原文：AVFoundation Programming Guide
要讀取和寫入視聽資源，您必須使用AVFoundation框架提供的導(dǎo)出API。AVAssetExportSession類提供了一個(gè)簡(jiǎn)單導(dǎo)出需求的接口，例如修改文件格式或修剪assets的長度(請(qǐng)參考Trimming and Transcoding a Movie)。對(duì)于更高的導(dǎo)出需求，請(qǐng)使用AVAssetReader和AVAssetWriter類。
當(dāng)您要對(duì)資源(asset)的內(nèi)容執(zhí)行操作時(shí)，請(qǐng)使用AVAssetReader。 例如，您可以讀取資源(asset)的音軌以產(chǎn)生波形的可視化表示。 要從媒體（如樣本緩沖區(qū)或靜態(tài)圖像）生成資源(asset)，請(qǐng)使用AVAssetWriter對(duì)象。
注意: 資源的讀寫器類不能用于實(shí)時(shí)處理。 實(shí)際上，資源的讀取器甚至不能用于從HTTP實(shí)時(shí)流中實(shí)時(shí)讀取數(shù)據(jù)。 但是，如果您正在使用具有實(shí)時(shí)數(shù)據(jù)源的資源編寫器（例如AVCaptureOutput 對(duì)象)，需要將 expectsMediaDataInRealTime屬性設(shè)置為YES。對(duì)于非實(shí)時(shí)數(shù)據(jù)源，將此屬性設(shè)置為YES將導(dǎo)致文件不正確交叉。
讀取資源
每個(gè)AVAssetReader對(duì)象一次只能與單個(gè)資源相關(guān)聯(lián)，但此資源可能包含多個(gè)軌道。 因此，在開始閱讀之前，您必須將 AVAssetReaderOutput類的具體子類分配給資源讀取器，以配置媒體數(shù)據(jù)的讀取方式。 AVAssetReaderOutput基類的三個(gè)具體子類可用于資源讀取需求：AVAssetReaderTrackOutput，AVAssetReaderAudioMixOutput 和AVAssetReaderVideoCompositionOutput。
創(chuàng)建一個(gè)讀取器
初始化一個(gè)AVAssetReader對(duì)象所需要的就是你想要讀取的的資源。
NSError *outError;
AVAsset *someAsset = <#AVAsset that you want to read#>;
AVAssetReader *assetReader = [AVAssetReader assetReaderWithAsset:someAsset error:&outError];
BOOL success = (assetReader != nil);
注意: 始終檢查資產(chǎn)讀取器的返回值是否為nil，以確保資產(chǎn)讀取器已初始化成功。 否則，錯(cuò)誤參數(shù)（前面的例子中的outError）將包含相關(guān)的錯(cuò)誤信息。
設(shè)置資源讀取器的輸出
創(chuàng)建資源讀取器后，設(shè)置至少一個(gè)輸出以接收正在讀取的媒體數(shù)據(jù)。 設(shè)置輸出時(shí)，請(qǐng)確保將alwaysCopiesSampleData屬性設(shè)置為NO。 通過這種方式，您可以獲得性能改進(jìn)的好處。 在本章中的所有示例中，此屬性可以并且應(yīng)該設(shè)置為NO。
如果您只想從一個(gè)或多個(gè)軌道中讀取媒體數(shù)據(jù)，并可能將該數(shù)據(jù)轉(zhuǎn)換為不同的格式，請(qǐng)使用AVAssetReaderTrackOutput類，資源讀取的每個(gè)AVAssetTrack對(duì)象對(duì)應(yīng)一個(gè)單獨(dú)的軌道輸出對(duì)象。 要使用資產(chǎn)讀取器將音軌解壓縮到Linear PCM，您可以按如下方式設(shè)置音軌輸出：
AVAsset *localAsset = assetReader.asset;
// Get the audio track to read.
AVAssetTrack *audioTrack = [[localAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
// Decompression settings for Linear PCM
NSDictionary *decompressionAudioSettings = @{ AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatLinearPCM] };
// Create the output with the audio track and decompression settings.
AVAssetReaderOutput *trackOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:audioTrack outputSettings:decompressionAudioSettings];
// Add the output to the reader if possible.
if ([assetReader canAddOutput:trackOutput])
[assetReader addOutput:trackOutput];
注意：要以存儲(chǔ)格式從特定資源軌道讀取媒體數(shù)據(jù)，請(qǐng)將outputSettings參數(shù)設(shè)置為nil。
您可以使用AVAssetReaderAudioMixOutput和AVAssetReaderVideoCompositionOutput類來讀取混合或合成在一起的媒體數(shù)據(jù)，分別使用AVAudioMix對(duì)象或AVVideoComposition對(duì)象。 通常，當(dāng)您的資源讀取器從AVComposition對(duì)象讀取時(shí)，將使用這些輸出。
對(duì)于單個(gè)音頻混合輸出，您可以使用AVAudioMix對(duì)象從資源中讀取已混合在一起的多個(gè)音軌。 要指定音軌如何混合，請(qǐng)?jiān)诔跏蓟髮⒒煲舴峙浣oAVAssetReaderAudioMixOutput對(duì)象。 以下代碼展示了如何使用資源中的所有音軌創(chuàng)建音頻混合輸出，將音軌解壓縮為Linear PCM，并將音頻混合對(duì)象分配給輸出。 有關(guān)如何配置音頻混合的詳細(xì)信息，請(qǐng)參閱 Editing。
AVAudioMix *audioMix = <#An AVAudioMix that specifies how the audio tracks from the AVAsset are mixed#>;
// Assumes that assetReader was initialized with an AVComposition object.
