Add a resources_present run condition

[alice-i-cecile]

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

The .run_if(resource_exists::<LateResource>()) run condition pattern is very helpful for avoiding silly panics when doing deferred resource initialization.

However, the exact identity of the resource type(s) rarely matters. When you have multiple, you need to either combine run conditions, use an Option<Res<T>> system parameter, or write your own one-off run condition.

What solution would you like?

Add a resources_present run condition, exported in common_conditions (and demonstrated with an example).

This checks all of the resources requested, and validates that they each exist. If they all do, it returns true. Otherwise, false.

What alternative(s) have you considered?

It may be easier to check if all of the system parameters can be fetched. In that case, we should change the name to something like system_param_exist to better match the functionality.

[AnthonyKalaitzis]

@alice-i-cecile Wouldn't mind giving this one a go if no one else is doing it

[alice-i-cecile]

Awesome, please feel free! Ask around in #ecs-dev if you get stuck.