Media.Streaming.GStreamer.Core.Bus

An asynchronous message bus subsystem.

Detail

The Bus is resposible for delivering Messages in a first-in, first-out order, from the streaming threads to the application.

Since the application typically only wants to deal with delivery of these messages from one thread, the Bus will marshal the messages between different threads. This is important since the actual streaming of media is done in a thread separate from the application.

The Bus provides support for Source based notifications. This makes it possible to handle the delivery in the GLib Source.

A message is posted on the bus with the busPost method. With the busPeek and busPop methods one can look at or retrieve a previously posted message.

The bus can be polled with the busPoll method. This methods blocks up to the specified timeout value until one of the specified messages types is posted on the bus. The application can then pop the messages from the bus to handle them. Alternatively the application can register an asynchronous bus function using busAddWatch. This function will install a Source in the default GLib main loop and will deliver messages a short while after they have been posted. Note that the main loop should be running for the asynchronous callbacks.

It is also possible to get messages from the bus without any thread marshalling with the busSetSyncHandler method. This makes it possible to react to a message in the same thread that posted the message on the bus. This should only be used if the application is able to deal with messages from different threads.

Every Pipeline has one bus.

Note that a Pipeline will set its bus into flushing state when changing from StateReady to StateNull.

Types

data Bus

Instances

Eq Bus

Methods

(==) :: Bus -> Bus -> Bool

(/=) :: Bus -> Bus -> Bool

Ord Bus

Methods

compare :: Bus -> Bus -> Ordering

(<) :: Bus -> Bus -> Bool

(<=) :: Bus -> Bus -> Bool

(>) :: Bus -> Bus -> Bool

(>=) :: Bus -> Bus -> Bool

max :: Bus -> Bus -> Bus

min :: Bus -> Bus -> Bus

GObjectClass Bus

Methods

toGObject :: Bus -> GObject

unsafeCastGObject :: GObject -> Bus

BusClass Bus

ObjectClass Bus

class ObjectClass o => BusClass o

Instances

BusClass Bus

The result of a BusSyncHandler.

data BusSyncReply

Instances

Enum BusSyncReply

Methods

succ :: BusSyncReply -> BusSyncReply

pred :: BusSyncReply -> BusSyncReply

toEnum :: Int -> BusSyncReply

fromEnum :: BusSyncReply -> Int

enumFrom :: BusSyncReply -> [BusSyncReply]

enumFromThen :: BusSyncReply -> BusSyncReply -> [BusSyncReply]

enumFromTo :: BusSyncReply -> BusSyncReply -> [BusSyncReply]

enumFromThenTo :: BusSyncReply -> BusSyncReply -> BusSyncReply -> [BusSyncReply]

Eq BusSyncReply

Methods

(==) :: BusSyncReply -> BusSyncReply -> Bool

(/=) :: BusSyncReply -> BusSyncReply -> Bool

Show BusSyncReply

Methods

showsPrec :: Int -> BusSyncReply -> ShowS

show :: BusSyncReply -> String

showList :: [BusSyncReply] -> ShowS

A handler that will be invoked synchronously when a new message is injected into the bus. This function is mostly used internally. Only one sync handler may be attached to a given bus.

type BusSyncHandler = Bus -> Message -> IO BusSyncReply

castToBus :: GObjectClass obj => obj -> Bus

gTypeBus :: GType

Bus Operations

busGetFlags

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO [BusFlags]

the flags set on bus

Get the flags set on this bus.

busSetFlags

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> [BusFlags]

the flags to set on bus

-> IO ()

Set flags on this bus.

busUnsetFlags

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> [BusFlags]

the flags to unset on bus

-> IO ()

Unset flags on this bus.

busNew

Arguments

:: IO Bus

the newly created Bus object

Create a new bus.

busPost

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> Message

message - the message to post

-> IO Bool

True if the message was posted, or False if the bus is flushing

Post a message to the bus.

busHavePending

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO Bool

True if there are messages on the bus to be handled, otherwise False

Check if there are pending messages on the bus.

busPeek

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO (Maybe Message)

the first Message on the bus, or Nothing if the bus is empty

Get the message at the front of the queue. Any message returned will remain on the queue.

busPop

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO (Maybe Message)

the first Message on the bus, or Nothing if the bus is empty

Get the message at the front of the queue. It will be removed from the queue.

busTimedPop

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> Maybe ClockTime

timeoutM - the time to wait for, or Nothing to wait forever

-> IO (Maybe Message)

the first message recieved, or Nothing if the timeout has expired

Get a message from the bus, waiting up to the specified timeout. If the time given is Nothing, the function will wait forever. If the time given is 0, the function will behave like busPop.

