Allow users to customize history length in FrameTimeDiagnosticsPlugin

[rparrett]

Objective

I have an application where I'd like to measure average frame rate over the entire life of the application, and it would be handy if I could just configure this on the existing FrameTimeDiagnosticsPlugin.

Probably fixes #10948?

Solution

Add max_history_length to FrameTimeDiagnosticsPlugin, and because smoothing_factor seems to be based on history length, add that too.

Discussion

I'm not totally sure that DEFAULT_MAX_HISTORY_LENGTH is a great default for FrameTimeDiagnosticsPlugin (or any diagnostic?). That's 1/3 of a second at typical game frame rates. Moreover, the default print interval for LogDiagnosticsPlugin is 1 second. So when the two are combined, you are printing the average over the last third of the duration between now and the previous print, which seems a bit wonky. (related: #11429)

I'm pretty sure this default value discussed and the current value wasn't totally arbitrary though.

Maybe it would be nice for Diagnostic to have a with_max_history_length_and_also_calculate_a_good_default_smoothing_factor method? And then make an explicit smoothing factor in FrameTimeDiagnosticsPlugin optional?

Or add a new(max_history_length: usize) method to FrameTimeDiagnosticsPlugin that sets a reasonable default smoothing_factor? edit: This one seems like a no-brainer, doing it.

Alternatives

It's really easy to roll your own FrameTimeDiagnosticsPlugin, but that might not be super interoperable with, for example, third party FPS overlays. Still, might be the right call.

Testing

cargo run --example many_sprites (modified to use a custom max_history_length)

Migration Guide

FrameTimeDiagnosticsPlugin now contains two fields. Use FrameTimeDiagnosticsPlugin::default() to match Bevy's previous behavior or, for example, FrameTimeDiagnosticsPlugin::new(60) to configure it.

[alice-i-cecile]

I think that DEFAULT_MAX_HISTORY_LENGTH is a bad default, but that change should be split out IMO. The existing changes are nicely uncontroversial :)