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design: states: section breakdown, markup, readability and grammar fixes
This commit is contained in:
Reynaldo H. Verdejo Pinochet
parent
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commit
3e315b6b2e
1 changed files with 97 additions and 101 deletions
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@ -4,84 +4,84 @@ Both elements and pads can be in different states. The states of the
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pads are linked to the state of the element so the design of the states
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is mainly focused around the element states.
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An element can be in 4 states. NULL, READY, PAUSED and PLAYING. When an
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An element can be in 4 states. `NULL`, `READY`, `PAUSED` and `PLAYING`. When an
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element is initially instantiated, it is in the NULL state.
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## State definitions
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- NULL: This is the initial state of an element.
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- `NULL`: This is the initial state of an element.
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- READY: The element should be prepared to go to PAUSED.
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- `READY`: The element should be prepared to go to PAUSED.
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- PAUSED: The element should be ready to accept and process data. Sink
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elements however only accept one buffer and then block.
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- `PAUSED`: The element should be ready to accept and process data. Sink
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elements, however, only accept one buffer and then block.
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- PLAYING: The same as PAUSED except for live sources and sinks. Sinks
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- `PLAYING`: The same as PAUSED except for live sources and sinks. Sinks
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accept and render data. Live sources produce data.
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We call the sequence NULL→PLAYING an upwards state change and
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PLAYING→NULL a downwards state change.
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We call the sequence `NULL→PLAYING` an upwards state change and
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`PLAYING→NULL` a downwards state change.
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## State transitions
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the following state changes are possible:
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* *NULL -> READY*:
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* `NULL -> READY`:
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- The element must check if the resources it needs are available.
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Device sinks and sources typically try to probe the device to constrain
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their caps.
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- The element opens the device, this is needed if the previous step requires
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the device to be opened.
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* *READY -> PAUSED*:
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- The element pads are activated in order to receive data in PAUSED.
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* `READY -> PAUSED`:
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- The element pads are activated in order to receive data in `PAUSED`.
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Streaming threads are started.
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- Some elements might need to return `ASYNC` and complete the state change
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when they have enough information. It is a requirement for sinks to
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return `ASYNC` and complete the state change when they receive the first
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buffer or EOS event (preroll). Sinks also block the dataflow when in PAUSED.
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- A pipeline resets the running_time to 0.
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- Live sources return NO_PREROLL and don't generate data.
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- A pipeline resets the `running_time` to 0.
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- Live sources return `NO_PREROLL` and don't generate data.
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* *PAUSED -> PLAYING*:
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* `PAUSED -> PLAYING`:
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- Most elements ignore this state change.
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- The pipeline selects a clock and distributes this to all the children
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before setting them to PLAYING. This means that it is only allowed to
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synchronize on the clock in the PLAYING state.
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- The pipeline uses the clock and the running_time to calculate the base_time.
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The base_time is distributed to all children when performing the state
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change.
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before setting them to `PLAYING`. This means that it is only allowed to
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synchronize on the clock in the `PLAYING` state.
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- The pipeline uses the clock and the `running_time` to calculate the
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`base_time`. This `base_time` is distributed to all children when
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performing the state change.
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- Sink elements stop blocking on the preroll buffer or event and start
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rendering the data.
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- Sinks can post the EOS message in the PLAYING state. It is not allowed to
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post EOS when not in the PLAYING state.
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- While streaming in PAUSED or PLAYING elements can create and remove
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- Sinks can post the EOS message in the `PLAYING` state. It is not allowed to
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post EOS when not in the `PLAYING` state.
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- While streaming in `PAUSED` or `PLAYING` elements can create and remove
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sometimes pads.
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- Live sources start generating data and return SUCCESS.
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- Live sources start generating data and return `SUCCESS`.
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* *PLAYING -> PAUSED*:
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* `PLAYING -> PAUSED`:
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- Most elements ignore this state change.
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- The pipeline calculates the running_time based on the last selected clock
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and the base_time. It stores this information to continue playback when
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going back to the PLAYING state.