Since 0.10.12.

busSetFlushing

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> Bool

flushing - the new flushing state

-> IO ()

If flushing is True, the bus will flush out any queued messages, as well as any future messages, until the function is called with flushing set to False.

busSetSyncHandler

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> Maybe BusSyncHandler

busSyncHandlerM - the new BusSyncHandler

-> IO ()

Set the synchronous message handler on the bus. The function will be called every time a new message is posted to the bus. Note that the function will be called from the thread context of the poster.

Calling this function will replace any previously set sync handler. If Nothing is passed to this function, it will unset the handler.

busUseSyncSignalHandler

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO ()

Use a synchronous message handler that converts all messages to signals.

busCreateWatch

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO Source

the new event Source

Create a watch for the bus. The Source will dispatch a signal whenever a message is on the bus. After the signal is dispatched, the message is popped off the bus.

busAddWatch

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> Priority

priority - the priority of the watch

-> BusFunc

func - the action to perform when a message is recieved

-> IO HandlerId

the event source ID

Adds a bus watch to the default main context with the given priority. This function is used to receive asynchronous messages in the main loop.

The watch can be removed by calling sourceRemove.

busDisableSyncMessageEmission

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO ()

Instructs GStreamer to stop emitting the busSyncMessage signal for this bus. See busEnableSyncMessageEmission for more information.

In the event that multiple pieces of code have called busEnableSyncMessageEmission, the sync-message emissions will only be stopped after all calls to busEnableSyncMessageEmission were "cancelled" by calling this function.

busEnableSyncMessageEmission

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO ()

Instructs GStreamer to emit the busSyncMessage signal after running the bus's sync handler. This function is here so that programmers can ensure that they can synchronously receive messages without having to affect what the bin's sync handler is.

This function may be called multiple times. To clean up, the caller is responsible for calling busDisableSyncMessageEmission as many times as this function is called.

While this function looks similar to busAddSignalWatch, it is not exactly the same -- this function enables synchronous emission of signals when messages arrive; busAddSignalWatch adds an idle callback to pop messages off the bus asynchronously. The busSyncMessage signal comes from the thread of whatever object posted the message; the busMessage signal is marshalled to the main thread via the main loop.

busAddSignalWatch

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> Priority

priority - the priority of the watch

-> IO ()

Adds a bus signal watch to the default main context with the given priority. After calling this method, the bus will emit the busMessage signal for each message posted on the bus.

This function may be called multiple times. To clean up, the caller is responsible for calling busRemoveSignalWatch as many times.

busRemoveSignalWatch

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> IO ()

Remove the signal watch that was added with busAddSignalWatch.

busPoll

Arguments

:: BusClass busT

=> busT

bus - a Bus

-> [MessageType]

events - the set of messages to poll for

-> ClockTimeDiff

timeout - the time to wait, or -1 to wait indefinitely

-> IO Message

Poll the bus for a message. Will block while waiting for messages to come. You can specify the maximum amount of time to wait with the timeout parameter. If timeout is negative, the function will wait indefinitely.

Messages not in events will be popped off the bus and ignored.

Because busPoll is implemented using the busMessage signal enabled by busAddSignalWatch, calling busPoll will cause the busMessage signal to be emitted for every message that the function sees. Thus, a busMessage signal handler will see every message that busPoll sees -- neither will steal messages from the other.

This function will run a main loop in the default main context while polling.

Bus Signals

busMessage :: BusClass busT => Signal busT (Message -> IO ())

A message has been posted on the bus. This signal is emitted from a Source added to the MainLoop, and only when it is running.

busSyncMessage :: BusClass busT => Signal busT (Message -> IO ())

A message has been posted on the bus. This signal is emitted from the thread that posted the message so one has to be careful with locking.

This signal will not be emitted by default, you must first call busUseSyncSignalHandler if you want this signal to be emitted when a message is posted on the bus.