Kivy：每次触摸后（on_touch_down）在FloatLayout中添加图像，但以前的所有图像都消失了

Question

I am trying to plot an image ( Image object) each time the on_touch_down method is triggered. Each image has the same source. This is done inside the FloatLayout and the plotting is done by self.add_widget(theImage) . I got no error, but can only display one image after each touch (the previous images disappeared after each touch).

But if i write print(self.children) inside the on_touch_down , we can see that the list is updated at every touch .

Why is this? Thanks.

(*The image size is such that the screen can fit 20 images without colliding each other)

The code :

from kivy.app import App
from kivy.uix.image import Image
from kivy.uix.floatlayout import FloatLayout
import random

class Screen(FloatLayout):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
    def on_touch_down(self, touch):           
        super().on_touch_down(touch)
        plot = Image(source='bottle.jpg')
        plot.pos_hint['x'] = random.uniform(0,1) - 0.5*plot.size_hint_x
        plot.pos_hint['y'] = random.uniform(0,1) - 0.5*plot.size_hint_y
        self.add_widget(plot)

class MyApp(App):
    def build(self):
        return Screen()

app = MyApp()

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

Answer 1

According to what is observed if a pos_hint is not established when the widget is created, it has the value of an empty dictionary that is a property of the class and not of the object, meaning that if a value is established an object is established to the others objects.

To verify this, the id of pos_hint is printed obtaining the same value:

def on_touch_down(self, touch):           
    super().on_touch_down(touch)
    plot = Image(source='bottle.jpg')
    plot.pos_hint['x'] = random.uniform(0,1) - 0.5*plot.size_hint_x
    plot.pos_hint['y'] = random.uniform(0,1) - 0.5*plot.size_hint_y
    print(id(plot.pos_hint))
    self.add_widget(plot)

Output:

140252781257784
140252781257784
140252781257784
140252781257784
...

So the solution is to pass a new dictionary:

def on_touch_down(self, touch):           
    super().on_touch_down(touch)
    plot = Image(source='bottle.jpg', pos_hint={'x':.0, 'y':.0}) # <-- default value
    plot.pos_hint['x'] = random.uniform(0,1) - 0.5*plot.size_hint_x
    plot.pos_hint['y'] = random.uniform(0,1) - 0.5*plot.size_hint_y
    self.add_widget(plot)

Or pass directly the new dictionary:

def on_touch_down(self, touch):           
    super().on_touch_down(touch)
    plot = Image(source='bottle.jpg')
    plot.pos_hint = {'x': random.uniform(0,1) - 0.5*plot.size_hint_x, 'y': random.uniform(0,1) - 0.5*plot.size_hint_y}
    self.add_widget(plot)