BTimeSource

Constructor and Destructor

BTimeSource()

BTimeSource::BTimeSource()

The standard BTimeSource constructor. You should never directly instantiate a BTimeSource; instead, you should create a node class derived from BTimeSource (and possibly other BMediaNode-derived classes as well) and use the BMediaRoster to instantiate the node as desired.

Member Functions

BroadcastTimeWarp()

void BTimeSource::BroadcastTimeWarp(bigtime_t atRealTime, bigtime_t newPerformanceTime)

Whenever time jumps suddenly (for instance, when a seek operation occurs), call this function to inform all slaved nodes that time has jumped in a discontinuous manner. You should also call this function if the time from which your time source derives its time jumps.

At the real time specified by atRealTime, the performance time will instantaneously jump to newPerformanceTime.

GetStartLatency()

status_t BTimeSource::GetStartLatency(bigtime_t *outLatency)

Returns in outLatency the amount of time, in microseconds, needed for the time source to start up, including the time needed to start up any slaved nodes that are started.

Return Code	Description
B_OK	The latency value was returned successfully.
B_NO_INIT	The time source has not been initialized.

GetTime()

status_t BTimeSource::GetTime(bigtime_t *performanceTime, bigtime_t *realTime, float *drift)

Returns the most recently published time information for this time source; this information specifies the performance time and real time of the last published time stamp, as well as the drift value, which can be used to interpolate the true current performance time, given a more accurate real time, as follows:

bigtime_t performanceTime;
bigtime_t realTime;
float drift;

while (GetTime(&performanceTime, &realTime, &drift) != B_OK);
performanceTime = performanceTime + (RealTime() - realTime) * drift;

Note, however, that the resulting performanceTime is the same value you would have gotten by calling PerformanceTimeFor(), which you should use instead.

Return Code	Description
B_OK	The returned information is accurate.
Other values.	The returned information is unreliable.

HandleMessage()

status_t BTimeSource::HandleMessage(int32 code, const void *message, size_t size)

Given a message received on the control port, this function dispatches the message to the appropriate BTimeSource hook function. If the message doesn’t correspond to a hook function, an appropriate error be returned.

If your node derives from BTimeSource, your implementation of HandleMessage() should call all inherited forms of HandleMessage().

Return Code	Description
B_OK	The message was dispatched.
Other values.	The message couldn’t be dispatched, possibly because it doesn’t correspond to a hook function.

IsRunning()

bool BTimeSource::IsRunning()

Returns true if the BTimeSource is currently progressing through time or false if it’s stopped.

Now(), PerformanceTimeFor()

bigtime_t BTimeSource::Now()

bigtime_t BTimeSource::PerformanceTimeFor(bigtime_t realTime)
bigtime_t BTimeSource::PerformanceTimeFor(bigtime_t performanceTime, bigtime_t withLatency)

Now() returns an approximation of what the current performance time is.

PerformanceTimeFor() returns an estimate of the performance time represented by the specified real time (as returned by BTimeSource::RealTime()).

RealTimeFor(), given a performance time, returns an approximation of the corresponding real time, adjusted by the given latency.

PublishTime()

void BTimeSource::PublishTime(bigtime_t performanceTime, bigtime_t realTime, float drift)

While your time source is running, you should repeatedly call this function in order to constantly refresh the mapping between real time and performance time. When your time source is stopped, you should call this function once with values of zero for all three arguments.

The arguments have the following meanings:

Parameter	Description
performanceTime.	Is the precise current performance time.
realTime.	Is the current real time.
drift.	Indicates the value which indicates the rate at which the performance time changes compared to real time.

The drift value makes it possible to interpolate intermediate values. For instance, if playback of a video source is progressing at normal speed, the drift value would be 1.0, indicating that performance time and real time progress at the same rate.

However, if the movie is playing at half-speed, drift would be 0.5, so that for every one unit of real time that passes, only a half-unit of performance time would pass. This information is used to compute times without having to query the time source each time an update must occur.

The data furnished by this function (which you should try to call about 20 times a second, although variations in frequency are acceptable as long as the drift value doesn’t change much) may be the only information the rest of the world sees from your node.

RealTime()

static bigtime_t BTimeSource::RealTime()

Returns the current absolute real time reference that all time sources measure themselves against. This is the only call that you should rely upon to obtain this value (don’t use the Kernel Kit’s system_time() function).

