Python速成课程135侧面射击手Pt 2，添加外星人

Question

回到第12章，我成功地完成了这个练习的第一部分，没有任何问题，但现在我正试图将外星人加入其中，我遇到了麻烦

我不能让他们在屏幕的右上角画，他们一直从左上角开始；他们一点也不动

其他一切似乎都很正常，包括船只移动和子弹碰撞

那么，我如何让外星人从右上角开始画，在列中，让他们上下移动呢

sideways\u shooter.py

import sys
from time import sleep

import pygame

from settings import Settings
from game_stats import GameStats
from ship import Ship
from bullet import Bullet
from alien import Alien

class SidewaysShooter:
    """Overall class to manage game assets and behavior"""

    def __init__(self):
        """Initialize the game and create game resources."""
        pygame.init()
        self.settings = Settings()

        self.screen = pygame.display.set_mode(
            (self.settings.screen_width, self.settings.screen_height))
        pygame.display.set_caption("Sideways Shooter")

        self.stats = GameStats(self)

        self.ship = Ship(self)
        self.bullets = pygame.sprite.Group()
        self.aliens = pygame.sprite.Group()

        self._create_fleet()

    def run_game(self):
        """Start the main loop for the game"""
        while True:
            self._check_events()

            if self.stats.game_active:
                self.ship.update()
                self._update_bullets()
                #self._update_aliens()

            self._update_screen()

    def _check_events(self):
        #Respond to keypresses and mouse events
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                self._check_keydown_events(event)
            elif event.type == pygame.KEYUP:
                self._check_keyup_events(event)

    def _check_keydown_events(self, event):
        #Respond to keypresses
        if event.key == pygame.K_UP:
            self.ship.moving_up = True
        elif event.key == pygame.K_DOWN:
            self.ship.moving_down = True
        elif event.key == pygame.K_q:
            sys.exit()
        elif event.key == pygame.K_SPACE:
            self._fire_bullet()

    def _check_keyup_events(self, event):
        #Respond to key releases
        if event.key == pygame.K_UP:
            self.ship.moving_up = False
        elif event.key == pygame.K_DOWN:
            self.ship.moving_down = False

    def _update_aliens(self):
        #Check if the fleet is at an edge, update the position of all the aliens
        self._check_fleet_edges()
        self.aliens.update()

        #Check for alien-ship collisions
        if pygame.sprite.spritecollideany(self.ship, self.aliens):
            self._ship_hit()

        #Look for aliens hitting the bottom
        self._check_aliens_bottom()

    def _fire_bullet(self):
        #Create a new bullet and add it to the bullets group
        if len(self.bullets) < self.settings.bullets_allowed:
            new_bullet = Bullet(self)
            self.bullets.add(new_bullet)

    def _update_bullets(self):
        #Update position of bullets and get rid of old bullets
        #Update bullet positions
        self.bullets.update()

        #Get rid of bullets that have left the screen
        for bullet in self.bullets.copy():
            if bullet.rect.left >= self.settings.screen_width:
                self.bullets.remove(bullet)

        self._check_bullet_alien_collisions()

    def _check_bullet_alien_collisions(self):
        #Respond to alien bullet collsions
        #Remove any bullets and aliens that have collided

        collisions = pygame.sprite.groupcollide(
            self.bullets, self.aliens, True, True)

        if not self.aliens:
            #Destroy existing bullets and create new fleet
            self.bullets.empty()
            self._create_fleet()


    def _update_screen(self):
        #Update images on the screen, and flip to the new screen
        self.screen.fill(self.settings.bg_color)
        self.ship.blitme()
        for bullet in self.bullets.sprites():
            bullet.draw_bullet()

        self.aliens.draw(self.screen)

        pygame.display.flip()

    def _create_fleet(self):
        #Create the fleet of aliens
        #Make an alien and find the number of aliens in a column
        #Spacing between each alien should be equal to one alien height
        alien = Alien(self)
        alien_width, alien_height = alien.rect.size
        available_space_y = self.settings.screen_height - (2 * alien_height)
        number_aliens_y = available_space_y // (2 * alien_height)

        #Determine the number of columns that can fit on the screen
        ship_width = self.ship.rect.width
        available_space_x = (self.settings.screen_width -
                                (4 * alien_width) - ship_width)
        number_rows = available_space_x // (2 * alien_width)

        #Create the full fleet of aliens
        for row_number in range(number_rows):
            for alien_number in range(number_aliens_y):
                self._create_alien(alien_number, row_number)

    def _create_alien(self, alien_number, row_number):
        #Create an alien and place it in the column
        alien = Alien(self)
        alien_width, alien_height = alien.rect.size
        alien.y = alien_height + 2 * alien_height * alien_number
        alien.rect.y = alien.y
        alien.rect.x = alien.rect.width + 2 * alien.rect.width * row_number
        self.aliens.add(alien) 

    def _check_fleet_edges(self):
        #Respond appropriately if any aliens have reached an edge
        for alien in self.aliens.sprites():
            if alien.check_edges():
                self._change_fleet_direction()
                break

    def _change_fleet_direction(self):
        #Drop the entire fleet and change direction
        for alien in self.aliens.sprites():
            alien.rect.x += self.settings.fleet_drop_speed
        self.settings.fleet_direction *= -1

    def _check_aliens_bottom(self):
        #Check if any aliens have reached the left of the screen
        screen_rect = self.screen.get_rect()
        for alien in self.aliens.sprites():
            if alien.rect.left >= screen_rect.left:
                #Treat this the same as if the ship was hit
                self._ship_hit()
                break

    def _ship_hit(self):
        #Respond to the ship being hit by an alien
        if self.stats.ships_left > 0:
            #Decrement ships_left
            self.stats.ships_left -= 1

            #Get rid of any remaining aliens and bullets            
            self.aliens.empty()
            self.bullets.empty()

            #Create a new fleet and center the ship
            self._create_fleet()
            self.ship.center_ship()

            #Pause
            sleep(0.5)

        else:
            self.stats.game_active = False 


if __name__ == '__main__':
    #Make a game instance, and run the game
    ai = SidewaysShooter()
    ai.run_game()

alien.py

import pygame
from pygame.sprite import Sprite

class Alien(Sprite):
    #A class to represent an alien

    def __init__(self, ai_game):
        #Initialize the alien and set its starting position
        super().__init__()
        self.screen = ai_game.screen
        self.settings = ai_game.settings

        #Load the alien image and set its rect attribute
        self.image = pygame.image.load('images/alien.bmp')
        self.rect = self.image.get_rect()

        #Start each new alien near the top right of the screen
        self.rect.x = self.settings.screen_width - self.rect.width
        self.rect.y = self.rect.height

        #Store the alien's exact vertical position
        self.y = float(self.rect.y)

    def check_edges(self):
        #Return True if alien is at the edge of screen
        screen_rect = self.screen.get_rect()
        if self.rect.bottom >= screen_rect.bottom or self.rect.top <= 0:
            return True

    def update(self):
        #Move the alien up or down
        self.y += (self.settings.alien_speed *
                    self.settings.fleet_direction)
        self.rect.y = self.y

Answer 1

我相信你的外星人出现在屏幕左侧是因为这一行：

alien.rect.x = alien.rect.width + 2 * alien.rect.width * row_number

当行号为0时，这里的第二项为0，外星人的x位置将等于其宽度

如果你想让它们开始出现在屏幕的右侧，你可以使它的初始位置等于屏幕宽度减去一定量，就像外星人宽度的倍数