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- The pipeline calculates the `running_time` based on the last selected clock
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and the `base_time`. It stores this information to continue playback when
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going back to the `PLAYING` state.
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- Sinks unblock any clock wait calls.
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- When a sink does not have a pending buffer to play, it returns `ASYNC` from
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this state change and completes the state change when it receives a new
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buffer or an EOS event.
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- Any queued EOS messages are removed since they will be reposted when going
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back to the PLAYING state. The EOS messages are queued in GstBins.
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- Live sources stop generating data and return NO_PREROLL.
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back to the PLAYING state. The EOS messages are queued in `GstBins`.
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- Live sources stop generating data and return `NO_PREROLL`.
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* *PAUSED -> READY*:
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* `PAUSED -> READY`:
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- Sinks unblock any waits in the preroll.
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- Elements unblock any waits on devices
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- Chain or get_range functions return FLUSHING.
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- Chain or `get_range()` functions return `FLUSHING`.
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- The element pads are deactivated so that streaming becomes impossible and
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all streaming threads are stopped.
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- The sink forgets all negotiated formats
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- Elements remove all sometimes pads
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* *READY -> NULL*:
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* `READY -> NULL`:
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- Elements close devices
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- Elements reset any internal state.
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@ -89,82 +89,81 @@ the following state changes are possible:
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An element has 4 state variables that are protected with the object LOCK:
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- *STATE*
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- *STATE_NEXT*
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- *STATE_PENDING*
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- *STATE_RETURN*
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- `STATE`
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- `STATE_NEXT`
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- `STATE_PENDING`
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- `STATE_RETURN`
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The STATE always reflects the current state of the element. The
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STATE\_NEXT reflects the next state the element will go to. The
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STATE\_PENDING always reflects the required state of the element. The
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STATE\_RETURN reflects the last return value of a state change.
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The `STATE` always reflects the current state of the element. The
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`STATE_NEXT` reflects the next state the element will go to. The
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`STATE_PENDING` always reflects the required state of the element. The
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`STATE_RETURN` reflects the last return value of a state change.
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The STATE\_NEXT and STATE\_PENDING can be VOID\_PENDING if the element
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The `STATE_NEXT` and `STATE_PENDING` can be `VOID_PENDING` if the element
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is in the right state.
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An element has a special lock to protect against concurrent invocations
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of set\_state(), called the STATE\_LOCK.
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of `set_state()`, called the `STATE_LOCK`.
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## Setting state on elements
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The state of an element can be changed with \_element\_set\_state().
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The state of an element can be changed with `_element_set_state()`.
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When changing the state of an element all intermediate states will also
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be set on the element until the final desired state is set.
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The `set\_state()` function can return 3 possible values:
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The `set_state()` function can return 3 possible values:
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* *GST_STATE_FAILURE*: The state change failed for some reason. The plugin should
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have posted an error message on the bus with information.
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* `GST_STATE_FAILURE`: The state change failed for some reason. The plugin should have posted an error message on the bus with information.
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* *GST_STATE_SUCCESS*: The state change is completed successfully.
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* `GST_STATE_SUCCESS`: The state change is completed successfully.
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* *GST_STATE_ASYNC*: The state change will complete later on. This can happen
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* `GST_STATE_ASYNC`: The state change will complete later on. This can happen
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when the element needs a long time to perform the state change or for sinks
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that need to receive the first buffer before they can complete the state change
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(preroll).
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* *GST_STATE_NO_PREROLL*: The state change is completed successfully but the
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element will not be able to produce data in the PAUSED state.
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* `GST_STATE_NO_PREROLL`: The state change is completed successfully but the
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element will not be able to produce data in the `PAUSED` state.
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In the case of an `ASYNC` state change, it is possible to proceed to the
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next state before the current state change completed, however, the
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next state before the current state change completes, however, the
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element will only get to this next state before completing the previous
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`ASYNC` state change. After receiving an `ASYNC` return value, you can use
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`element\_get\_state()` to poll the status of the element. If the
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`element_get_state()` to poll the status of the element. If the
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polling returns `SUCCESS`, the element completed the state change to the
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last requested state with `set\_state()`.
