How can I create a script that runs continuously but does not block the interface?

Question

I would like to create a script that runs constantly throughout the life of a Blender session. The problem is that Blender's interface "blocks" while the script is running. As a trivial example, if I run the code below:

while True:
    continue

Then Blender freezes up, and I have to use xkill to stop it. How may I create a script that may be running constantly but does not stop Blender from functioning normally?

Answer 1

What you are looking for is to create a modal operator. From the Wiki:

A modal operator defines a Operator.modal function which running, handling events until it returns {'FINISHED'} or {'CANCELLED'}. Grab, Rotate, Scale and Fly-Mode are examples of modal operators. They are especially useful for interactive tools, your operator can have its own state where keys toggle options as the operator runs.

There is a template for the Modal Operator accessible from the Blender Text Editor.

Answer 2

Instead of running your code continuously all of the time, consider either running it periodically (every x seconds, etc), or only calling it when a specific event comes up.

The standard technique to use to get your code snippet to run periodically is to use the threading module. For example:

import threading

def my_func():
    threading.Timer(number_of_seconds, my_func).start()

This will repeat the my_func function in intervals every number_of_seconds.

The other option is to instead link your code snippet to a specific action in Blender, by simply putting the call for your function in a well-selected location. I personally recommend this option, if it works in your particular case, since it certainly a more efficient way of handling code.