As of this time, RealTime() and system_time() do return the same value; however, you shouldn’t rely upon this relationship. When doing media stuff, you should always use RealTime().

Seek()

virtual void BTimeSource::Seek(bigtime_t performanceTime, bigtime_t atRealTime)

Implement this function to handle a seek request. When a BTimeSource’s performance time is adjusted, it needs to broadcast the change to all nodes slaved to it; call BroadcastTimeWarp() to do this.

Be sure to queue at least one seek request, so seek operations can be requested in advance. The seek request should occur at the real time specified by atRealTime.

SendRunMode()

void BTimeSource::SendRunMode(run_mode mode)

This function transmits the specified mode to all the nodes slaved to this node, so they know that their time source’s run mode has changed. This function is called by SetRunMode(); it may or may not make sense to call it elsewhere, depending on your BTimeSource implementation.

SetRunMode()

virtual void BTimeSource::SetRunMode(run_mode mode)

This hook function is called when someone requests that your node’s run mode be changed. Be sure to call through to either BMediaNode::SetRunMode() or BTimeSource::SetRunMode().

SnoozeUntil()

virtual void BTimeSource::SnoozeUntil(bigtime_t performanceTime, bigtime_t withLatency = 0, bool retrySignals = false)

Waits until the specified performanceTime (specified in performance time) arrives in real time. If withLatency is non-zero, SnoozeUntil() waits until withLatency microseconds (in real time) prior to the specified performance time; this lets you have time for setup prior to starting some operation.

Note

Because performanceTime is specified in performance time, and withLatency is specified in real time, you can’t just subtract withLatency microseconds from performanceTime and snooze until that time, because performanceTime may be passing at a rate faster than or slower than real time (if, for example, the time source is running at double-speed).

If retrySignals is false (as is the default), SnoozeUntil() returns B_INTERRUPTED if it’s interrupted by a signal before performanceTime; if retrySignals is true, the function will go right back to sleep and not return until the time has actually arrived.

Return Code	Description
B_OK	No error; the snooze has completed without incident.
B_MEDIA_TIME_SOURCE_STOPPED	The time source is stopped; snoozing on a stopped time source is a bad thing.

Start()

virtual void BTimeSource::Start(bigtime_t realTime)

This function is called when someone wants the BTimeSource to begin running; the realTime argument indicates the real time at which the time source should start. Implement the function to queue the request as necessary.

Be sure to queue at least one start request, so that start requests can be filed in advance.

Note

A BTimeSource measures start and stop times in real time, not in performance time, because BTimeSources are the objects used to perform the mapping between real time and performance time.

Stop()

virtual void BTimeSource::Stop(bigtime_t realTime)

Stop() is called when someone wants the BTimeSource to stop; the realTime argument indicates the real time at which the time source should stop running. Implement the function to queue the request.

Be sure to queue at least one stop request, so stop requests can be filed in advance.

Note

A BTimeSource measures start and stop times in real time, not in performance time, because BTimeSources are the objects used to perform the mapping between real time and performance time.

TimeSourceOp()

virtual status_t BTimeSource::TimeSourceOp(const time_source_op_info &op, void *_reserved = 0)

This function is called by the BMediaNode::Start(), BMediaNode::Stop(), and BMediaNode::Seek() functions to perform the requested activities.

You must implement this function to handle these requests.

Return B_OK if the request was handled successfully; otherwise, return an appropriate error code.

Constants

time_source_op

Constant	Description
B_TIMESOURCE_START	Start the time source.
B_TIMESOURCE_STOP	Stop the time source.
B_TIMESOURCE_STOP_IMMEDIATELY	Stop the time source immediately.
B_TIMESOURCE_SEEK	Seek the time source.

These constants identify the various operations TimeSourceOp() will be called upon to handle.

Defined Types

time_source_op_info

struct time_source_op_info {
   time_source_op op;
   int32 _reserved1;
   bigtime_t real_time;
   bigtime_t performance_time;
   int32 _reserved2[6];
};

Describes a request to TimeSourceOp().

Field	Description
op	Indicates the operation to be performed.
real_time	Indicates the time at which the request is to be filled (this is the atRealTime argument passed to the BMediaRoster functions SeekTimeSource(), StopTimeSource(), or StartTimeSource()).
performance_time	Is the newPerformanceTime argument passed to SeekTimeSource().