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326d36b8d8
Original commit message from CVS: Added state change code. Added/updated docs. Added sink base class, make fakesink extend the base class. Small cleanups in GstPipeline.
151 lines
6.1 KiB
Text
151 lines
6.1 KiB
Text
States
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======
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Both elements and pads can be in different states. The states of the pads are
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linked to the state of the element so the design of the states is mainly
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focused around the element states.
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An element can be in 4 states. NULL, READY, PAUSED and PLAYING. When an element
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is initially instantiated, it is in the NULL state.
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State definitions
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-----------------
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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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- 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 sinks, who are now accepting
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and rendering data.
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We call the sequence NULL->PLAYING an upwards state change and PLAYING->NULL
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a downwards state change.
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State variables
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---------------
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An element has a special lock to manage the state changes. This lock is called
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the STATE_LOCK.
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The STATE_LOCK protects 3 element variables:
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- STATE
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- PENDING_STATE
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- STATE_ERROR flag
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The STATE always reflects the current state of the element. The PENDING_STATE
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always reflects the required state of the element. The PENDING_STATE can be
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VOID_PENDING if the element is in the right state. The STATE_ERROR flag
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indicates that an error occured while doing the last state change.
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Setting state on elements
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-------------------------
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The state of an element can be changed with _element_set_state(). When chaning
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the state of an element all intermediate states will also be set on the element
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until the final desired state is set.
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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_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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When the element needs a long time to perform the state
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change or for sinks that need to receive the first buffer
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before they can complete the state change (preroll).
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In the case of an async state change, it is not possible to proceed to the next
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state until the current state change completed. After receiving an ASYNC return
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value, you can use _element_get_state() to poll the status of the element.
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When setting the state of an element, the PENDING_STATE is set to the required
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state and the STATE_ERROR flag is cleared. Then the state change function of the
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element is called and the result of that function is used to update the STATE,
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PENDING_STATE and STATE_ERROR flags. If the function returned ASYNC, this result
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is immediatly returned to the caller.
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Getting state of elements
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-------------------------
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The _get_state() function takes 3 arguments, two pointers that will hold the
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current and pending state and one GTimeVal that holds a timeout value. The
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function returns a GstElementStateReturn.
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- If the element returned SUCCESS to the previous _set_state() function, this
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function will return the last state set on the element and VOID_PENDING in
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the pending state value.
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- If the element returned FAILURE to the previous _set_state() call, this
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funciton will return FAILURE with the state set to the current state of
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the element and the pending state set to the value used in the last call
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of _set_state().
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- If the element returned ASYNC to the previous _set_state() call, this function
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will wait for the element to complete its state change up to the amount of time
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specified in the GTimeVal.
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* If the element does not complete the state change in the specified amount of
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time, this function will return ASYNC with the state set to the current state
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and the pending state set to the pending state.
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* If the element completes the state change within the specified timeout, this
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function returns the updated state and VOID_PENDING as the pending state.
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* If the element aborts the ASYNC state change due to an error within the
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specified timeout, this function returns FAILURE with the state set to last
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successfull state and pending set to the last attempt. The element should
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also post an error message on the bus with more information about the problem.
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States in GstBin
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----------------
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A GstBin manages the state of its children. It does this by propagating the state
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changes performed on it to all of its children. The _set_state() function on a
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bin will call the _set_state() function on all of its children.
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The children are iterated from the sink elements to the source elements. This makes
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sure that when changing the state of an element, the downstream elements are in
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the correct state to process the eventual buffers. In the case of a downwards
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state change, the sink elements will shut down first which makes the upstream
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elements shut down as well since the _push() function returns a GST_FLOW_WRONG_STATE
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error.
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If all the children return SUCCESS, the function returns SUCCESS as well.
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If one of the children returns FAILURE, the function returns FAILURE as well. In
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this state it is possible that some elements successfuly changed state. The
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application can check which elements have a changed state, which were in error
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and which were not affected by iterating the elements and calling _get_state()
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on the elements.
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If after calling the state function on all children, one of the children returned
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ASYNC, the function returns ASYNC as well.
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The current state of the bin can be retrieved with _get_state(). This function will
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call the _get_state() function on all the elements. If one of the children returns
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FAILURE or ASYNC, the bin reports FAILURE or ASYNC respectively. The bin also
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updates its state variables after polling its children, this means that the state
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variables of the bin are only updated after calling _get_state() on the bin.
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The _get_state() function will be called on the children with the same timout value
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so the function can potentially block timeout*num_children.
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Implementing states in elements
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-------------------------------
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READY
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-----
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