20. Touch Coordinates

on_touch_down is fired when a touch, on mobile, or a mouse click, on desktop, is detected. on_size is called whenever the size of the window changes, as well as at startup.

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We use BoxLayout as base for the root class. We import graphics classes since drawings will be done in the Python file. The only function of on_touch_down, here, is to print the coordinates that it receives.

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The on_size function is called whenever the size property changes. In the first half of this function, the lists px and py are created. They hold the coordinates for 9 horizontal lines and 9 vertical lines.

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First the canvas is cleared so the old instructions do not effect the screen. Then, we create a red background color as well as 18 blue lines, half horizontal and half vertical.

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The app class is created and run.

# ex20.py

from kivy.uix.boxlayout import BoxLayout
from kivy.app import App
from kivy.graphics import Color, Line, Rectangle

class Ex20(BoxLayout):
    def on_touch_down(self, touch):
        print("pos: {}\nspos: {}\n".format(touch.pos,touch.spos))
    def on_size(self,*args):
        self.px = []
        self.py = []
        w = self.width
        h = self.height
        for r in range(1,10):
            rw = r / 10.0
            self.px.extend([rw*w,0,rw*w,h])
            self.py.extend([0,rw*h,w,rw*h])
        self.canvas.clear()
        with self.canvas:
            Color(1,0,0)
            Rectangle(size=self.size)
            Color(0,0,1)
            for i in range(9):
                Line(points=self.px[i*4:i*4+4],width=2)
                Line(points=self.py[i*4:i*4+4],width=2)
                
class Ex20App(App):
    def build(self):
        return Ex20()

if __name__=='__main__':
    Ex20App().run()

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This is the result of the execution. Due to the lines, we create a grid and we can click at any point and see the Python console.

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The Python console shows the results from the printouts in the on_touch_down function. The first three are due to clicks in the upper-right-corner, and the last three clicks are in the lower-left corner of the screen.

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