In addition to notification messages at discrete locations in the media, periodic notification of elapsed media time can also be requested. These periodic messages, referred to as "position advise" messages, are requested for a device context based on a specified time interval. Position advise messages are requested by issuing the MCI_SET_POSITION_ADVISE message to a device context as shown in the following code fragment.
MCI_OPEN_PARMS mop;
static MCI_PLAY_PARMS mpp; /* parms for MCI_PLAY */
static MCI_POSITION_PARMS mppPos; /* parms for
MCI_SET_POSITION_ADVISE */
iState = ST_PLAYING; /* Set state to reflect play mode */
mppPos.hwndCallback = hwndMainDlg;
mppPos.ulUnits = 1500; /* Request position advise messages */
mppPos.usUserParm = usPositionUP;
mppPos.Reserved0 = 0;
mciSendCommand ( mop.usDeviceID,
MCI_SET_POSITION_ADVISE,
MCI_NOTIFY | MCI_SET_POSITION_ADVISE_ON,
(PVOID) &mppPos,
UP_POSITION );
This causes MM_MCIPOSITIONCHANGE messages to be returned to the application window specified in the MCI_POSITION_PARMS structure at the requested frequency as media time passes in the device context. Only one position advise frequency may be active for a device context, and having position advise notification active in one device context does not cause messages to be generated in other device contexts. Position advise messages can be set only when a device element is loaded in the device context, and are reset when a new device element is loaded.
Like MM_MCICUEPOINT messages, MM_MCIPOSITIONCHANGE message parameters contain the device ID of the device context that generated the position advise message, as well as the media position and an additional application-defined parameter that can be specified when the position advise notification is requested. Although the media position interval (frequency) specified by the application on the MCI_SET_POSITION_ADVISE message is in the currently set device units, the media position returned on the MM_MCIPOSITIONCHANGE message is always in MMTIME units. MMTIME units are used because the time format set when the position advise notification is set might not be the same time format set when the position advise notification messages are returned.
Position advise notifications are generated only during playback or recording. MM_MCIPOSITIONCHANGE messages are usually not generated during seek operations initiated by the application. The exception is a device, such as a tape recorder, that has a discernible position during the seek operation. A device like this can generate position advise messages as the media is traversed, to indicate the progress of the seek operation.
The following code fragment shows how the Caption Sample application handles the MM_MCIPOSITIONCHANGE message. When the Caption Sample application receives a position change message, it updates its media position slider arm allowing the application to advance the media position slider smoothly as the audio plays.
case MM_MCIPOSITIONCHANGE:
/*
* This message will be returned (in MMTIME) to the application
* whenever the audio position changes. This time will be used to
* increment the audio position slider. This message is only
* generated during playback.
*/
if ( eState == ST_PLAYING )
{
ulTime = (ULONG) LONGFROMMP(mp2);
/*
* Get the new slider arm position and set it.
*/
sArmPosition =
(SHORT) ( ( ulTime * ( sAudioArmRange - 1) ) / ulAudioLength );
WinSendMsg(
hwndAudioSlider,
SLM_SETSLIDERINFO,
MPFROM2SHORT( SMA_SLIDERARMPOSITION, SMA_RANGEVALUE ),
MPFROMSHORT( sArmPosition ));
}
return 0;