AVComposition *composition = (AVComposition *)assetReader.asset;
// Get the audio tracks to read.
NSArray *audioTracks = [composition tracksWithMediaType:AVMediaTypeAudio];
// Get the decompression settings for Linear PCM.
NSDictionary *decompressionAudioSettings = @{ AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatLinearPCM] };
// Create the audio mix output with the audio tracks and decompression setttings.
AVAssetReaderOutput *audioMixOutput = [AVAssetReaderAudioMixOutput assetReaderAudioMixOutputWithAudioTracks:audioTracks audioSettings:decompressionAudioSettings];
// Associate the audio mix used to mix the audio tracks being read with the output.
audioMixOutput.audioMix = audioMix;
// Add the output to the reader if possible.
if ([assetReader canAddOutput:audioMixOutput])
[assetReader addOutput:audioMixOutput];
注意: 通過將audioSettings參數(shù)設(shè)置為nil可以讓資源讀取器以方便的未壓縮格式返回樣本。 對(duì)于AVAssetReaderVideoCompositionOutput類也是如此。
視頻合成輸出的行為方式大致相同：您可以使用AVVideoComposition對(duì)象從資源中讀取已經(jīng)合成在一起的多個(gè)視頻軌道。 要從多個(gè)合成視頻軌道讀取媒體數(shù)據(jù)并將其解壓縮到ARGB，請(qǐng)按如下所示設(shè)置輸出：
AVVideoComposition *videoComposition = <#An AVVideoComposition that specifies how the video tracks from the AVAsset are composited#>;
// Assumes assetReader was initialized with an AVComposition.
AVComposition *composition = (AVComposition *)assetReader.asset;
// Get the video tracks to read.
NSArray *videoTracks = [composition tracksWithMediaType:AVMediaTypeVideo];
// Decompression settings for ARGB.
NSDictionary *decompressionVideoSettings = @{ (id)kCVPixelBufferPixelFormatTypeKey : [NSNumber numberWithUnsignedInt:kCVPixelFormatType_32ARGB], (id)kCVPixelBufferIOSurfacePropertiesKey : [NSDictionary dictionary] };
// Create the video composition output with the video tracks and decompression setttings.
AVAssetReaderOutput *videoCompositionOutput = [AVAssetReaderVideoCompositionOutput assetReaderVideoCompositionOutputWithVideoTracks:videoTracks videoSettings:decompressionVideoSettings];
// Associate the video composition used to composite the video tracks being read with the output.
videoCompositionOutput.videoComposition = videoComposition;
// Add the output to the reader if possible.
if ([assetReader canAddOutput:videoCompositionOutput])
[assetReader addOutput:videoCompositionOutput];
讀取資源的媒體數(shù)據(jù)
在設(shè)置所需的所有輸出后，可以在資源讀取器上調(diào)用startReading方法開始讀取。 接下來，使用copyNextSampleBuffer方法從每個(gè)輸出中單獨(dú)檢索媒體數(shù)據(jù)。 要使用單個(gè)輸出啟動(dòng)資源讀取器并讀取其所有媒體數(shù)據(jù)，請(qǐng)執(zhí)行以下操作：
// Start the asset reader up.
[self.assetReader startReading];
BOOL done = NO;
while (!done)
{
// Copy the next sample buffer from the reader output.
CMSampleBufferRef sampleBuffer = [self.assetReaderOutput copyNextSampleBuffer];
if (sampleBuffer)
{
// Do something with sampleBuffer here.
CFRelease(sampleBuffer);
sampleBuffer = NULL;
}
else
{
// Find out why the asset reader output couldn't copy another sample buffer.
if (self.assetReader.status == AVAssetReaderStatusFailed)
{
NSError *failureError = self.assetReader.error;
// Handle the error here.
}
else
{
// The asset reader output has read all of its samples.
done = YES;
}
}
}
寫資源
AVAssetWriter類將媒體數(shù)據(jù)從多個(gè)源寫入指定文件格式的單個(gè)文件。 您不需要將資源寫入對(duì)象與特定資源相關(guān)聯(lián)，但必須為要?jiǎng)?chuàng)建的每個(gè)輸出文件使用單獨(dú)的資源寫入器。 由于資源寫入對(duì)象可以從多個(gè)源寫入媒體數(shù)據(jù)，因此必須為要寫入輸出文件的每個(gè)單獨(dú)的軌道創(chuàng)建一個(gè)AVAssetWriterInput對(duì)象。 每個(gè)AVAssetWriterInput對(duì)象都希望接收CMSampleBufferRef對(duì)象格式的數(shù)據(jù)，但如果要將CVPixelBufferRef對(duì)象附加到資源寫入中，請(qǐng)使用AVAssetWriterInputPixelBufferAdaptor類。
創(chuàng)建資源寫入對(duì)象
要?jiǎng)?chuàng)建資源寫入對(duì)象，請(qǐng)指定輸出文件的URL和所需的文件類型。 以下代碼顯示如何初始化資源寫入對(duì)象以創(chuàng)建一個(gè)QuickTime電影：
NSError *outError;
NSURL *outputURL = <#NSURL object representing the URL where you want to save the video#>;
AVAssetWriter *assetWriter = [AVAssetWriter assetWriterWithURL:outputURL fileType:AVFileTypeQuickTimeMovie error:&outError];
BOOL success = (assetWriter != nil);
設(shè)置Asset Writer Inputs
要使資產(chǎn)寫入對(duì)象能夠編寫媒體數(shù)據(jù)，必須至少設(shè)置一個(gè)資源寫入對(duì)象輸入(asset writer input)。 例如，如果您的媒體數(shù)據(jù)源已經(jīng)將媒體樣本作為CMSampleBufferRef對(duì)象，那么只需使用AVAssetWriterInput類。 要設(shè)置資源寫入對(duì)象輸入(asset writer input)，將音頻媒體數(shù)據(jù)壓縮為128 kbps AAC并將其連接到資源寫入對(duì)象，請(qǐng)執(zhí)行以下操作：
// Configure the channel layout as stereo.
