Callbacks

Description¶

If a function is passed to another function as an argument, it is known as a callback.

We define a function to use as the callback with this signature: def my_callback(obj, ev):. Then, to pass client data to it, we simply do: my_callback.my_client_data = my_client_data. This relies on the fact that Python functions are in fact objects and we are simply adding new attributes such as my_client_data in this case.

An alternative method is to define a class passing the needed variables in the __init__ function and then implement a __call__ function that does the work.

The simplest implementation of a callback is CameraPosition where we do not even pass client data to it.

An implementation passing client data for both a function and a class is CallBack, the skeleton code of this is outlined here.

To use the snippet, click the Copy to clipboard at the upper right of the code blocks.

Implementation¶

def get_orientation(caller, ev):
    """
    Print out the orientation.

    We must do this before we register the callback in the calling function.
    Add the active camera as an attribute:
        get_orientation.cam = ren.active_camera

    :param caller: The caller.
    :param ev: The event.
    :return:
    """
    # Just do this to demonstrate who called callback and the event that triggered it.
    print(f'Caller: {caller.class_name}, Event Id: {ev}')
    # Verify that we have a camera in this case.
    print(f'Camera: {get_orientation.cam.class_name}')
    # Now print the camera orientation.
    # camera_orientation(get_orientation.cam)


class OrientationObserver:
    def __init__(self, cam):
        self.cam = cam

    def __call__(self, caller, ev):
        # Just do this to demonstrate who called callback and the event that triggered it.
        print(f'Caller: {caller.class_name}, Event Id: {ev}')
        # Verify that we have a camera in this case.
        print(f'Camera: {self.cam.class_name}')
        # Now print the camera orientation.
        # camera_orientation(self.cam)

Usage¶

    # Set up the callback.
    if use_function_callback:
        # We are going to output the camera position when the event
        #   is triggered, so we add the active camera as an attribute.
        get_orientation.cam = ren.active_camera
        # Register the callback with the object that is observing.
        iren.AddObserver('EndInteractionEvent', get_orientation)
    else:
        iren.AddObserver('EndInteractionEvent', OrientationObserver(ren.active_camera))
        # Or:
        # observer = OrientationObserver(ren.active_camera)
        # iren.AddObserver('EndInteractionEvent', observer)