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Figure 13-1: The alien we’ll use to build the fleet Creating the Alien Class
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Python Crash Course, 2nd Edition
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- Creating an Instance of the Alien
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Figure 13-1: The alien we’ll use to build the fleet Creating the Alien Class Now we’ll write the Alien class and save it as alien.py: import pygame from pygame.sprite import Sprite class Alien(Sprite): """A class to represent a single alien in the fleet.""" def __init__(self, ai_game): """Initialize the alien and set its starting position.""" super().__init__() self.screen = ai_game.screen # 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 left of the screen. u self.rect.x = self.rect.width self.rect.y = self.rect.height # Store the alien's exact horizontal position. v self.x = float(self.rect.x) Most of this class is like the Ship class except for the aliens’ placement on the screen. We initially place each alien near the top-left corner of the screen; we add a space to the left of it that’s equal to the alien’s width and a space above it equal to its height u so it’s easy to see. We’re mainly alien.py 258 Chapter 13 concerned with the aliens’ horizontal speed, so we’ll track the horizontal position of each alien precisely v. This Alien class doesn’t need a method for drawing it to the screen; instead, we’ll use a Pygame group method that automatically draws all the elements of a group to the screen. Creating an Instance of the Alien We want to create an instance of Alien so we can see the first alien on the screen. Because it’s part of our setup work, we’ll add the code for this instance at the end of the __init__() method in AlienInvasion . Eventually, we’ll create an entire fleet of aliens, which will be quite a bit of work, so we’ll make a new helper method called _create_fleet() . The order of methods in a class doesn’t matter, as long as there’s some consistency to how they’re placed. I’ll place _create_fleet() just before the _update_screen() method, but anywhere in AlienInvasion will work. First, we’ll import the Alien class. Here are the updated import statements for alien_invasion.py: --snip-- from bullet import Bullet from alien import Alien And here’s the updated __init__() method: def __init__(self): --snip-- self.ship = Ship(self) self.bullets = pygame.sprite.Group() self.aliens = pygame.sprite.Group() self._create_fleet() We create a group to hold the fleet of aliens, and we call _create_fleet() , which we’re about to write. Here’s the new _create_fleet() method: def _create_fleet(self): """Create the fleet of aliens.""" # Make an alien. alien = Alien(self) self.aliens.add(alien) In this method, we’re creating one instance of Alien , and then adding it to the group that will hold the fleet. The alien will be placed in the default upper-left area of the screen, which is perfect for the first alien. alien_invasion.py alien_invasion.py alien_invasion.py Aliens! 259 To make the alien appear, we need to call the group’s draw() method in _update_screen() : def _update_screen(self): --snip-- for bullet in self.bullets.sprites(): bullet.draw_bullet() self.aliens.draw(self.screen) pygame.display.flip() When you call draw() on a group, Pygame draws each element in the group at the position defined by its rect attribute. The draw() method requires one argument: a surface on which to draw the elements from the group. Figure 13-2 shows the first alien on the screen. Figure 13-2: The first alien appears. Now that the first alien appears correctly, we’ll write the code to draw an entire fleet. Download 4.21 Mb. Do'stlaringiz bilan baham: |
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