A radial progress bar with a gradient!

main.py

from kivy.animation import Animation
from kivy.app import App
from kivy.lang import Builder
from kivy.properties import AliasProperty, ColorProperty, NumericProperty
from kivy.uix.effectwidget import AdvancedEffectBase, EffectWidget


class TangentGradient(AdvancedEffectBase):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.glsl = '''
uniform vec4 start_color;
uniform vec4 end_color;

float pi = 3.141592653589793;

vec4 effect(vec4 color, sampler2D texture, vec2 tex_coords, vec2 coords)
{
    float alpha = (.33 * color.r + .5 * color.g + .17 * color.b) * color.a;
    return vec4(mix(start_color, end_color, atan(
        tex_coords.x * 2.0 - 1.0,
        tex_coords.y * 2.0 - 1.0
    ) / (2. * pi) + .5).rgb, alpha);
}
        '''


class RadialProgressBar(EffectWidget):
    progress = NumericProperty()
    start_color = ColorProperty()
    end_color = ColorProperty()
    bar_width = NumericProperty(25)

    def _get_uniforms(self):
        return {
            'start_color': tuple(float(rgba) for rgba in self.start_color),
            'end_color': tuple(float(rgba) for rgba in self.end_color),
        }

    uniforms = AliasProperty(_get_uniforms, bind=['start_color', 'end_color'])

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        gradient = TangentGradient(uniforms=self.uniforms)
        self.bind(uniforms=gradient.setter('uniforms'))
        self.effects = [gradient]


KV = '''
RadialProgressBar:
    progress: app.progress
    start_color: 0, 1, 0, 1
    end_color: 0, 0, 1, 1

    Label:
        text: 'Radial progress:'
        font_size: 64


<RadialProgressBar>:
    Widget:
        pos: root.pos
        size: root.size

        canvas:
            Color:
            Line:
                circle:
                    [
                    *root.center,
                    min(root.size) / 2 - root.bar_width,
                    -180,
                    360 * root.progress - 180,
                    ]
                width: root.bar_width
                cap: 'none'
'''


class Test(App):
    progress = NumericProperty()

    def build(self):
        animation = (
            Animation(progress=1, t='out_expo')
            + Animation(progress=0, t='out_expo')
        )
        animation.repeat = True
        animation.start(self)
        return Builder.load_string(KV)


Test().run()