从 iPhone 中的音乐文件中获取 NSData

Question

我已从我的 iPhone 设备中检索到所有音乐和视频。我现在被困在将它们保存到我的应用程序中，我无法从文件中获取原始数据。任何人都可以帮我找到解决方案。这是我用来获取音乐文件的代码。

MPMediaQuery *deviceiPod = [[MPMediaQuery alloc] init];
NSArray *itemsFromGenericQuery = [deviceiPod items];
for (MPMediaItem *media in itemsFromGenericQuery){
 //i get the media item here.
}

如何将其转换为 NSData ?? 这是我试图获取数据的原因

audioURL = [media valueForProperty:MPMediaItemPropertyAssetURL];//here i get the asset url
NSData *soundData = [NSData dataWithContentsOfURL:audioURL];

使用它对我来说毫无用处。我力图从LocalAssestURL 获取数据。对此的任何解决方案。提前致谢

Answer 1

这不是一项简单的任务——Apple 的 SDK 通常无法为简单任务提供简单的 API。这是我在其中一项调整中使用的代码，以便从资产中获取原始 PCM 数据。您需要将 AVFoundation 和 CoreMedia 框架添加到您的项目中才能使其正常工作：

#import <AVFoundation/AVFoundation.h>
#import <CoreMedia/CoreMedia.h>

MPMediaItem *item = // obtain the media item
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

// Get raw PCM data from the track
NSURL *assetURL = [item valueForProperty:MPMediaItemPropertyAssetURL];
NSMutableData *data = [[NSMutableData alloc] init];

const uint32_t sampleRate = 16000; // 16k sample/sec
const uint16_t bitDepth = 16; // 16 bit/sample/channel
const uint16_t channels = 2; // 2 channel/sample (stereo)

NSDictionary *opts = [NSDictionary dictionary];
AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:assetURL options:opts];
AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset error:NULL];
NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
    [NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey,
    [NSNumber numberWithFloat:(float)sampleRate], AVSampleRateKey,
    [NSNumber numberWithInt:bitDepth], AVLinearPCMBitDepthKey,
    [NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
    [NSNumber numberWithBool:NO], AVLinearPCMIsFloatKey,
    [NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey, nil];

AVAssetReaderTrackOutput *output = [[AVAssetReaderTrackOutput alloc] initWithTrack:[[asset tracks] objectAtIndex:0] outputSettings:settings];
[asset release];
[reader addOutput:output];
[reader startReading];

// read the samples from the asset and append them subsequently
while ([reader status] != AVAssetReaderStatusCompleted) {
    CMSampleBufferRef buffer = [output copyNextSampleBuffer];
    if (buffer == NULL) continue;

    CMBlockBufferRef blockBuffer = CMSampleBufferGetDataBuffer(buffer);
    size_t size = CMBlockBufferGetDataLength(blockBuffer);
    uint8_t *outBytes = malloc(size);
    CMBlockBufferCopyDataBytes(blockBuffer, 0, size, outBytes);
    CMSampleBufferInvalidate(buffer);
    CFRelease(buffer);
    [data appendBytes:outBytes length:size];
    free(outBytes);
}

[output release];
[reader release];
[pool release];

这里data 将包含曲目的原始PCM数据；您可以使用某种编码来压缩它，例如我使用 FLAC 编解码器库。

请参阅original source code here。