Warn when no camera exists to render an object

[alice-i-cecile]

What problem does this solve or what need does it fill?

Beginners commonly attempt to get started with adding 2D/3D/UI graphics, forget to add a camera, and then are confused when nothing appears.

What solution would you like?

Warn when no camera of the appropriate type exists.

What alternative(s) have you considered?

Improve documentation further, especially in the Bevy book.

Additional context

The exact timing / details of when this should trigger need some work by others who understand the rendering pipeline more clearly.

We should also be careful not to create unsilenceable warnings in cases where the lack of camera may actually be intended.

[cart]

From a comment I left in #1440:

I think my ideal solution is to make the UiCamera completely optional (and fall back to a default value). Most people don't need control over the UI camera, so theres no real reason to dictate one (other than the fact that its easier to require one because that behavior is consistent with the other render passes)

[cart]

The same argument could be made for 2D render passes. But 3d has no "reasonable default", so it might make sense to warn if you try to draw a 3d object when there is no camera.

[wduminy]

I am here because this issue is marked as a good first issue.

However, related items indicate that this issue is possibly resolved.

Is there any more work to be done here?

[bas-ie]

Hrm 🤔 The associated PR was closed because it added a check on each frame. The goal is to achieve a warning without using a system. I'll take a poke around, see what the current state is, and whether there might be a semi-obvious solution in 2024.