Let’s say you’ve got the Raspberry Pi camera module, and you’d like it to snap a photo when you press a button. However, you don’t want your code to poll that button constantly, and you certainly don’t want to wait for an edge because you may have other code running simultaneously.
The best way to execute this code is using a callback function. This is a function that is attached to a specific GPIO pin and run whenever that edge is detected. Let’s try one:
import RPi.GPIO as GPIO
GPIO.setup(23, GPIO.IN, pull_up_down = GPIO.PUD_DOWN)
GPIO.setup(24, GPIO.IN, pull_up_down = GPIO.PUD_UP)
print(“Button 1 pressed!”)
print(“Note how the bouncetime affects the button press”)
GPIO.add_event_detect(23, GPIO.RISING, callback=printFunction, bouncetime=300)
print(“Button 2 Pressed”)
print(“Button 2 Released”)
You’ll notice here that button 1 will consistently trigger the printFunction, even while the main loop is waiting for an edge on button 2. This is because the callback function is in a separate thread. Threads are important in programming because they allow things to happen simultaneously without affecting other functions. Pressing button 1 will not affect what happens in our main loop.
Events are also great, because you can remove them from a pin just as easily as you can add them:
Now you’re free to add a different function to the same pin!