AudioChannelLayout stereoChannelLayout = {
.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo,
.mChannelBitmap = 0,
.mNumberChannelDescriptions = 0
};
// Convert the channel layout object to an NSData object.
NSData *channelLayoutAsData = [NSData dataWithBytes:&stereoChannelLayout length:offsetof(AudioChannelLayout, mChannelDescriptions)];
// Get the compression settings for 128 kbps AAC.
NSDictionary *compressionAudioSettings = @{
AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatMPEG4AAC],
AVEncoderBitRateKey : [NSNumber numberWithInteger:128000],
AVSampleRateKey : [NSNumber numberWithInteger:44100],
AVChannelLayoutKey : channelLayoutAsData,
AVNumberOfChannelsKey : [NSNumber numberWithUnsignedInteger:2]
};
// Create the asset writer input with the compression settings and specify the media type as audio.
AVAssetWriterInput *assetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:compressionAudioSettings];
// Add the input to the writer if possible.
if ([assetWriter canAddInput:assetWriterInput])
[assetWriter addInput:assetWriterInput];
注意: 如果要以存儲(chǔ)格式寫入媒體數(shù)據(jù)，可以將outputSettings參數(shù)設(shè)置為nil。 只有當(dāng)資源寫入對(duì)象是使用AVFileTypeQuickTimeMovie的文件類型初始化時(shí)，才可以設(shè)置為nil。
您的資源寫入對(duì)象輸入可以分別使用metadata和transform屬性來選擇性地包括一些元數(shù)據(jù)或?yàn)樘囟ǖ能壍乐付ú煌淖儞Q。 對(duì)于數(shù)據(jù)源是視頻軌道的資源寫入對(duì)象輸入，您可以通過執(zhí)行以下操作在輸出文件中維持視頻的原始變換：
AVAsset *videoAsset = <#AVAsset with at least one video track#>;
AVAssetTrack *videoAssetTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
assetWriterInput.transform = videoAssetTrack.preferredTransform;
注意: 在開始寫入之前，設(shè)置元數(shù)據(jù)和變換屬性，使其生效。
將媒體數(shù)據(jù)寫入輸出文件時(shí)，有時(shí)您可能需要分配像素緩沖區(qū)。 為此，請(qǐng)使用AVAssetWriterInputPixelBufferAdaptor類。 為了最大的效率，不要使用單獨(dú)的池分配的像素緩沖區(qū)，而是使用像素緩沖適配器提供的像素緩沖池。 以下代碼創(chuàng)建一個(gè)在RGB域中工作的像素緩沖區(qū)對(duì)象，它將使用CGImage對(duì)象創(chuàng)建其像素緩沖區(qū)。
NSDictionary *pixelBufferAttributes = @{
kCVPixelBufferCGImageCompatibilityKey : [NSNumber numberWithBool:YES],
kCVPixelBufferCGBitmapContextCompatibilityKey : [NSNumber numberWithBool:YES],
kCVPixelBufferPixelFormatTypeKey : [NSNumber numberWithInt:kCVPixelFormatType_32ARGB]
};
AVAssetWriterInputPixelBufferAdaptor *inputPixelBufferAdaptor = [AVAssetWriterInputPixelBufferAdaptor assetWriterInputPixelBufferAdaptorWithAssetWriterInput:self.assetWriterInput sourcePixelBufferAttributes:pixelBufferAttributes];
注意: 所有AVAssetWriterInputPixelBufferAdaptor對(duì)象必須連接到單個(gè)的資產(chǎn)寫入對(duì)象輸入。 資源寫入對(duì)象輸入必須接受AVMediaTypeVideo類型的媒體數(shù)據(jù)。
寫入媒體數(shù)據(jù)
當(dāng)您配置了資源寫入所需的所有輸入后，即可開始寫入媒體數(shù)據(jù)。 與資源讀取器一樣，通過調(diào)用startWriting方法啟動(dòng)寫入過程。 然后，您需要通過調(diào)用startSessionAtSourceTime: 方法啟動(dòng)示例寫入會(huì)話。 資源寫入器進(jìn)行的所有寫入都必須在這些會(huì)話之一內(nèi)進(jìn)行，每個(gè)會(huì)話的時(shí)間范圍定義了從源中包含的媒體數(shù)據(jù)的時(shí)間范圍。 例如，如果您的來源是資源讀取器，它提供從AVAsset對(duì)象讀取的媒體數(shù)據(jù)，并且你不希望包含資源的前半部分媒體數(shù)據(jù)，則可以執(zhí)行以下操作：
CMTime halfAssetDuration = CMTimeMultiplyByFloat64(self.asset.duration, 0.5);
[self.assetWriter startSessionAtSourceTime:halfAssetDuration];
//Implementation continues.
