Hit Points and Health Bars

This example demonstrates a reasonably efficient way of drawing a health bar above a character.

The enemy at the center of the screen shoots bullets at the player, while the player attempts to dodge the bullets by moving the mouse. Each bullet that hits the player reduces the player’s health, which is shown by the bar above the player’s head. When the player’s health bar is empty (zero), the game ends.

sprite_health.py

"""
Sprite Health Bars

Artwork from https://kenney.nl

If Python and Arcade are installed, this example can be run from the command line with:
python -m arcade.examples.sprite_health
"""
from __future__ import annotations

import math
from typing import Tuple

import arcade
from arcade.resources import (
    image_female_person_idle,
    image_laser_blue01,
    image_zombie_idle,
)
from arcade.types import Color

SPRITE_SCALING_PLAYER = 0.5
SPRITE_SCALING_ENEMY = 0.5
SPRITE_SCALING_BULLET = 1
INDICATOR_BAR_OFFSET = 32
ENEMY_ATTACK_COOLDOWN = 1
BULLET_SPEED = 150
BULLET_DAMAGE = 1
PLAYER_HEALTH = 5

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
SCREEN_TITLE = "Sprite Health Bars"


def sprite_off_screen(
    sprite: arcade.Sprite,
    screen_height: int = SCREEN_HEIGHT,
    screen_width: int = SCREEN_WIDTH,
) -> bool:
    """Checks if a sprite is off-screen or not."""
    return (
        sprite.top < 0
        or sprite.bottom > screen_height
        or sprite.right < 0
        or sprite.left > screen_width
    )


class Player(arcade.Sprite):
    def __init__(self, bar_list: arcade.SpriteList) -> None:
        super().__init__(
            image_female_person_idle,
            scale=SPRITE_SCALING_PLAYER,
        )
        self.indicator_bar: IndicatorBar = IndicatorBar(
            self, bar_list, (self.center_x, self.center_y), scale=1.5,
        )
        self.health: int = PLAYER_HEALTH


class Bullet(arcade.Sprite):
    def __init__(self) -> None:
        super().__init__(
            image_laser_blue01,
            scale=SPRITE_SCALING_BULLET,
        )

    def on_update(self, delta_time: float = 1 / 60) -> None:
        """Updates the bullet's position."""
        self.position = (
            self.center_x + self.change_x * delta_time,
            self.center_y + self.change_y * delta_time,
        )


class IndicatorBar:
    """
    Represents a bar which can display information about a sprite.

    :param owner: The owner of this indicator bar.
    :param sprite_list: The sprite list used to draw the indicator
        bar components.
    :param Tuple[float, float] position: The initial position of the bar.
    :param full_color: The color of the bar.
    :param background_color: The background color of the bar.
    :param width: The width of the bar.
    :param height: The height of the bar.
    :param border_size: The size of the bar's border.
    :param scale: The scale of the indicator bar.
    """

    def __init__(
        self,
        owner: Player,
        sprite_list: arcade.SpriteList,
        position: Tuple[float, float] = (0, 0),
        full_color: Color = arcade.color.GREEN,
        background_color: Color = arcade.color.BLACK,
        width: int = 100,
        height: int = 4,
        border_size: int = 4,
        scale: float = 1.0,
    ) -> None:
        # Store the reference to the owner and the sprite list
        self.owner: Player = owner
        self.sprite_list: arcade.SpriteList = sprite_list

        # Set the needed size variables
        self._bar_width: int = width
        self._bar_height: int = height
        self._center_x: float = 0.0
        self._center_y: float = 0.0
        self._fullness: float = 0.0
        self._scale: float = 1.0

        # Create the boxes needed to represent the indicator bar
        self._background_box: arcade.SpriteSolidColor = arcade.SpriteSolidColor(
            self._bar_width + border_size,
            self._bar_height + border_size,
            color=background_color,
        )
        self._full_box: arcade.SpriteSolidColor = arcade.SpriteSolidColor(
            self._bar_width,
            self._bar_height,
            color=full_color,
        )
        self.sprite_list.append(self._background_box)
        self.sprite_list.append(self._full_box)

