对于我的应用程序，我需要解码存储在 NSData 对象中的 MP3 文件。
出于安全原因，将 NSData 对象写入磁盘并使用系统 URL 引用重新打开它是不可取的，即使它只是在本地存储了片刻。
我想利用扩展音频文件服务（或音频文件服务）来做到这一点，但我无法获得 NSData 的表示，它只存在于内存中，可以由这些音频文件服务读取。
编辑：我想解码 MP3 数据，以便可以访问线性 PCM 音频样本进行操作。从 NSData 对象回放不是问题。
我的代码如下：
decryptedData; //an NSData object which has already been initialized
const void *dataBytes = decryptedData.bytes; //pointer to the bytes in my NSData object
//this creates a CFURLRef from the pointer to the byte data
//I have printed out the resulting CFURL and have confirmed that it is indeed reading the bytes correctly
CFURLRef audioFileURLFromBytes = CFURLCreateWithBytes (kCFAllocatorDefault,
dataBytes,
decryptedData.length,
kCFStringEncodingASCII,
NULL);
//attempt to open the the URL using Extended Audio File Services
ExtAudioFileRef outExtAudioFile;
OSStatus err = 0;
err = ExtAudioFileOpenURL(audioFileURLFromBytes, &outExtAudioFile);
if (err != noErr) {
NSLog(@"ExtAudioFileOpenURL failed with OSStatus Code %i \n", err);
}
//Attempt to open the URL using Audio File Services
AudioFileID audioFile;
OSStatus res = 0;
res = AudioFileOpenURL(audioFileURLFromBytes, kAudioFileReadPermission, kAudioFileMP3Type, &audioFile);
if (res != noErr) {
NSLog(@"AudioFileOpenURL failed with OSStatus Code %i \n", res);
}
两次打开 URL 的尝试都会导致 OSStatus 代码 43，即“找不到文件”。
我已经验证我的指针指向 NSData 内存中的正确地址，并且可以正确读取字节。
扩展音频文件服务是否有一些限制，禁止引用存储在内存中的字节？
感谢您的任何帮助，您可以提供。
编辑：我想出了如何使用 Sbooth 的建议来做到这一点。下面的代码： 这个函数接受一个 NSData 对象，其中包含一个音频文件的 mp3 表示。它将其解码为线性 PCM，因此您可以获得样本，然后将其重新编码为 AAC。我不认为所有平台（移动/桌面）的 CoreAudio 都可以使用 MP3 编码。这段代码在我的 Mac 上进行了测试并完成了工作。
-(void) audioFileReaderWithData: (NSData *) audioData {
AudioFileID refAudioFileID;
ExtAudioFileRef inputFileID;
ExtAudioFileRef outputFileID;
OSStatus result = AudioFileOpenWithCallbacks(audioData, readProc, 0, getSizeProc, 0, kAudioFileMP3Type, &refAudioFileID);
if(result != noErr){
NSLog(@"problem in theAudioFileReaderWithData function: result code %i \n", result);
}
result = ExtAudioFileWrapAudioFileID(refAudioFileID, false, &inputFileID);
if (result != noErr){
NSLog(@"problem in theAudioFileReaderWithData function Wraping the audio FileID: result code %i \n", result);
}
// Client Audio Format Description
AudioStreamBasicDescription clientFormat;
memset(&clientFormat, 0, sizeof(clientFormat));
clientFormat.mFormatID = kAudioFormatLinearPCM;
clientFormat.mFramesPerPacket = 1;
clientFormat.mChannelsPerFrame = 2;
clientFormat.mBitsPerChannel = 32;
clientFormat.mBytesPerPacket = clientFormat.mBytesPerFrame = 4 * clientFormat.mChannelsPerFrame;
clientFormat.mFormatFlags = kAudioFormatFlagsNativeFloatPacked;
clientFormat.mSampleRate = 44100;
//Output Audio Format Description
AudioStreamBasicDescription outputFormat;
memset(&outputFormat, 0, sizeof(outputFormat));
outputFormat.mChannelsPerFrame = 2;
outputFormat.mSampleRate = 44100;
outputFormat.mFormatID = kAudioFormatMPEG4AAC;
outputFormat.mFormatFlags = kMPEG4Object_AAC_Main;
outputFormat.mBitsPerChannel = 0;
outputFormat.mBytesPerFrame = 0;
outputFormat.mBytesPerPacket = 0;
outputFormat.mFramesPerPacket = 1024;
// create the outputFile that we're writing to here....
