Pygame Space Invaders Unfinish

import pygame

class State:
    def __init__(self, engine):
        self.engine = engine

    def on_draw(self, surface): pass
    def on_event(self, event): pass
    def on_update(self, delta, ticks): pass

    def on_quit(self):
        self.engine.running = False

    def next_state(self, state):
        self.engine.next_state = state

class DisplayEngine:
    def __init__(self, caption, width, height, flags=0):
        pygame.display.set_caption(caption)
        self.surface = pygame.display.set_mode((width, height), flags)
        self.rect = self.surface.get_rect()
        self.clock = pygame.time.Clock()
        self.running = False
        self.delta = 0
        self.fps = 60

        self.state = State(self)
        self.next_state = None

    def mainloop(self, in_state):
        if in_state:
            self.state = in_state

        self.running = True
        while self.running:
            if self.next_state:
                self.state = self.next_state
                self.next_state = None

            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    self.state.on_quit()
                else:
                    self.state.on_event(event)

            self.state.on_draw(self.surface)
            self.state.on_update(self.delta,
                                 pygame.time.get_ticks())

            pygame.display.flip()
            self.delta = self.clock.tick(self.fps) * 0.001

class Entity(pygame.sprite.Sprite):
    def __init__(self, image, position, anchor):
        super().__init__()
        self.image = image
        self.rect = self.image.get_rect(**{anchor: position})
        self.position = pygame.Vector2(self.rect.center)

    def move(self, x, y):
        self.position += (x, y)
        self.rect.center = self.position

    def move_vector(self, vector):
        self.position += vector
        self.rect.center = self.position

class AnimateEntity(Entity):
    def __init__(self, images, frame, frame_rate, position, anchor):
        super().__init__(images[frame], position, anchor)
        self.frame_rate = frame_rate
        self.images = images
        self.frame = frame
        self.ticks = 0

    def update(self, delta):
        self.ticks += delta
        if self.ticks > self.frame_rate:
            self.ticks -= self.frame_rate
            self.frame = (self.frame + 1) % len(self.images)
            self.image = self.images[self.frame]

class Images:
    def __init__(self):
        self.aliens = []
        self.player = pygame.Surface((20, 10))
        self.player.fill(pygame.Color("gray40"))
        self.create_alien1()
        self.create_alien2()

    def add(self, alien, animate):
        self.aliens.append((
            pygame.transform.scale2x(alien),
            pygame.transform.scale2x(animate)
        ))

    def create_alien1(self):
        alien = self.create_surface((9, 9))
        eye = pygame.Color(0, 0, 0, 0)
        color = pygame.Color("orange")
        pygame.draw.line(alien, color, (3, 0), (5, 0))
        pygame.draw.line(alien, color, (2, 1), (6, 1))
        pygame.draw.line(alien, color, (1, 2), (7, 2))
        pygame.draw.line(alien, color, (0, 3), (8, 3))
        pygame.draw.line(alien, color, (0, 4), (8, 4))
        pygame.draw.rect(alien, eye, (2, 3, 2, 2))
        pygame.draw.rect(alien, eye, (5, 3, 2, 2))
        pygame.draw.line(alien, color, (1, 5), (7, 5))
        alien.set_at((2, 6), color)
        alien.set_at((4, 6), color)
        alien.set_at((6, 6), color)
        alien.set_at((1, 7), color)
        alien.set_at((4, 7), color)
        alien.set_at((7, 7), color)
        animate = alien.copy()
        alien.set_at((0, 8), color)
        alien.set_at((8, 8), color)
        animate.set_at((2, 8), color)
        animate.set_at((6, 8), color)

        self.add(alien, animate)

    def create_alien2(self):
        alien = self.create_surface((8, 8))
        eye = pygame.Color(0, 0, 0, 0)
        color = pygame.Color("skyblue")
        alien.set_at((2, 0), color)
        alien.set_at((5, 0), color)
        pygame.draw.line(alien, color, (3, 1), (4, 1))
        pygame.draw.line(alien, color, (2, 2), (5, 2))
        pygame.draw.line(alien, color, (1, 3), (6, 3))
        alien.set_at((2, 3), eye)
        alien.set_at((5, 3), eye)
        pygame.draw.line(alien, color, (1, 4), (6, 4))
        pygame.draw.line(alien, color, (2, 5), (5, 5))
        pygame.draw.line(alien, color, (3, 6), (4, 6))
        animate = alien.copy()
        pygame.draw.line(alien, color, (0, 2), (0, 7))
        pygame.draw.line(alien, color, (7, 2), (7, 7))
        alien.set_at((1, 7), color)
        alien.set_at((6, 7), color)

        pygame.draw.line(animate, color, (0, 1), (0, 6))
        pygame.draw.line(animate, color, (7, 1), (7, 6))
        animate.set_at((1, 6), color)
        animate.set_at((6, 6), color)

        self.add(alien, animate)

    def create_surface(self, size):
        surface = pygame.Surface(size, pygame.SRCALPHA)
        surface.fill((0, 0, 0, 0))
        return surface

class Intro(State):
    def __init__(self, engine):
        super().__init__(engine)
        self.images = Images()
        font = pygame.font.Font(None, 30)
        self.space = font.render("SPACE", 1, pygame.Color("yellow"))
        self.invaders = font.render("INVADERS", 1, pygame.Color("yellow"))
        pos = self.engine.rect.centerx, 100
        self.space_rect = self.space.get_rect(midtop=pos)
        pos = self.engine.rect.centerx, 100 + font.get_linesize()
        self.invaders_rect = self.invaders.get_rect(midtop=pos)

    def on_draw(self, surface):
        surface.fill(pygame.Color("black"))
        surface.blit(self.space, self.space_rect)
        surface.blit(self.invaders, self.invaders_rect)

    def on_event(self, event):
        if event.type == pygame.KEYDOWN:
            self.next_state(Game(self.engine, self.images))

class Game(State):
    def __init__(self, engine, images, level=0):
        super().__init__(engine)
        self.level = level
        self.images = images
        self.aliens = pygame.sprite.Group()
        self.player_bullets = pygame.sprite.Group()
        position = self.engine.rect.centerx, self.engine.rect.bottom - 40
        self.player = Entity(images.player, position, "midbottom")
        self.build_level()

    def build_level(self):
        if self.level == 0:
            for x in range(11):
                position = 30 + x * 30, 100
                images = self.images.aliens[1]
                alien1 = AnimateEntity(images, x % 2, 1, position, "midtop")
                position = 30 + x * 30, 140
                alien2 = AnimateEntity(images, (x + 1) % 2, 1, position, "midtop")
                position = 30 + x * 30, 180
                images = self.images.aliens[0]
                alien3 = AnimateEntity(images, x % 2, 1, position, "midtop")
                position = 30 + x * 30, 220
                alien4 = AnimateEntity(images, (x + 1) % 2, 1, position, "midtop")
                self.aliens.add(alien1, alien2, alien3, alien4)

    def on_draw(self, surface):
        surface.fill(pygame.Color("black"))
        self.aliens.draw(surface)

    def on_update(self, delta, ticks):
        self.aliens.update(delta)

if __name__ == "__main__":
    pygame.init()
    engine = DisplayEngine("Space Invaders", 30 * 12, 400)
    engine.mainloop(Intro(engine))
    pygame.quit()