`Stage::run::<StateTransitionStage>()` causes the initial state to exit and enter again on first run about iyes_loopless HOT 5 CLOSED

If we're asking the user to insert a resource to transition, I would personally expect the existence of the NextState resource to indicate that a transition should occur. IMO we should simply not insert this resource by default, and remove it once the transition has processed.

from iyes_loopless.

OK, reverted back to the 0.1 states behavior, except now without the next != current check (transitioning to the same state as the current is allowed). The NextState behaves as you described: insert it to trigger a transition, and it gets auto-removed when the transition is performed.

I consider this bug important enough that I decided to release a 0.3.0 immediately, just for this.

from iyes_loopless.

Ways to fix this:

1. Treat switching a state to itself as an edge-case and require users to set another resource (e.g. insert_resource(NextState(same_state)) + insert_resource(SameStateSwitch(true))) in order to execute exit and enter stages.
   1.1. Pros: Only need to check for resource SameStateSwitch(true) if current and new state are equal.
   1.2. Cons: Clunky if you need to switch state to itself intentionally. New users need to discover the need for SameStateSwitch resource.
2. Turn NextState<T>(pub T) into NextState<T>(pub Option<T>).
   2.1. Pros: Simple to use.
   2.2. Cons: Matching an Option is not free. (It doesn't cost much though)

from iyes_loopless.

That was the behavior in 0.1. I changed it, because "repeatedly inserting and removing a resource" (effectively "using a resource as an event") felt weird to me. But you are right, why not, it makes sense. In retrospect, if we are gonna use a resource to trigger a transition, that makes more sense. I guess I should bring it back.

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I confirmed that the fix works. Only the initial state's enter stage is run at the start, and it only ran once.

from iyes_loopless.