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61 lines
2.4 KiB
Markdown
61 lines
2.4 KiB
Markdown
# Context
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`GstContext` is a container object, containing a type string and a generic
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`GstStructure`. It is used to store and propagate context information in a
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pipeline, like device handles, display server connections and other
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information that should be shared between multiple elements in a
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pipeline.
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For sharing context objects and distributing them between application
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and elements in a pipeline, there are downstream queries, upstream
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queries, messages and functions to set a context on a complete pipeline.
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## Context types
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Context type names should be unique and be put in appropriate
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namespaces, to prevent name conflicts, e.g. "gst.egl.EGLDisplay". Only
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one specific type is allowed per context type name.
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## Elements
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Elements that need a specific context for their operation would do the
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following steps until one succeeds:
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1) Check if the element already has a context of the specific type,
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i.e. it was previously set via `gst_element_set_context()`.
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2) Query downstream with `GST_QUERY_CONTEXT` for the context and check if
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downstream already has a context of the specific type
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3) Query upstream with `GST_QUERY_CONTEXT` for the context and check if
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upstream already has a context of the specific type
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4) Post a `GST_MESSAGE_NEED_CONTEXT` message on the bus with the required
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context types and afterwards check if a usable context was set now
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as in 1). The message could be handled by the parent bins of the
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element and the application.
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4) Create a context by itself and post a `GST_MESSAGE_HAVE_CONTEXT` message
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on the bus.
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Bins will propagate any context that is set on them to their child
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elements via `gst_element_set_context()`. Even to elements added after
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a given context has been set.
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Bins can handle the `GST_MESSAGE_NEED_CONTEXT` message, can filter both
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messages and can also set different contexts for different pipeline
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parts.
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## Applications
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Applications can set a specific context on a pipeline or elements inside
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a pipeline with `gst_element_set_context()`.
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If an element inside the pipeline needs a specific context, it will post
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a `GST_MESSAGE_NEED_CONTEXT` message on the bus. The application can
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now create a context of the requested type or pass an already existing
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context to the element (or to the complete pipeline).
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Whenever an element creates a context internally it will post a
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`GST_MESSAGE_HAVE_CONTEXT` message on the bus. Bins will cache these
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contexts and pass them to any future element that requests them.
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