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design: Update states architectural documentation

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We now have STATE_TARGET, which is subtly different from STATE_PENDING

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-docs/-/merge_requests/149>
This commit is contained in:
Nirbheek Chauhan 2021-03-15 16:07:30 +05:30
parent c1fa62f971
commit 3b2f3939a4
1 changed files with 16 additions and 6 deletions
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markdown/additional/design/states.md
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@ -87,20 +87,30 @@ the following state changes are possible:
## State variables
An element has 4 state variables that are protected with the object LOCK:
An element has 5 state variables that are protected with the object LOCK:
- `STATE`
- `STATE_NEXT`
- `STATE_PENDING`
- `STATE_TARGET`
- `STATE_RETURN`
The `STATE` always reflects the current state of the element. The
`STATE_NEXT` reflects the next state the element will go to. The
`STATE_PENDING` always reflects the required state of the element. The
`STATE` always reflects the current state of the element.
`STATE_NEXT` reflects the next state the element will go to.
`STATE_PENDING` always reflects the final state that the element is going to.
This is different than `STATE_NEXT` when the final state involves going through
multiple state changes, like from `PLAYING -> NULL`.
`STATE_TARGET` is the final state that the element should go to as set by the
application. `STATE_PENDING` can diverge from `STATE_TARGET` during `ASYNC`
state changes when the element does state transitions.
`STATE_RETURN` reflects the last return value of a state change.
The `STATE_NEXT` and `STATE_PENDING` can be `VOID_PENDING` if the element
is in the right state.
`STATE_NEXT` and `STATE_PENDING` can be `VOID_PENDING` if the element is
already in the right state.
An element has a special lock to protect against concurrent invocations
of `set_state()`, called the `STATE_LOCK`.