我找到的唯一100%可靠的方法是通过CoreAudio或AudioToolbox进行设置:
https://developer.apple.com/documentation/audiotoolbox
一种音频流数据提供程序,由iOS以固定的时间间隔调用,以向音频系统提供音频样本。
一开始它可能看起来令人望而生畏,但一旦你设置好了它,你就可以完全精确地控制为音频生成的内容。
这是我使用音频工具箱设置音频单元时使用的代码:
static AudioComponentInstance _audioUnit;
static int _outputAudioBus;
…
#pragma mark - Audio Unit
+(void)_activateAudioUnit
{
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryAmbient error:nil];
if([self _createAudioUnitInstance]
&& [self _setupAudioUnitOutput]
&& [self _setupAudioUnitFormat]
&& [self _setupAudioUnitRenderCallback]
&& [self _initializeAudioUnit]
&& [self _startAudioUnit]
)
{
[self _adjustOutputLatency];
// NSLog(@"Audio unit initialized");
}
}
+(BOOL)_createAudioUnitInstance
{
// Describe audio component
AudioComponentDescription desc;
desc.componentType = kAudioUnitType_Output;
desc.componentSubType = kAudioUnitSubType_RemoteIO;
desc.componentFlags = 0;
desc.componentFlagsMask = 0;
desc.componentManufacturer = kAudioUnitManufacturer_Apple;
AudioComponent inputComponent = AudioComponentFindNext(NULL, &desc);
// Get audio units
OSStatus status = AudioComponentInstanceNew(inputComponent, &_audioUnit);
[self _logStatus:status step:@"instantiate"];
return (status == noErr );
}
+(BOOL)_setupAudioUnitOutput
{
UInt32 flag = 1;
OSStatus status = AudioUnitSetProperty(_audioUnit,
kAudioOutputUnitProperty_EnableIO,
kAudioUnitScope_Output,
_outputAudioBus,
&flag,
sizeof(flag));
[self _logStatus:status step:@"set output bus"];
return (status == noErr );
}
+(BOOL)_setupAudioUnitFormat
{
AudioStreamBasicDescription audioFormat = {0};
audioFormat.mSampleRate = 44100.00;
audioFormat.mFormatID = kAudioFormatLinearPCM;
audioFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
audioFormat.mFramesPerPacket = 1;
audioFormat.mChannelsPerFrame = 2;
audioFormat.mBitsPerChannel = 16;
audioFormat.mBytesPerPacket = 4;
audioFormat.mBytesPerFrame = 4;
OSStatus status = AudioUnitSetProperty(_audioUnit,
kAudioUnitProperty_StreamFormat,
kAudioUnitScope_Input,
_outputAudioBus,
&audioFormat,
sizeof(audioFormat));
[self _logStatus:status step:@"set audio format"];
return (status == noErr );
}
+(BOOL)_setupAudioUnitRenderCallback
{
AURenderCallbackStruct audioCallback;
audioCallback.inputProc = playbackCallback;
audioCallback.inputProcRefCon = (__bridge void *)(self);
OSStatus status = AudioUnitSetProperty(_audioUnit,
kAudioUnitProperty_SetRenderCallback,
kAudioUnitScope_Global,
_outputAudioBus,
&audioCallback,
sizeof(audioCallback));
[self _logStatus:status step:@"set render callback"];
return (status == noErr);
}
+(BOOL)_initializeAudioUnit
{
OSStatus status = AudioUnitInitialize(_audioUnit);
[self _logStatus:status step:@"initialize"];
return (status == noErr);
}
+(void)start
{
[self clearFeeds];
[self _startAudioUnit];
}
+(void)stop
{
[self _stopAudioUnit];
}
+(BOOL)_startAudioUnit
{
OSStatus status = AudioOutputUnitStart(_audioUnit);
[self _logStatus:status step:@"start"];
return (status == noErr);
}
+(BOOL)_stopAudioUnit
{
OSStatus status = AudioOutputUnitStop(_audioUnit);
[self _logStatus:status step:@"stop"];
return (status == noErr);
}
+(void)_logStatus:(OSStatus)status step:(NSString *)step
{
if( status != noErr )
{
NSLog(@"AudioUnit failed to %@, error: %d", step, (int)status);
}
}
最后,一旦启动,我注册的音频回调将是提供音频的回调:
static OSStatus playbackCallback(void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList *ioData) {
@autoreleasepool {
AudioBuffer *audioBuffer = ioData->mBuffers;
// .. fill in audioBuffer with Metronome sample data, fill the in-between ticks with 0s
}
return noErr;
}
您可以使用声音编辑器,如Audacity:
https://www.audacityteam.org/download/mac/
要编辑您的文件并将其保存到原始的PCM单声道/立体声数据文件中,或者您可以使用AVFoundation库之一从任何支持的音频文件格式中检索音频样本。将您的示例加载到缓冲区中,跟踪您在音频回调帧之间的中断位置,并将节拍器示例与0交错输入。
这样做的好处是,您现在可以依靠iOS的音频工具箱来确定代码的优先级,这样音频动画和视图动画就不会相互干扰。
干杯,祝你好运!
