanism is a manual frame animation: change the image of the Actor every 5 frames. This is a simple but effective technique.

Expert: invincibility Invincibility stops you from getting hit again straight after an explosion. Use a counter that counts down every frame.

Cliffhanger script

“Today you dodged asteroids and watched your ship blow up. Next time: your spaceship SHOOTS BACK. Enemies in formation, and you have lasers. Ready to attack?”