通常，要結(jié)束寫入會(huì)話，您必須調(diào)用endSessionAtSourceTime:方法。 但是，如果您的寫作會(huì)話到了文件的末尾，您可以簡(jiǎn)單地通過調(diào)用finishWriting方法來結(jié)束寫入會(huì)話。 要開始一個(gè)包含單個(gè)輸入的資源寫入器，并寫入其所有媒體數(shù)據(jù)，請(qǐng)執(zhí)行以下操作：
// Prepare the asset writer for writing.
[self.assetWriter startWriting];
// Start a sample-writing session.
[self.assetWriter startSessionAtSourceTime:kCMTimeZero];
// Specify the block to execute when the asset writer is ready for media data and the queue to call it on.
[self.assetWriterInput requestMediaDataWhenReadyOnQueue:myInputSerialQueue usingBlock:^{
while ([self.assetWriterInput isReadyForMoreMediaData])
{
// Get the next sample buffer.
CMSampleBufferRef nextSampleBuffer = [self copyNextSampleBufferToWrite];
if (nextSampleBuffer)
{
// If it exists, append the next sample buffer to the output file.
[self.assetWriterInput appendSampleBuffer:nextSampleBuffer];
CFRelease(nextSampleBuffer);
nextSampleBuffer = nil;
}
else
{
// Assume that lack of a next sample buffer means the sample buffer source is out of samples and mark the input as finished.
[self.assetWriterInput markAsFinished];
break;
}
}
}];
上面代碼中的copyNextSampleBufferToWrite方法只是一個(gè)存根。 此存根的位置是您需要插入一些邏輯以返回表示要寫入的媒體數(shù)據(jù)的CMSampleBufferRef對(duì)象。 樣本緩沖區(qū)的一個(gè)可能來源是資源讀取器輸出。
重新編碼資源
您可以一起使用資源讀取器和資產(chǎn)寫入器將資源從一種表示形式轉(zhuǎn)換為另一種。 使用這些對(duì)象，您可以比使用AVAssetExportSession對(duì)象更能控制轉(zhuǎn)換。 例如，您可以選擇要輸出到文件的軌道，指定您自己的輸出格式，或在轉(zhuǎn)換過程中修改資源。 此過程的第一步都是根據(jù)需要設(shè)置資源讀取器輸出和資產(chǎn)寫入器輸入。 當(dāng)資源讀寫器配置好后，您可以分別啟動(dòng)startReading和startWriting方法。 以下代碼片段顯示了如何使用單個(gè)資源寫入器輸入來寫入單個(gè)資源讀取器輸出提供的媒體數(shù)據(jù)：
NSString *serializationQueueDescription = [NSString stringWithFormat:@"%@ serialization queue", self];
// Create a serialization queue for reading and writing.
dispatch_queue_t serializationQueue = dispatch_queue_create([serializationQueueDescription UTF8String], NULL);
// Specify the block to execute when the asset writer is ready for media data and the queue to call it on.
[self.assetWriterInput requestMediaDataWhenReadyOnQueue:serializationQueue usingBlock:^{
while ([self.assetWriterInput isReadyForMoreMediaData])
{
// Get the asset reader output's next sample buffer.
CMSampleBufferRef sampleBuffer = [self.assetReaderOutput copyNextSampleBuffer];
if (sampleBuffer != NULL)
{
// If it exists, append this sample buffer to the output file.
BOOL success = [self.assetWriterInput appendSampleBuffer:sampleBuffer];
CFRelease(sampleBuffer);
sampleBuffer = NULL;
// Check for errors that may have occurred when appending the new sample buffer.
if (!success && self.assetWriter.status == AVAssetWriterStatusFailed)
{
NSError *failureError = self.assetWriter.error;
//Handle the error.
}
}
else
{
// If the next sample buffer doesn't exist, find out why the asset reader output couldn't vend another one.
if (self.assetReader.status == AVAssetReaderStatusFailed)
{
NSError *failureError = self.assetReader.error;
//Handle the error here.
}
else
{
// The asset reader output must have vended all of its samples. Mark the input as finished.
[self.assetWriterInput markAsFinished];
break;
}
}
}
}];
整合: 使用資源讀寫器來重新編碼資源
這個(gè)簡(jiǎn)短的代碼示例說明了如何使用資源讀取器和寫入器將資源的第一個(gè)視頻和音軌重新編碼為新的文件。步驟如下：
- 使用串行隊(duì)列來處理讀和寫視聽數(shù)據(jù)
- 初始化資源讀取器并配置兩個(gè)資源讀取器輸出，一個(gè)用于音頻，另一個(gè)用于視頻
- 初始化資源寫入器并配置兩個(gè)資源寫入器輸入，一個(gè)用于音頻，一個(gè)用于視頻
- 使用資源讀取器通過兩種不同的輸出/輸入組合異步地向資源寫入器提供媒體數(shù)據(jù)
- 使用dispatch group通知處理重新編碼過程的完成
- 允許用戶取消重新編碼過程
注意：為了專注于最相關(guān)的代碼，本示例省略了一個(gè)完整應(yīng)用程序的幾個(gè)方面。要使用AVFoundation，您需要有足夠的Cocoa編程經(jīng)驗(yàn)，能夠推斷出缺少的部分。
初始設(shè)置
在創(chuàng)建資源讀寫器并配置其輸出和輸入之前，需要做一些初始設(shè)置。 此設(shè)置的第一部分是創(chuàng)建三個(gè)單獨(dú)的串行隊(duì)列來處理讀寫過程。
NSString *serializationQueueDescription = [NSString stringWithFormat:@"%@ serialization queue", self];
// Create the main serialization queue.
