Call a function in main thread once threaded function is completed

Question

I am building a Tkinter GUI and I have a process that takes a while to complete, so I threaded it to prevent the GUI from hanging. Lets call the threaded function foo. Once foo is completed, I need to call another function, bar. bar needs to be called from the main thread (it uses a matplotlib method that does not work inside of a thread).

I can't seem to wrap my head around how I might do this. I thought about joining the thread, but that just causes the GUI to hang. I also thought about using a signal variable that I would change in the last line of foo to tell the rest of my program that it is done and its time to execute bar, but then I couldn't figure out how I could continuously check that variable in the main thread without hanging the GUI. Any ideas?

Using Python 3.7

Answer 1

You can use threading.Event() object to notify the main thread and use after() to call a function periodically to check the Event() object to determine when to call bar().

Below is a simple example:

import tkinter as tk
import threading
import time

def foo(event):
    print('foo started')
    time.sleep(5)
    print('foo done')
    # notify main thread
    event.set()

def bar():
    print('hello')

def check_event(event, callback):
    print('.', end='')
    if event.is_set():
        # thread task is completed
        callback()
    else:
        # check again 100 ms (adjust this to suit your case) later
        root.after(100, check_event, event, callback)

root = tk.Tk()

# create the `Event()` object
event = threading.Event()
# start the checking
check_event(event, bar)
# start the thread task
threading.Thread(target=foo, args=(event,)).start()

root.mainloop()