如何在轨道中间启动AUAudioFilePlayer但循环到开头？

Question

在iOS中，我正在使用AUAudioFilePlayer ，我试图在循环整个音轨之前进行10秒的引导。 我将ScheduledAudioFileRegion设置如下，以非零帧开始（详情请参阅： https ： //developer.apple.com/library/ios/qa/qa1786/_index.html ）

ScheduledAudioFileRegion playRegion;
playRegion.mTimeStamp.mFlags = kAudioTimeStampSampleTimeValid;
playRegion.mTimeStamp.mSampleTime = 0;
playRegion.mCompletionProc = NULL;
playRegion.mCompletionProcUserData = NULL;
playRegion.mAudioFile = audioFileId;
playRegion.mLoopCount = UInt32(-1);
playRegion.mStartFrame = startFrame;
playRegion.mFramesToPlay = UInt32(-1);  // i.e. all of it
_(AudioUnitSetProperty(_audioUnit,
                       kAudioUnitProperty_ScheduledFileRegion,
                       kAudioUnitScope_Global,
                       0,
                       &playRegion,
                       sizeof(playRegion)),
  @"Error setting Audio File Region on the FilePlayer AU");

问题是，当它循环时，它会在startFrame再次启动。 我是如何从一开始就开始的？

Answer 1

答案是做两个时间表。 mTimeStamp.mSampleTime是相对于等待播放的预定开始时间的长度：

/////////////////////////////////////////
// PRE-AMBLE
/////////////////////////////////////////

// Tell it to play the whole file with optional looping...
// more details: https://developer.apple.com/library/ios/qa/qa1786/_index.html
ScheduledAudioFileRegion playRegion;
playRegion.mTimeStamp.mFlags = kAudioTimeStampSampleTimeValid;
playRegion.mTimeStamp.mSampleTime = 0;  // PLAY IMMEDIATELY...
playRegion.mCompletionProc = NULL;
playRegion.mCompletionProcUserData = NULL;
playRegion.mAudioFile = audioFileId;
playRegion.mLoopCount = 0;
playRegion.mStartFrame = startFrame;   // ...FROM REQUEST START FRAME
playRegion.mFramesToPlay = UInt32(-1);  // i.e. all of it
_(AudioUnitSetProperty(_audioUnit,
                       kAudioUnitProperty_ScheduledFileRegion,
                       kAudioUnitScope_Global,
                       0,
                       &playRegion,
                       sizeof(playRegion)),
  @"Error setting Audio File Region on the FilePlayer AU");


/////////////////////////////////////////
// LOOP
/////////////////////////////////////////

playRegion.mTimeStamp.mFlags = kAudioTimeStampSampleTimeValid;
playRegion.mTimeStamp.mSampleTime = lengthInFrames - startFrame; // WAIT UNTIL PREAMBLE IS DONE...
playRegion.mCompletionProc = NULL;
playRegion.mCompletionProcUserData = NULL;
playRegion.mAudioFile = audioFileId;
playRegion.mLoopCount = UInt32(-1);
playRegion.mStartFrame = 0;         // ...THEN PLAY FROM THE BEGINNING
playRegion.mFramesToPlay = UInt32(-1);  // i.e. all of it
_(AudioUnitSetProperty(_audioUnit,
                       kAudioUnitProperty_ScheduledFileRegion,
                       kAudioUnitScope_Global,
                       0,
                       &playRegion,
                       sizeof(playRegion)),
  @"Error setting Audio File Region on the FilePlayer AU");