self.mainSerializationQueue = dispatch_queue_create([serializationQueueDescription UTF8String], NULL);
NSString *rwAudioSerializationQueueDescription = [NSString stringWithFormat:@"%@ rw audio serialization queue", self];
// Create the serialization queue to use for reading and writing the audio data.
self.rwAudioSerializationQueue = dispatch_queue_create([rwAudioSerializationQueueDescription UTF8String], NULL);
NSString *rwVideoSerializationQueueDescription = [NSString stringWithFormat:@"%@ rw video serialization queue", self];
// Create the serialization queue to use for reading and writing the video data.
self.rwVideoSerializationQueue = dispatch_queue_create([rwVideoSerializationQueueDescription UTF8String], NULL);
主串行隊(duì)列用于處理資源讀寫器的啟動(dòng)和停止(取消)，其他兩個(gè)串行隊(duì)列用于序列化每個(gè)輸出/輸入組合的讀取和寫入，為了以后可能存在的取消行為。
現(xiàn)在您有一些序列化隊(duì)列，可以加載資源軌道并開始重新編碼過程。
self.asset = <#AVAsset that you want to reencode#>;
self.cancelled = NO;
self.outputURL = <#NSURL representing desired output URL for file generated by asset writer#>;
// Asynchronously load the tracks of the asset you want to read.
[self.asset loadValuesAsynchronouslyForKeys:@[@"tracks"] completionHandler:^{
// Once the tracks have finished loading, dispatch the work to the main serialization queue.
dispatch_async(self.mainSerializationQueue, ^{
// Due to asynchronous nature, check to see if user has already cancelled.
if (self.cancelled)
return;
BOOL success = YES;
NSError *localError = nil;
// Check for success of loading the assets tracks.
success = ([self.asset statusOfValueForKey:@"tracks" error:&localError] == AVKeyValueStatusLoaded);
if (success)
{
// If the tracks loaded successfully, make sure that no file exists at the output path for the asset writer.
NSFileManager *fm = [NSFileManager defaultManager];
NSString *localOutputPath = [self.outputURL path];
if ([fm fileExistsAtPath:localOutputPath])
success = [fm removeItemAtPath:localOutputPath error:&localError];
}
if (success)
success = [self setupAssetReaderAndAssetWriter:&localError];
if (success)
success = [self startAssetReaderAndWriter:&localError];
if (!success)
[self readingAndWritingDidFinishSuccessfully:success withError:localError];
});
}];
當(dāng)軌道加載過程完成時(shí)，無論成功與否，其余的工作都將被分派到主序列化隊(duì)列，以確保所有這些工作都被序列化并可以取消。 現(xiàn)在剩下的是在上一個(gè)代碼結(jié)束時(shí)實(shí)現(xiàn)取消流程和三個(gè)自定義方法。
初始化資源讀寫器
自定義setupAssetReaderAndAssetWriter：方法用來初始化讀寫器并配置兩個(gè)輸出/輸入組合，一個(gè)用于音軌，一個(gè)用于視頻軌道。 在此示例中，音頻使用資源讀取器解壓縮為Linear PCM，并使用資源寫入器壓縮回128 kbps AAC。 使用資源讀取器將視頻解壓縮為YUV，并使用資源寫入器將其壓縮為H.264。
- (BOOL)setupAssetReaderAndAssetWriter:(NSError **)outError
{
// Create and initialize the asset reader.
self.assetReader = [[AVAssetReader alloc] initWithAsset:self.asset error:outError];
BOOL success = (self.assetReader != nil);
if (success)
{
// If the asset reader was successfully initialized, do the same for the asset writer.
self.assetWriter = [[AVAssetWriter alloc] initWithURL:self.outputURL fileType:AVFileTypeQuickTimeMovie error:outError];
success = (self.assetWriter != nil);
}
if (success)
{
// If the reader and writer were successfully initialized, grab the audio and video asset tracks that will be used.
AVAssetTrack *assetAudioTrack = nil, *assetVideoTrack = nil;
NSArray *audioTracks = [self.asset tracksWithMediaType:AVMediaTypeAudio];
if ([audioTracks count] > 0)
assetAudioTrack = [audioTracks objectAtIndex:0];
NSArray *videoTracks = [self.asset tracksWithMediaType:AVMediaTypeVideo];
if ([videoTracks count] > 0)
assetVideoTrack = [videoTracks objectAtIndex:0];
if (assetAudioTrack)
{
// If there is an audio track to read, set the decompression settings to Linear PCM and create the asset reader output.
