When listening to the iOS development lecture today, I entered the code according to the demo of the lecture and encountered the error “this class is not key value coding-compliant for the key digitPressed.'” when I tried to run it. After some searching and troubleshooting, I found it was my own When setting up IBAction and IBOutlet, there are redundant wires. Press the ctrl key to check the connection of the control and delete the redundant ones continuously.