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Classpad Dev

Touchscreen Drawing

In this short tutorial, you’ll learn how to use the touchscreen to interact with your program.

We’ll make a simple drawing app:

  • Drag your finger to draw lines
  • Press [EXE] to clear
  • Press [CLEAR] to exit

How it works

ClassPad touch input is handled through the event system, just like keys.
You can listen to:

  • KEYEV_TOUCH_DOWN: when your finger touches the screen
  • KEYEV_TOUCH_DRAG: while you’re dragging
  • KEYEV_TOUCH_UP: when you lift your finger

Each event has .x and .y fields for the position (in pixels).

Step 1: Start with a blank screen

We create a function clear() that:

  • Clears the screen
  • Draws a helpful label at the top
def clear():
    dclear(C_WHITE)
    dtext(2, 2, C_BLACK, "Touch: draw | EXE: clear | Clear: quit")

We then call it once at the beginning:

clear()
dupdate()

Step 2: Track Touch Position

We use two variables to remember the last touch location:

x, y = -1, -1

Every time the screen is touched, we save where it started.

Step 3: Event Loop

We use pollevent() to listen to input:

ev = pollevent()
while ev.type != KEYEV_NONE:
    # Handle events...
    ev = pollevent()

This loop catches every type of input — key presses and touches.

Step 4: Drawing with your finger

if ev.type == KEYEV_TOUCH_DOWN:
    x, y = ev.x, ev.y

This saves the starting point when you first touch the screen.

if ev.type == KEYEV_TOUCH_DRAG:
    dline(x, y, ev.x, ev.y, C_BLACK)
    x, y = ev.x, ev.y

This draws a line between the last known point and the new one, creating a fluid pen stroke.

Step 5: Keyboard controls

if ev.type == KEYEV_DOWN and ev.key == KEY_EXE:
    clear()

Press [EXE] to clear the screen and restart drawing.

if ev.type == KEYEV_DOWN and ev.key == KEY_EXIT:
    break

You can optionally add this to allow quitting with [CLEAR].

Final Code

from gint import *

def clear():
    dclear(C_WHITE)
    dtext(2, 2, C_BLACK, "Touch: draw | EXE: clear | Clear: quit")

clear()
dupdate()

x, y = -1, -1

while True:
    ev = pollevent()
    while ev.type != KEYEV_NONE:
        if ev.type == KEYEV_DOWN and ev.key == KEY_EXE:
            clear()
        if ev.type == KEYEV_TOUCH_DOWN:
            x, y = ev.x, ev.y
        if ev.type == KEYEV_TOUCH_DRAG:
            dline(x, y, ev.x, ev.y, C_BLACK)
            x, y = ev.x, ev.y
        ev = pollevent()
    dupdate()

Want to try more touch projects? Try:

  • A touch paint app with colors
  • A grid tile editor
  • A basic drag & drop UI

Need help with ideas? Ask the 👨‍💻 PythonExtra ChatGPT!