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last requested state with `set_state()`.
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When setting the state of an element, the STATE\_PENDING is set to the
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When setting the state of an element, the `STATE_PENDING` is set to the
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required state. Then the state change function of the element is called
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and the result of that function is used to update the STATE and
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STATE\_RETURN fields, STATE\_NEXT, STATE\_PENDING and STATE\_RETURN
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and the result of that function is used to update the `STATE` and
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`STATE_RETURN` fields, `STATE_NEXT`, `STATE_PENDING` and `STATE_RETURN`
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fields. If the function returned `ASYNC`, this result is immediately
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returned to the caller.
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## Getting state of elements
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## Getting the state of elements
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The get\_state() function takes 3 arguments, two pointers that will
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hold the current and pending state and one GstClockTime that holds a
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timeout value. The function returns a GstElementStateReturn.
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The `get_state()` function takes 3 arguments, two pointers that will
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hold the current and pending state and one `GstClockTime` that holds a
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timeout value. The function returns a `GstElementStateReturn`.
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- If the element returned `SUCCESS` to the previous \_set\_state()
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- If the element returned `SUCCESS` to the previous `_set_state()`
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function, this function will return the last state set on the
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element and VOID\_PENDING in the pending state value. The function
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returns GST\_STATE\_SUCCESS.
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element and `VOID_PENDING` in the pending state value. The function
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returns `GST_STATE_SUCCESS`.
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- If the element returned NO\_PREROLL to the previous \_set\_state()
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- If the element returned `NO_PREROLL` to the previous `_set_state()`
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function, this function will return the last state set on the
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element and VOID\_PENDING in the pending state value. The function
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returns GST\_STATE\_NO\_PREROLL.
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element and `VOID_PENDING` in the pending state value. The function
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returns `GST_STATE_NO_PREROLL`.
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- If the element returned FAILURE to the previous \_set\_state() call,
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this function will return FAILURE with the state set to the current
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- If the element returned `FAILURE` to the previous `_set_state()` call,
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this function will return `FAILURE` with the state set to the current
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state of the element and the pending state set to the value used in
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the last call of \_set\_state().
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the last call of `_set_state()`.
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- If the element returned `ASYNC` to the previous \_set\_state() call,
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- If the element returned `ASYNC` to the previous `_set_state()` call,
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this function will wait for the element to complete its state change
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up to the amount of time specified in the GstClockTime.
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up to the amount of time specified in the `GstClockTime`.
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- If the element does not complete the state change in the
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specified amount of time, this function will return `ASYNC` with
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@ -173,19 +172,19 @@ timeout value. The function returns a GstElementStateReturn.
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- If the element completes the state change within the specified
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timeout, this function returns the updated state and
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VOID\_PENDING as the pending state.
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`VOID_PENDING` as the pending state.
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- If the element aborts the `ASYNC` state change due to an error
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within the specified timeout, this function returns FAILURE with
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within the specified timeout, this function returns `FAILURE` with
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the state set to last successful state and pending set to the
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last attempt. The element should also post an error message on
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the bus with more information about the problem.
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## States in GstBin
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A GstBin manages the state of its children. It does this by propagating
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A `GstBin` manages the state of its children. It does this by propagating
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the state changes performed on it to all of its children. The
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\_set\_state() function on a bin will call the \_set\_state() function
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`_set_state()` function on a bin will call the `_set_state()` function
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on all of its children, that are not already in the target state or in a
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change state to the target state.
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@ -194,27 +193,27 @@ This makes sure that when changing the state of an element, the
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downstream elements are in the correct state to process the eventual
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buffers. In the case of a downwards state change, the sink elements will
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shut down first which makes the upstream elements shut down as well
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since the \_push() function returns a GST\_FLOW\_FLUSHING error.
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since the `_push()` function returns a `GST_FLOW_FLUSHING` error.