NSDictionary *decompressionAudioSettings = @{ AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatLinearPCM] };
self.assetReaderAudioOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:assetAudioTrack outputSettings:decompressionAudioSettings];
[self.assetReader addOutput:self.assetReaderAudioOutput];
// Then, set the compression settings to 128kbps AAC and create the asset writer input.
AudioChannelLayout stereoChannelLayout = {
.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo,
.mChannelBitmap = 0,
.mNumberChannelDescriptions = 0
};
NSData *channelLayoutAsData = [NSData dataWithBytes:&stereoChannelLayout length:offsetof(AudioChannelLayout, mChannelDescriptions)];
NSDictionary *compressionAudioSettings = @{
AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatMPEG4AAC],
AVEncoderBitRateKey : [NSNumber numberWithInteger:128000],
AVSampleRateKey : [NSNumber numberWithInteger:44100],
AVChannelLayoutKey : channelLayoutAsData,
AVNumberOfChannelsKey : [NSNumber numberWithUnsignedInteger:2]
};
self.assetWriterAudioInput = [AVAssetWriterInput assetWriterInputWithMediaType:[assetAudioTrack mediaType] outputSettings:compressionAudioSettings];
[self.assetWriter addInput:self.assetWriterAudioInput];
}
if (assetVideoTrack)
{
// If there is a video track to read, set the decompression settings for YUV and create the asset reader output.
NSDictionary *decompressionVideoSettings = @{
(id)kCVPixelBufferPixelFormatTypeKey : [NSNumber numberWithUnsignedInt:kCVPixelFormatType_422YpCbCr8],
(id)kCVPixelBufferIOSurfacePropertiesKey : [NSDictionary dictionary]
};
self.assetReaderVideoOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:assetVideoTrack outputSettings:decompressionVideoSettings];
[self.assetReader addOutput:self.assetReaderVideoOutput];
CMFormatDescriptionRef formatDescription = NULL;
// Grab the video format descriptions from the video track and grab the first one if it exists.
NSArray *videoFormatDescriptions = [assetVideoTrack formatDescriptions];
if ([videoFormatDescriptions count] > 0)
formatDescription = (__bridge CMFormatDescriptionRef)[formatDescriptions objectAtIndex:0];
CGSize trackDimensions = {
.width = 0.0,
.height = 0.0,
};
// If the video track had a format description, grab the track dimensions from there. Otherwise, grab them direcly from the track itself.
if (formatDescription)
trackDimensions = CMVideoFormatDescriptionGetPresentationDimensions(formatDescription, false, false);
else
trackDimensions = [assetVideoTrack naturalSize];
NSDictionary *compressionSettings = nil;
// If the video track had a format description, attempt to grab the clean aperture settings and pixel aspect ratio used by the video.
if (formatDescription)
{
NSDictionary *cleanAperture = nil;
NSDictionary *pixelAspectRatio = nil;
CFDictionaryRef cleanApertureFromCMFormatDescription = CMFormatDescriptionGetExtension(formatDescription, kCMFormatDescriptionExtension_CleanAperture);
if (cleanApertureFromCMFormatDescription)
{
cleanAperture = @{
AVVideoCleanApertureWidthKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureWidth),
AVVideoCleanApertureHeightKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureHeight),
AVVideoCleanApertureHorizontalOffsetKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureHorizontalOffset),
AVVideoCleanApertureVerticalOffsetKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureVerticalOffset)
};
}
CFDictionaryRef pixelAspectRatioFromCMFormatDescription = CMFormatDescriptionGetExtension(formatDescription, kCMFormatDescriptionExtension_PixelAspectRatio);
if (pixelAspectRatioFromCMFormatDescription)
{
pixelAspectRatio = @{
AVVideoPixelAspectRatioHorizontalSpacingKey : (id)CFDictionaryGetValue(pixelAspectRatioFromCMFormatDescription, kCMFormatDescriptionKey_PixelAspectRatioHorizontalSpacing),
AVVideoPixelAspectRatioVerticalSpacingKey : (id)CFDictionaryGetValue(pixelAspectRatioFromCMFormatDescription, kCMFormatDescriptionKey_PixelAspectRatioVerticalSpacing)
};
}
// Add whichever settings we could grab from the format description to the compression settings dictionary.
if (cleanAperture || pixelAspectRatio)
{
NSMutableDictionary *mutableCompressionSettings = [NSMutableDictionary dictionary];
if (cleanAperture)
[mutableCompressionSettings setObject:cleanAperture forKey:AVVideoCleanApertureKey];
if (pixelAspectRatio)
[mutableCompressionSettings setObject:pixelAspectRatio forKey:AVVideoPixelAspectRatioKey];
compressionSettings = mutableCompressionSettings;
}
}
// Create the video settings dictionary for H.264.