UInt32 outputFormatSize = sizeof(outputFormat);
result = 0;
result = AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, 0, NULL, &outputFormatSize, &outputFormat);
if(result != noErr)
NSLog(@"could not set the output format with status code %i \n",result);
NSMutableString *outputFilePath = [NSMutableString stringWithCapacity: 100];
[outputFilePath setString:@"/Users/You/Desktop/testAudio.m4a"];
NSURL *sourceURL = [NSURL fileURLWithPath:outputFilePath];
result = 0;
result = ExtAudioFileCreateWithURL((CFURLRef)sourceURL, kAudioFileM4AType, &outputFormat, NULL, kAudioFileFlags_EraseFile, &outputFileID);
if(result != noErr){
NSLog(@"ExtAudioFileCreateWithURL failed for outputFileID with status %i \n", result);
}
int size = sizeof(clientFormat);
result = 0;
result = ExtAudioFileSetProperty(inputFileID, kExtAudioFileProperty_ClientDataFormat, size, &clientFormat);
if(result != noErr)
NSLog(@"error on ExtAudioFileSetProperty for input File with result code %i \n", result);
size = sizeof(clientFormat);
result = 0;
result = ExtAudioFileSetProperty(outputFileID, kExtAudioFileProperty_ClientDataFormat, size, &clientFormat);
if(result != noErr)
NSLog(@"error on ExtAudioFileSetProperty for output File with result code %i \n", result);
int totalFrames = 0;
UInt32 outputFilePacketPosition = 0; //in bytes
UInt32 encodedBytes = 0;
while (1) {
UInt32 bufferByteSize = 22050 * 4 * 2;
char srcBuffer[bufferByteSize];
UInt32 numFrames = (bufferByteSize/clientFormat.mBytesPerFrame);
AudioBufferList fillBufList;
fillBufList.mNumberBuffers = 1;
fillBufList.mBuffers[0].mNumberChannels = clientFormat.mChannelsPerFrame;
fillBufList.mBuffers[0].mDataByteSize = bufferByteSize;
fillBufList.mBuffers[0].mData = srcBuffer;
result = 0;
result = ExtAudioFileRead(inputFileID, &numFrames, &fillBufList);
if (result != noErr) {
NSLog(@"Error on ExtAudioFileRead with result code %i \n", result);
totalFrames = 0;
break;
}
if (!numFrames)
break;
totalFrames = totalFrames + numFrames;
result = 0;
result = ExtAudioFileWrite(outputFileID,
numFrames,
&fillBufList);
if(result!= noErr){
NSLog(@"ExtAudioFileWrite failed with code %i \n", result);
}
encodedBytes += numFrames * clientFormat.mBytesPerFrame;
}
//Clean up
ExtAudioFileDispose(inputFileID);
ExtAudioFileDispose(outputFileID);
AudioFileClose(refAudioFileID);
}
你也需要这些功能......
static OSStatus readProc(void* clientData,
SInt64 position,
UInt32 requestCount,
void* buffer,
UInt32* actualCount)
{
NSData *inAudioData = (NSData *) clientData;
size_t dataSize = inAudioData.length;
size_t bytesToRead = 0;
if(position < dataSize) {
size_t bytesAvailable = dataSize - position;
bytesToRead = requestCount <= bytesAvailable ? requestCount : bytesAvailable;
[inAudioData getBytes: buffer range:NSMakeRange(position, bytesToRead)];
} else {
NSLog(@"data was not read \n");
bytesToRead = 0;
}
if(actualCount)
*actualCount = bytesToRead;
return noErr;
}
static SInt64 getSizeProc(void* clientData) {
NSData *inAudioData = (NSData *) clientData;
size_t dataSize = inAudioData.length;
return dataSize;
}