Part 8: Working with multiple scenes¶
So far we have had just one Scene in our game, the GameplayScene. Let’s add two more: for the title screen
and for the pause screen. Even though we’ll have 3 scenes created in the game, only one of them can be active at a time.
It means that only active scene will render its nodes on the screen, run the update() loop and receive input
events. The other scenes will become “freezed” until one of them is activated again. Their update() loops won’t
be called, no input events will be published to them, no nodes present in those scenes will be drawn on the screen etc.
How to activate a new scene¶
To make another scene active, get an engine object first, and then call change_scene(new_scene) method.
To get an engine:
from kaa.engine import get_engine
engine = get_engine()
engine.change_scene(some_new_scene)
Each scene has the engine object stored under self.engine so you can get it from there as well:
# .... inside kaa.engine.Scene class method ....
self.engine.change_scene(some_new_scene)
How to create a new scene¶
Let’s write two more scenes:
GameTitleScene- Will be activated when the game starts. The scene will be a welcome screen, showing a logo and allowing to start the game or exit it.PauseScene- Will be activated when pressing ESC during gameplay. Will show a simple screen allowing to abort game (return to title screen) or resume game (return to gameplay scene)
import registry
import settings
from kaa.engine import Scene
from kaa.input import Keycode, MouseButton
from kaa.nodes import Node
from kaa.geometry import Vector, Alignment
from kaa.fonts import TextNode
class TitleScreenScene(Scene):
def __init__(self):
super().__init__()
self.root.add_child(Node(sprite=registry.global_controllers.assets_controller.title_screen_background_img,
z_index=0, position=Vector(0,0), origin_alignment=Alignment.top_left))
self.root.add_child(TextNode(font=registry.global_controllers.assets_controller.font_2, font_size=30,
position=Vector(settings.VIEWPORT_WIDTH/2, 500), text="Click to start the game",
z_index=1, origin_alignment=Alignment.center))
self.root.add_child(TextNode(font=registry.global_controllers.assets_controller.font_2, font_size=30,
position=Vector(settings.VIEWPORT_WIDTH/2, 550), text="Press ESC to exit",
z_index=1, origin_alignment=Alignment.center))
def update(self, dt):
for event in self.input.events():
if event.keyboard_key:
if event.keyboard_key.is_key_down and event.keyboard_key.key == Keycode.escape:
self.engine.quit()
if event.mouse_button:
if event.mouse_button.is_button_down and event.mouse_button.button == MouseButton.left:
self.engine.change_scene(registry.scenes.gameplay_scene)
Nothing unusual here, just the stuff we already know: the scene is pretty static, with just a background image and two labels. Mouse click changes the scene to gameplay and ESC quits the game. It won’t work yet, because registry object does not store gameplay_scene yet, but we’ll get there.
For now, let’s add the pause scene. It is very similar to the title screen scene:
import registry
import settings
from kaa.engine import Scene
from kaa.input import Keycode
from kaa.geometry import Vector, Alignment
from kaa.fonts import TextNode
class PauseScene(Scene):
def __init__(self):
super().__init__()
self.root.add_child(TextNode(font=registry.global_controllers.assets_controller.font_2, font_size=40,
position=Vector(settings.VIEWPORT_WIDTH/2, 300), text="GAME PAUSED",
z_index=1, origin_alignment=Alignment.center))
self.root.add_child(TextNode(font=registry.global_controllers.assets_controller.font_2, font_size=30,
position=Vector(settings.VIEWPORT_WIDTH/2, 550), text="Press ESC to resume",
z_index=1, origin_alignment=Alignment.center))
self.root.add_child(TextNode(font=registry.global_controllers.assets_controller.font_2, font_size=30,
position=Vector(settings.VIEWPORT_WIDTH/2, 650), text="Press q to abort",
z_index=1, origin_alignment=Alignment.center))
def update(self, dt):
for event in self.input.events():
if event.keyboard_key and event.keyboard_key.is_key_down:
if event.keyboard_key.key == Keycode.escape:
self.engine.change_scene(registry.scenes.gameplay_scene)
if event.keyboard_key.key == Keycode.q:
self.engine.change_scene(registry.scenes.title_screen_scene)
Let’s now make a small modification to the GameplayScene allowing to change scene to pause, when player
presses ESC.
def update(self, dt):
# .... other code ....
for event in self.input.events():
# .... other code ....
if event.keyboard_key and event.keyboard_key.is_key_down:
if event.keyboard_key.key == Keycode.escape:
self.engine.change_scene(registry.scenes.pause_scene)
Finally, let’s create all our scenes in the main.py and add them to the registry to make the change_scene
calls work!
from scenes.pause import PauseScene
from scenes.title_screen import TitleScreenScene
with Engine(virtual_resolution=Vector(settings.VIEWPORT_WIDTH, settings.VIEWPORT_HEIGHT)) as engine:
# .... rest of the function ....
# initialize scenes and remember them in the registry
registry.scenes.gameplay_scene = GameplayScene()
registry.scenes.title_screen_scene = TitleScreenScene()
registry.scenes.pause_scene = PauseScene()
engine.run(registry.scenes.title_screen_scene)
Run the game. Isn’t it much better with all those different screens? I think it is!
Starting a new game¶
If you test the flow of the game, you’ll notice the following bug: aborting game and then starting new game just returns to the
previous state of the scene: all monsters are where they were left, frag count is not reset and so on. It’s because
change_scene does not destroy scene state it just runs a new scene and freezes all other scenes, as we stated earlier.
A bug needs fixing! Let’s refactor the TitleScreenScene a little bit:
from scenes.gameplay import GameplayScene
class TitleScreenScene(Scene):
# .... rest of the class ....
def start_new_game(self):
registry.scenes.gameplay_scene = GameplayScene()
self.engine.change_scene(registry.scenes.gameplay_scene)
def update(self, dt):
for event in self.input.events():
# ... other code ...
if event.mouse_button and event.mouse_button.is_button_down and event.mouse_button.button == MouseButton.left:
self.start_new_game()
We simply create the new instance of GameplayScene before telling engine to change to that scene. Run the game again and enjoy the full experience of multiple scenes :)
Scene’s on_enter and on_exit methods¶
Scene has two methods on_enter and on_exit. They’re being used when you call change_scene so
you can do some additional initialization or cleanup before the scene loads.
class Gameplay(Scene):
def on_enter(self):
# do something when active scene changes TO this scene.
def on_exit(self):
# do something when active scene changes FROM this scene.
Let’s move on to the next part of the tutorial where we’ll learn few things about the camera.