        # Set the fullness, position and scale of the bar
        self.fullness = 1.0
        self.position = position
        self.scale = scale

    def __repr__(self) -> str:
        return f"<IndicatorBar (Owner={self.owner})>"

    @property
    def background_box(self) -> arcade.SpriteSolidColor:
        """Returns the background box of the indicator bar."""
        return self._background_box

    @property
    def full_box(self) -> arcade.SpriteSolidColor:
        """Returns the full box of the indicator bar."""
        return self._full_box

    @property
    def bar_width(self) -> int:
        """Gets the width of the bar."""
        return self._bar_width

    @property
    def bar_height(self) -> int:
        """Gets the height of the bar."""
        return self._bar_height

    @property
    def center_x(self) -> float:
        """Gets the x position of the bar."""
        return self._center_x

    @property
    def center_y(self) -> float:
        """Gets the y position of the bar."""
        return self._center_y

    @property
    def top(self) -> float:
        """Gets the y coordinate of the top of the bar."""
        return self.background_box.top

    @property
    def bottom(self) -> float:
        """Gets the y coordinate of the bottom of the bar."""
        return self.background_box.bottom

    @property
    def left(self) -> float:
        """Gets the x coordinate of the left of the bar."""
        return self.background_box.left

    @property
    def right(self) -> float:
        """Gets the x coordinate of the right of the bar."""
        return self.background_box.right

    @property
    def fullness(self) -> float:
        """Returns the fullness of the bar."""
        return self._fullness

    @fullness.setter
    def fullness(self, new_fullness: float) -> None:
        """Sets the fullness of the bar."""
        # Check if new_fullness if valid
        if not (0.0 <= new_fullness <= 1.0):
            raise ValueError(
                f"Got {new_fullness}, but fullness must be between 0.0 and 1.0."
            )

        # Set the size of the bar
        self._fullness = new_fullness
        if new_fullness == 0.0:
            # Set the full_box to not be visible since it is not full anymore
            self.full_box.visible = False
        else:
            # Set the full_box to be visible incase it wasn't then update the bar
            self.full_box.visible = True
            self.full_box.width = self._bar_width * new_fullness * self.scale
            self.full_box.left = self._center_x - (self._bar_width / 2) * self.scale

    @property
    def position(self) -> Tuple[float, float]:
        """Returns the current position of the bar."""
        return self._center_x, self._center_y

    @position.setter
    def position(self, new_position: Tuple[float, float]) -> None:
        """Sets the new position of the bar."""
        # Check if the position has changed. If so, change the bar's position
        if new_position != self.position:
            self._center_x, self._center_y = new_position
            self.background_box.position = new_position
            self.full_box.position = new_position

            # Make sure full_box is to the left of the bar instead of the middle
            self.full_box.left = self._center_x - (self._bar_width / 2) * self.scale

    @property
    def scale(self) -> float:
        """Returns the scale of the bar."""
        return self._scale

    @scale.setter
    def scale(self, value: float) -> None:
        """Sets the new scale of the bar."""
        # Check if the scale has changed. If so, change the bar's scale
        if value != self.scale:
            self._scale = value
            self.background_box.scale = value
            self.full_box.scale = value


class MyGame(arcade.Window):
    def __init__(self) -> None:
        super().__init__(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_TITLE)

        # Create sprite lists
        self.bullet_list: arcade.SpriteList = arcade.SpriteList()
        self.bar_list: arcade.SpriteList = arcade.SpriteList()
        self.player_sprite_list: arcade.SpriteList = arcade.SpriteList()
        self.enemy_sprite_list: arcade.SpriteList = arcade.SpriteList()

        # Create player sprite
        self.player_sprite = Player(self.bar_list)
        self.player_sprite_list.append(self.player_sprite)

        # Create enemy Sprite
        self.enemy_sprite = arcade.Sprite(image_zombie_idle, scale=SPRITE_SCALING_ENEMY)
        self.enemy_sprite_list.append(self.enemy_sprite)

