# GstBin

`GstBin` is a container element for other `GstElements`. It makes possible to group elements together so that they can be treated as one single `GstElement`. A `GstBin` provides a `GstBus` for the children and collates messages from them.

## Adding/removing elements

The basic functionality of a bin is to add and remove `GstElement` to/from it. `gst_bin_add()` and `gst_bin_remove()` perform these operations respectively.

The bin maintains a parent-child relationship with its elements (see relations).

## Retrieving elements

`GstBin` provides a number of functions to retrieve one or more children from itself. A few examples of the provided functions:

- `gst_bin_get_by_name()` retrieves an element by name.
- `gst_bin_iterate_elements()` returns an iterator to all the children.

## Element management

The most important function of the `GstBin` is to distribute all `GstElement` operations on itself to all of its children. These operations include:

- state changes

- index get/set

- clock get/set

The state change distribution is the most complex and is explained in states.

## GstBus

The `GstBin` creates a `GstBus` for its children and distributes it when child elements are added to the bin. The bin attaches a sync handler to receive messages from children. The bus for receiving messages from children is distinct from the bin’s own externally-visible `GstBus`.

Messages received from children are forwarded intact onto the bin’s external message bus, except for EOS and `SEGMENT_START`/`DONE` which are handled specially.

`ASYNC_START`/`ASYNC_STOP` messages received from the children are used to trigger a recalculation of the current state of the bin, as described in states.

The application can retrieve the external `GstBus` and integrate it in the mainloop or it can just `pop()` messages off in its own thread.

When a bin goes to `READY` it will clear all cached messages.

## EOS

The sink elements will post an `EOS` message on the bus when they reach `EOS`. This message is only posted to the bus when the sink element is in `PLAYING`.

The bin collects all `EOS` messages and forwards it to the application as soon as all the sinks have posted an `EOS`.

The list of queued `EOS` messages is cleared when the bin goes to `PAUSED` again. This means that all elements should repost the `EOS` message when going to `PLAYING` again.

## SEGMENT_START/DONE

A bin collects `SEGMENT_START` messages but does not post them to the application. It counts the number of `SEGMENT_START` messages and posts a `SEGMENT_STOP` message to the application when an equal number of `SEGMENT_STOP` messages were received.

The cached `SEGMENT_START`/`STOP` messages are cleared when going to `READY`.

## DURATION

When a `DURATION` query is performed on a bin, it will forward the query to all its sink elements. The bin will calculate the total duration as the MAX of all returned durations and will then cache the result so that any further queries can use the cached version. The reason for caching the result is because the duration of a stream typically does not change that often.

A `GST_MESSAGE_DURATION_CHANGED` posted by an element will clear the cached duration value so that the bin will query the sinks again. This message is typically posted by elements that calculate the duration of the stream based on some average bitrate, which might change while playing the stream. The `DURATION_CHANGED` message is posted to the application, which can then fetch the updated `DURATION`.

## Subclassing

Subclasses of `GstBin` are free to implement their own add/remove functions. It is a good idea to update the `GList` of children so that the `_iterate()` functions can still be used if the custom bin allows access to its children.

Any bin subclass can also implement a custom message handler by overriding the default one.

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