NSMutableDictionary *videoSettings = (NSMutableDictionary *) @{
AVVideoCodecKey : AVVideoCodecH264,
AVVideoWidthKey : [NSNumber numberWithDouble:trackDimensions.width],
AVVideoHeightKey : [NSNumber numberWithDouble:trackDimensions.height]
};
// Put the compression settings into the video settings dictionary if we were able to grab them.
if (compressionSettings)
[videoSettings setObject:compressionSettings forKey:AVVideoCompressionPropertiesKey];
// Create the asset writer input and add it to the asset writer.
self.assetWriterVideoInput = [AVAssetWriterInput assetWriterInputWithMediaType:[videoTrack mediaType] outputSettings:videoSettings];
[self.assetWriter addInput:self.assetWriterVideoInput];
}
}
return success;
}
重新編碼資源
如果資產(chǎn)讀取器和編寫器已成功初始化和配置，則調(diào)用Handling the Initial Setup 的startAssetReaderAndWriter：方法。 這個(gè)方法實(shí)現(xiàn)了資源讀取和寫入。
- (BOOL)startAssetReaderAndWriter:(NSError **)outError
{
BOOL success = YES;
// Attempt to start the asset reader.
success = [self.assetReader startReading];
if (!success)
*outError = [self.assetReader error];
if (success)
{
// If the reader started successfully, attempt to start the asset writer.
success = [self.assetWriter startWriting];
if (!success)
*outError = [self.assetWriter error];
}
if (success)
{
// If the asset reader and writer both started successfully, create the dispatch group where the reencoding will take place and start a sample-writing session.
self.dispatchGroup = dispatch_group_create();
[self.assetWriter startSessionAtSourceTime:kCMTimeZero];
self.audioFinished = NO;
self.videoFinished = NO;
if (self.assetWriterAudioInput)
{
// If there is audio to reencode, enter the dispatch group before beginning the work.
dispatch_group_enter(self.dispatchGroup);
// Specify the block to execute when the asset writer is ready for audio media data, and specify the queue to call it on.
[self.assetWriterAudioInput requestMediaDataWhenReadyOnQueue:self.rwAudioSerializationQueue usingBlock:^{
// Because the block is called asynchronously, check to see whether its task is complete.
if (self.audioFinished)
return;
BOOL completedOrFailed = NO;
// If the task isn't complete yet, make sure that the input is actually ready for more media data.
while ([self.assetWriterAudioInput isReadyForMoreMediaData] && !completedOrFailed)
{
// Get the next audio sample buffer, and append it to the output file.
CMSampleBufferRef sampleBuffer = [self.assetReaderAudioOutput copyNextSampleBuffer];
if (sampleBuffer != NULL)
{
BOOL success = [self.assetWriterAudioInput appendSampleBuffer:sampleBuffer];
CFRelease(sampleBuffer);
sampleBuffer = NULL;
completedOrFailed = !success;
}
else
{
completedOrFailed = YES;
}
}
if (completedOrFailed)
{
// Mark the input as finished, but only if we haven't already done so, and then leave the dispatch group (since the audio work has finished).
BOOL oldFinished = self.audioFinished;
self.audioFinished = YES;
if (oldFinished == NO)
{
[self.assetWriterAudioInput markAsFinished];
}
dispatch_group_leave(self.dispatchGroup);
}
}];
}
if (self.assetWriterVideoInput)
{
// If we had video to reencode, enter the dispatch group before beginning the work.
dispatch_group_enter(self.dispatchGroup);
// Specify the block to execute when the asset writer is ready for video media data, and specify the queue to call it on.
[self.assetWriterVideoInput requestMediaDataWhenReadyOnQueue:self.rwVideoSerializationQueue usingBlock:^{
// Because the block is called asynchronously, check to see whether its task is complete.
if (self.videoFinished)
return;
BOOL completedOrFailed = NO;
// If the task isn't complete yet, make sure that the input is actually ready for more media data.
while ([self.assetWriterVideoInput isReadyForMoreMediaData] && !completedOrFailed)
{
// Get the next video sample buffer, and append it to the output file.
CMSampleBufferRef sampleBuffer = [self.assetReaderVideoOutput copyNextSampleBuffer];
if (sampleBuffer != NULL)
{
BOOL success = [self.assetWriterVideoInput appendSampleBuffer:sampleBuffer];
CFRelease(sampleBuffer);
sampleBuffer = NULL;
completedOrFailed = !success;
}
else
{
completedOrFailed = YES;
}
}
if (completedOrFailed)
{
// Mark the input as finished, but only if we haven't already done so, and then leave the dispatch group (since the video work has finished).
BOOL oldFinished = self.videoFinished;
self.videoFinished = YES;
if (oldFinished == NO)
{
[self.assetWriterVideoInput markAsFinished];
}
dispatch_group_leave(self.dispatchGroup);
}
}];
}
// Set up the notification that the dispatch group will send when the audio and video work have both finished.
dispatch_group_notify(self.dispatchGroup, self.mainSerializationQueue, ^{
BOOL finalSuccess = YES;
NSError *finalError = nil;
// Check to see if the work has finished due to cancellation.
if (self.cancelled)
{
// If so, cancel the reader and writer.
[self.assetReader cancelReading];
[self.assetWriter cancelWriting];
}
else
{
// If cancellation didn't occur, first make sure that the asset reader didn't fail.
if ([self.assetReader status] == AVAssetReaderStatusFailed)
{
finalSuccess = NO;
finalError = [self.assetReader error];
}
// If the asset reader didn't fail, attempt to stop the asset writer and check for any errors.
if (finalSuccess)
{
finalSuccess = [self.assetWriter finishWriting];
if (!finalSuccess)
finalError = [self.assetWriter error];
}
}
// Call the method to handle completion, and pass in the appropriate parameters to indicate whether reencoding was successful.
