The lifetime of Events seems to have changed in 0.12

[MJohnson459]

Bevy version

0.12.1

What you did

I just upgraded to 0.12.1 and I noticed that this code no longer works for panning the camera with a mouse drag:

fn pan_camera(
    mut ev_motion: EventReader<MouseMotion>,
    input_mouse: Res<Input<MouseButton>>,
) {
...
    if input_mouse.pressed(pan_button) {
        for ev in ev_motion.read() {
            pan += ev.delta;
        }
    }

Previously the ev_motion reader would only contain events from the last frame. Now it seems that there is a backlog of events stored and so clicking the mouse results in a large pan happening.

I believe previously bevy used to only store the delta for two frames but now it is unbounded? I didn't see anything in the release or migration notes about this but I am guessing it might be related to #7728 (@james7132 )?

The workaround for now is to just clean the events every frame. This will fix the symptoms for one system but doesn't fix the underlying change which has implications for memory usage etc.

    if input_mouse.pressed(pan_button) {
        for ev in ev_motion.read() {
            pan += ev.delta;
        }
    } else {
        ev_motion.clear();
    }

[mockersf]

seems related to #10877

[alice-i-cecile]

Closing as duplicate.

[james7132]

I believe previously bevy used to only store the delta for two frames but now it is unbounded? I didn't see anything in the release or migration notes about this but I am guessing it might be related to #7728 (@james7132 )?

#7728 will only stop running the update if both internal stores are empty. I don't think this is the cause of this issue.