Python pygame sprite在到达屏幕底部后不会向上移动

Question

I'm riffing on an exercise in Python Crash Course v2. I replicated Space Invaders with the alien fleet moving up and down (instead of left and right). Now I'm modifying parts of that to create another game with a dog that should move up and down as it reaches the top and bottom of the screen.

Problem: The dog starts at center right, then moves down to the bottom right corner, but does not move back up. It jitters down there like it thinks it's reached the top and moved back down really fast. I've been staring at this off and on for a couple of weeks and would really appreciate some help finding my mistake.

The code below isolates the dog element in a single .py file. (If you're not using an IDE, edit pygame.QUIT code so it will close gracefully.)

You can get the dog image from the images folder for my [Github for PlayBall pygame][1].Or you can substitute any image about 96 x 56 px.

# -*- coding: utf-8 -*-
"""
Created on Mon Aug 31 19:25:26 2020

@author: Cathig
"""

import sys
import pygame
import pygame.font

class Settings:
    """A class to store all settings for Play Catch."""

    def __init__(self):
        """Initialize the game's settings."""
        # Screen settings
        self.screen_width = 1200
        self.screen_height = 800
        self.bg_color = (20, 230, 80)

        # Dog settings
        self.dog_speed = 1.0
        # dog direction of 1 represents down; -1 represents up.
        self.dog_direction = 1

class Dog:
    def __init__(self, pb_game):
        """Initialize the dog and set its starting position."""
        self.screen = pb_game.screen
        self.settings = pb_game.settings
        self.screen_rect = pb_game.screen.get_rect()

        # Load the dog image and get its rect.
        self.image = pygame.image.load('images/dog.png')
        self.rect = self.image.get_rect()

        # Start the dog at the center right of the screen.
        self.rect.midright = self.screen_rect.midright

        # Store a decimal value for the dog's vertical position.
        self.y = float(self.rect.y)

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

    def update(self, dog_direction):
        """Move the dog down or up."""
        self.y += (self.settings.dog_speed * dog_direction)
        self.rect.y = float(self.y)

    def center_dog(self):
        self.rect.midright = self.screen_rect.midright
        self.y = float(self.rect.y)

    def blitme(self):
        """Draw the dog at its current location."""
        self.screen.blit(self.image, self.rect)

class PlayBall:
    """Overall class to manage game assets and behavior."""

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

        # Set the window size and title bar text
        # Windowed
        self.screen = pygame.display.set_mode(
            (self.settings.screen_width, self.settings.screen_height))
        # Full screen
        # self.screen = pygame.display.set_mode((0, 0), pygame.FULLSCREEN)
        # self.settings.screen_width = self.screen.get_rect().width
        # self.settings.screen_height = self.screen.get_rect().height
        pygame.display.set_caption("Dog test")

        self.dog = Dog(self)

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

            self._update_dog()

            self._update_screen()

    def _update_dog(self):
        """Respond appropriately if the dog has reached an edge."""
        dog_direction = self.settings.dog_direction
        if self.dog.check_edges():
            dog_direction *= -1
        self.dog.update(dog_direction)

    def _check_events(self):
        """Respond to key presses and mouse events."""
        # Gracefully exit when 'X' or alt+F4 close the window
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                pygame.quit() # Use with IDE
                # sys.exit() # If not using IDE, use sys.exit()

    def _start_game(self):
        """Start a new game."""
        self.dog.center_dog()

    def _update_screen(self):
        """Update images on the screen, and flip to the new screen."""
        self.screen.fill(self.settings.bg_color)
        self.dog.blitme()

        # Make the most recently drawn screen visible.
        pygame.display.flip()

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

Answer 1

In the _update_dog method, you are copying the direction to a temporary variable then reversing the temporary variable if an edge is hit. The change is lost on the next loop. To keep the change, update the settings variable also.

Here is the updated code:

def _update_dog(self):
    """Respond appropriately if the dog has reached an edge."""
    dog_direction = self.settings.dog_direction
    if self.dog.check_edges():
        dog_direction *= -1
        self.settings.dog_direction *= -1  # need to update settings also
    self.dog.update(dog_direction)