[self readingAndWritingDidFinishSuccessfully:finalSuccess withError:finalError];
});
}
// Return success here to indicate whether the asset reader and writer were started successfully.
return success;
}
在重新編碼時(shí)，音頻和視頻軌道在各個(gè)串行隊(duì)列中異步處理，以增加進(jìn)程的整體性能，但是兩個(gè)隊(duì)列都包含在同一個(gè)group中。 通過將每個(gè)軌道的工作放在同一個(gè)group中，group可以在完成所有工作并且可以確定重新編碼過程的成功時(shí)進(jìn)行相應(yīng)處理。
處理完成操作
為了處理讀寫過程的完成，readAndWritingDidFinishSuccessfully：方法被調(diào)用，其中的參數(shù)指示重新編碼是否成功完成。 如果進(jìn)程沒有成功完成，則資源讀取器和寫入器都將被取消，任何UI相關(guān)任務(wù)都將分派到主隊(duì)列中處理。
- (void)readingAndWritingDidFinishSuccessfully:(BOOL)success withError:(NSError *)error
{
if (!success)
{
// If the reencoding process failed, we need to cancel the asset reader and writer.
[self.assetReader cancelReading];
[self.assetWriter cancelWriting];
dispatch_async(dispatch_get_main_queue(), ^{
// Handle any UI tasks here related to failure.
});
}
else
{
// Reencoding was successful, reset booleans.
self.cancelled = NO;
self.videoFinished = NO;
self.audioFinished = NO;
dispatch_async(dispatch_get_main_queue(), ^{
// Handle any UI tasks here related to success.
});
}
}
處理取消行為
使用多個(gè)序列化隊(duì)列，您可以允許應(yīng)用程序的用戶輕松取消重新編碼過程。 在主序列化隊(duì)列中，消息被異步發(fā)送到每個(gè)資源重編碼序列化隊(duì)列以取消其讀取和寫入。 當(dāng)這兩個(gè)序列化隊(duì)列完成取消時(shí)，dispatch group會(huì)向主序列化隊(duì)列發(fā)送通知，其中已取消的屬性設(shè)置為YES。 您可以將以下代碼中的取消方法與UI上的按鈕相關(guān)聯(lián)。
- (void)cancel
{
// Handle cancellation asynchronously, but serialize it with the main queue.
dispatch_async(self.mainSerializationQueue, ^{
// If we had audio data to reencode, we need to cancel the audio work.
if (self.assetWriterAudioInput)
{
// Handle cancellation asynchronously again, but this time serialize it with the audio queue.
dispatch_async(self.rwAudioSerializationQueue, ^{
// Update the Boolean property indicating the task is complete and mark the input as finished if it hasn't already been marked as such.
BOOL oldFinished = self.audioFinished;
self.audioFinished = YES;
if (oldFinished == NO)
{
[self.assetWriterAudioInput markAsFinished];
}
// Leave the dispatch group since the audio work is finished now.
dispatch_group_leave(self.dispatchGroup);
});
}
if (self.assetWriterVideoInput)
{
// Handle cancellation asynchronously again, but this time serialize it with the video queue.
dispatch_async(self.rwVideoSerializationQueue, ^{
// Update the Boolean property indicating the task is complete and mark the input as finished if it hasn't already been marked as such.
BOOL oldFinished = self.videoFinished;
self.videoFinished = YES;
if (oldFinished == NO)
{
[self.assetWriterVideoInput markAsFinished];
}
// Leave the dispatch group, since the video work is finished now.
dispatch_group_leave(self.dispatchGroup);
});
}
// Set the cancelled Boolean property to YES to cancel any work on the main queue as well.
self.cancelled = YES;
});
}
資源輸出設(shè)置助手
AVOutputSettingsAssistant類有助于為資源讀取器或?qū)懭肫鲃?chuàng)建輸出設(shè)置字典。 這使得設(shè)置更簡(jiǎn)單，特別是對(duì)于具有多個(gè)特定預(yù)設(shè)的高幀率H264電影。 下面展示了使用輸出設(shè)置助手使用設(shè)置助手的示例。
AVOutputSettingsAssistant *outputSettingsAssistant = [AVOutputSettingsAssistant outputSettingsAssistantWithPreset:<some preset>];
CMFormatDescriptionRef audioFormat = [self getAudioFormat];
if (audioFormat != NULL)
[outputSettingsAssistant setSourceAudioFormat:(CMAudioFormatDescriptionRef)audioFormat];
CMFormatDescriptionRef videoFormat = [self getVideoFormat];
if (videoFormat != NULL)
[outputSettingsAssistant setSourceVideoFormat:(CMVideoFormatDescriptionRef)videoFormat];
CMTime assetMinVideoFrameDuration = [self getMinFrameDuration];
CMTime averageFrameDuration = [self getAvgFrameDuration]
[outputSettingsAssistant setSourceVideoAverageFrameDuration:averageFrameDuration];
[outputSettingsAssistant setSourceVideoMinFrameDuration:assetMinVideoFrameDuration];
AVAssetWriter *assetWriter = [AVAssetWriter assetWriterWithURL:<some URL> fileType:[outputSettingsAssistant outputFileType] error:NULL];
AVAssetWriterInput *audioInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:[outputSettingsAssistant audioSettings] sourceFormatHint:audioFormat];
AVAssetWriterInput *videoInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:[outputSettingsAssistant videoSettings] sourceFormatHint:videoFormat];