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If all the children return `SUCCESS`, the function returns `SUCCESS` as
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well.
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If one of the children returns FAILURE, the function returns FAILURE as
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If one of the children returns `FAILURE`, the function returns `FAILURE` as
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well. In this state it is possible that some elements successfully
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changed state. The application can check which elements have a changed
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state, which were in error and which were not affected by iterating the
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elements and calling \_get\_state() on the elements.
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elements and calling `_get_state()` on the elements.
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If after calling the state function on all children, one of the children
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returned `ASYNC`, the function returns `ASYNC` as well.
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If after calling the state function on all children, one of the children
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returned NO\_PREROLL, the function returns NO\_PREROLL as well.
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returned `NO_PREROLL`, the function returns `NO_PREROLL` as well.
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If both NO\_PREROLL and `ASYNC` children are present, NO\_PREROLL is
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If both `NO_PREROLL` and `ASYNC` children are present, `NO_PREROLL` is
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returned.
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The current state of the bin can be retrieved with \_get\_state().
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The current state of the bin can be retrieved with `_get_state()`.
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If the bin is performing an `ASYNC` state change, it will automatically
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update its current state fields when it receives state messages from the
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@ -222,11 +221,9 @@ children.
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## Implementing states in elements
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### READY
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### Upward state change
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## upward state change
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Upward state changes always return `ASYNC` either if the STATE\_PENDING is
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Upward state changes always return `ASYNC` either if the `STATE_PENDING` is
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reached or not.
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Element:
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@ -239,9 +236,9 @@ Element:
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- element commits state `ASYNC`
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* A -> B while `ASYNC`
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- update STATE_PENDING state
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- update `STATE_PENDING` state
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- no commit state
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- no change_state called on element
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- no `change_state()` called on element
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Bin:
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@ -254,11 +251,11 @@ Bin:
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- for each commit message, poll elements, this happens in another
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thread.
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- if no `ASYNC` elements, commit state, continue state change
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to STATE_PENDING
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to `STATE_PENDING`
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## downward state change
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### Downward state change
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Downward state changes only return `ASYNC` if the final state is ASYNC.
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Downward state changes only return `ASYNC` if the final state is `ASYNC`.
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This is to make sure that it’s not needed to wait for an element to
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complete the preroll or other `ASYNC` state changes when one only wants to
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shut down an element.
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@ -292,15 +289,14 @@ A -> B => `ASYNC` final state
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- Element committing `SUCCESS`
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- STATE\_LOCK is taken in set\_state
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- `STATE_LOCK` is taken in `set_state()`
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- change state is called if `SUCCESS`, commit state is called
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- commit state calls change\_state to next state change.
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- commit state calls `change_state()` to next state change.
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- if final state is reached, stack unwinds and result is returned
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to set\_state and
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caller.
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to `set_state()` and caller.
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```
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set_state(element) change_state (element) commit_state
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@ -334,7 +330,7 @@ STATE_UNLOCK
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- Element committing `ASYNC`
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- STATE\_LOCK is taken in set\_state
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- `STATE_LOCK` is taken in `set_state()`
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- change state is called and returns `ASYNC`
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@ -342,9 +338,9 @@ STATE_UNLOCK
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- element takes LOCK in streaming thread.
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- element calls commit\_state in streaming thread.
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- element calls `commit_state` in streaming thread.
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- commit state calls change\_state to next state
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- commit state calls `change_state()` to next state
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change.
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```
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@ -380,13 +376,13 @@ STATE_UNLOCK | |
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## Remarks
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set\_state cannot be called from multiple threads at the same time. The
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STATE\_LOCK prevents this.
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`set_state()` cannot be called from multiple threads at the same time. The
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`STATE_LOCK` prevents this.
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State variables are protected with the LOCK.
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Calling set\_state while gst\_state is called should unlock the
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get\_state with an error. The cookie will do that.
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Calling `set_state()` while `get_state()` is called should unlock the
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`get_state()` with an error. The cookie will do that.
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``` c
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set_state(element)
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