This PR was merged into the 3.4 branch.

Discussion
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[Workflow] Add transition completed event

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR | symfony/symfony-docs#8213

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Because the "entered" event is the only event dispatched after the new marking is applied, and publish's an event upon entering into a "Place" (as opposed to completing a transition), it is not sufficient for a lot of use cases and is causing bugs.

Example:
Enabled Transitions:
1. A -> B
2. B -> C
3. C -> B

Transition 1 and transition 3, will dispatch an "entered" event on Place B, forcing post transition behaviour to be the same for both transition 1 and 3.

A user might need different behaviour depending on the transition, rather the the destination.
A concrete use case would be when applying an "undo" transition to a subject. One may or may not want to re-trigger all the events associated with the original transition to that Place.

I propose adding a "completed" event (ie. Transition completed) in addition to the entered event.

Commits
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c254cac [Workflow] Added an transition completed event

…zzyp)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #8213).

Discussion
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[Workflow] Document the new transition completed event

as per PR - symfony/symfony#22587

Commits
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b8249ab [Workflow] Document the new transition completed event