Python PySDL2绘图顺序

下面的代码绘制了黑色背景、白色框和文本。然而，似乎绘图顺序还没有确定，我有时会用背景矩形覆盖文本或矩形。我想知道什么是处理这件事的合适方法

import sdl2.ext
import sdl2.sdlttf
from sdl2 import SDL_Color, SDL_Init

WHITE = SDL_Color(255, 255, 255)

class Entity(sdl2.ext.Entity):
    def __init__(self, world, sprite, posx=0, posy=0):
        self.sprite = sprite
        self.sprite.position = posx, posy

sdl2.ext.init()

window = sdl2.ext.Window("PySDL2", size=(320, 240))
window.show()
world = sdl2.ext.World()

texture_renderer = sdl2.ext.Renderer(window)
spriterenderer = sdl2.ext.TextureSpriteRenderSystem(texture_renderer)
factory = sdl2.ext.SpriteFactory(sdl2.ext.TEXTURE, renderer=texture_renderer)

world.add_system(spriterenderer)

sdl2.sdlttf.TTF_Init()
font = sdl2.sdlttf.TTF_OpenFont('resources/Helvetica.dfont',32)
text_surface = sdl2.sdlttf.TTF_RenderText_Blended(font, 'test', WHITE).contents
sdl2.sdlttf.TTF_CloseFont(font)

bg = factory.from_color(sdl2.ext.Color(0, 0, 0), size = (320,240))
Entity(world, bg, 0, 0)

c = factory.from_color(sdl2.ext.Color(255, 255, 255), size = (50,50))
Entity(world, c, 100, 100)

text_sprite = factory.from_surface(text_surface)
text_entity = Entity(world, text_sprite, 50, 50)

def run():
    running = True
    while running:
        events = sdl2.ext.get_events()
        for event in events:
            if event.type == sdl2.SDL_QUIT:
                running = False
                break
        world.process()
run()
sdl2.ext.quit()

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TextureScriteRenderSystem的默认实现使用Sprite对象的深度属性来确定绘图顺序。如果所有精灵具有相同的深度，则它们的绘制顺序将不确定，这将导致您不想要的行为

深度值为0的精灵将在深度值较高（例如10）的精灵下方或之前绘制。正在创建的每个精灵的默认深度为0。可以将背景和矩形的深度设置为负值，以强制绘图顺序：

bg = factory.from_color(sdl2.ext.Color(0, 0, 0), size = (320,240))
bg.depth = -99 # always below everything
Entity(world, bg, 0, 0)

c = factory.from_color(sdl2.ext.Color(255, 255, 255), size = (50,50))
c.depth = -1
Entity(world, c, 100, 100)

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