如何在Kivy按钮on_press事件中隐藏Canvas与Rectangle

Hey there! I see you're trying to hide the background Rectangle in your login screen when the "Login" button is pressed. The tricky part here is that Canvas instructions (like Color and Rectangle) aren't widgets, so you can't just set their opacity property like you did with the labels and text inputs. Let's fix this with a simple boolean flag approach.

Step 1: Add a Boolean Property to Your Root Widget #

First, in your Python code, define a BooleanProperty to control whether the background should be visible. This will let us toggle Canvas rendering directly from the button press.

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import BooleanProperty
from kivy.lang import Builder

Builder.load_file('your_login_ui.kv')  # Replace with your actual kv file name

class LoginScreen(Widget):
    show_background = BooleanProperty(True)  # Flag to control background visibility
    
    def validate(self, username, password):
        # Your existing login validation logic goes here
        pass

class MyLoginApp(App):
    def build(self):
        return LoginScreen()

if __name__ == '__main__':
    MyLoginApp().run()

Step 2: Update Your KV Code to Use the Flag #

Modify your kv code to only render the canvas.before instructions when show_background is True. We'll use Kivy's conditional syntax in the kv language for clean control:

<LoginScreen>:
    name: "cany"
    canvas.before:
        if root.show_background:
            Color:
                rgba: 1,1,1,1
            Rectangle:
                size: self.size
                pos: self.pos
                source: 'rect4190.png'
    Label:
        id:user_lbl
        text:"User Name: "
        color: .9,.1,.1,1
        pos: 40,400
    TextInput:
        id:username_txt
        size: cm(4),cm(0.66)
        pos: 125,437
        write_tab: False
        multiline: False
    Label:
        id:pass_lbl
        text: 'Password:'
        color: .9, 0.12, .1, 1
        pos: 40,350
    TextInput:
        id:password_txt
        size: cm(4),cm(0.66)
        pos: 125,387
        write_tab: False
        multiline: False
    Button:
        id:btn_login
        size: cm(2.33),cm(0.66)
        text: 'Login'
        pos: 160,200
        focus: True
        on_press:
            root.validate(username_txt.text,password_txt.text)
            user_lbl.opacity = 0
            pass_lbl.opacity = 0
            username_txt.opacity = 0
            password_txt.opacity = 0
            btn_login.opacity = 0
            root.show_background = False  # Toggle background off here

How This Works #

• The show_background boolean acts as a switch: when True, the Canvas draws your background; when False, those instructions are skipped entirely.
• Adding root.show_background = False to the button's on_press event ensures the background disappears right when the user clicks "Login", matching the behavior of your other UI elements.

If you ever wanted to bring the background back later (like after a failed login), you just need to set root.show_background = True somewhere in your code.

内容的提问来源于stack exchange，提问作者mohamadreza