        # Create text objects
        self.top_text: arcade.Text = arcade.Text(
            "Dodge the bullets by moving the mouse!",
            self.width // 2,
            self.height - 50,
            anchor_x="center",
        )
        self.bottom_text: arcade.Text = arcade.Text(
            "When your health bar reaches zero, you lose!",
            self.width // 2,
            50,
            anchor_x="center",
        )
        self.enemy_timer = 0

    def setup(self) -> None:
        """Set up the game and initialize the variables."""
        # Setup player and enemy positions
        self.player_sprite.position = self.width // 2, self.height // 4
        self.enemy_sprite.position = self.width // 2, self.height // 2

        # Set the background color
        self.background_color = arcade.color.AMAZON

    def on_draw(self) -> None:
        """Render the screen."""
        # Clear the screen. This command has to happen before we start drawing
        self.clear()

        # Draw all the sprites
        self.player_sprite_list.draw()
        self.enemy_sprite_list.draw()
        self.bullet_list.draw()
        self.bar_list.draw()

        # Draw the text objects
        self.top_text.draw()
        self.bottom_text.draw()

    def on_mouse_motion(self, x: float, y: float, dx: float, dy: float) -> None:
        """Called whenever the mouse moves."""
        self.player_sprite.position = x, y

    def on_update(self, delta_time) -> None:
        """Movement and game logic."""
        # Check if the player is dead. If so, exit the game
        if self.player_sprite.health <= 0:
            arcade.exit()

        # Increase the enemy's timer
        self.enemy_timer += delta_time

        # Update the player's indicator bar position
        self.player_sprite.indicator_bar.position = (
            self.player_sprite.center_x,
            self.player_sprite.center_y + INDICATOR_BAR_OFFSET,
        )

        # Call updates on bullet sprites
        self.bullet_list.on_update(delta_time)

        # Check if the enemy can attack. If so, shoot a bullet from the
        # enemy towards the player
        if self.enemy_timer >= ENEMY_ATTACK_COOLDOWN:
            self.enemy_timer = 0

            # Create the bullet
            bullet = Bullet()

            # Set the bullet's position
            bullet.position = self.enemy_sprite.position

            # Calculate the trajectory.
            # Zero degrees is up, 90 to the right.
            # atan returns 0 degrees to the right instead of up, so shift by 90 degrees.
            diff_x = self.player_sprite.center_x - self.enemy_sprite.center_x
            diff_y = self.player_sprite.center_y - self.enemy_sprite.center_y
            angle = -math.atan2(diff_y, diff_x) + 3.14 / 2
            angle_deg = math.degrees(angle)

            if angle_deg < 0:
                angle_deg += 360

            # Set the bullet's angle to face the player.
            # Bullet graphic isn't pointed up, so rotate 90
            bullet.angle = angle_deg - 90

            # Give the bullet a velocity towards the player
            bullet.change_x = math.sin(angle) * BULLET_SPEED
            bullet.change_y = math.cos(angle) * BULLET_SPEED

            # Add the bullet to the bullet list
            self.bullet_list.append(bullet)

        # Loop through each bullet
        for existing_bullet in self.bullet_list:
            # Check if the bullet has gone off-screen. If so, delete the bullet
            if sprite_off_screen(existing_bullet):
                existing_bullet.remove_from_sprite_lists()
                continue

            # Check if the bullet has hit the player
            if arcade.check_for_collision(existing_bullet, self.player_sprite):
                # Damage the player and remove the bullet
                self.player_sprite.health -= BULLET_DAMAGE
                existing_bullet.remove_from_sprite_lists()

                # Set the player's indicator bar fullness
                self.player_sprite.indicator_bar.fullness = (
                    self.player_sprite.health / PLAYER_HEALTH
                )


def main() -> None:
    """Main Program."""
    window = MyGame()
    window.setup()
    arcade.run()


if __name__ == "__main__":
    main()