AVAssetWriterInput Class Reference

Discussion

The timing information in the sample buffer, considered relative to the time passed to the asset writer’s startSessionAtSourceTime: will be used to determine the timing of those samples in the output file.

Do not modify sampleBuffer or its contents after you have passed it to this method.

Discussion

Each new input accepts data for a new track of the asset writer’s output file. You add an input to an asset writer using the AVAssetWriter method addInput:.

Passing nil for outputSettings instructs the input to pass through appended samples, doing no processing before they are written to the output file. This is useful if, for example, you are appending buffers that are already in a desirable compressed format. However, passthrough is currently supported only when writing to QuickTime Movie files (i.e. the AVAssetWriter was initialized with AVFileTypeQuickTimeMovie). For other file types, you must specify non-nil output settings.

When the mediaType parameter is AVMediaTypeAudio, the outputSettings dictionary does not support the AVEncoderAudioQualityKey and AVSampleRateConverterAudioQualityKey keys. When using this method, an audio settings dictionary must be fully specified, meaning that it must contain the AVFormatIDKey, AVSampleRateKey, and AVNumberOfChannelsKey keys. If no other channel layout information is available, a value of 1 for the AVNumberOfChannelsKey key results in mono output and a value of 2 results in stereo output. If the AVNumberOfChannelsKey key specifies a value greater than 2, the dictionary must also specify a value for the AVChannelLayoutKey key. For audio using the kAudioFormatLinearPCM format, include all relevant AVLinearPCM*Key keys. For the kAudioFormatAppleLossless format, include the AVEncoderBitDepthHintKey keys. To avoid specifying values for each of those keys, use the initWithMediaType:outputSettings:sourceFormatHint: method instead.

When the mediaType parameter is AVMediaTypeVideo, the outputSettings dictionary must request a compressed video format. This means that the values specified in the dictionary must follow the rules for compressed video output, as described in AVVideoSettings.h. When using this initializer, a video settings dictionary must be fully specified, meaning that it must contain the following keys: AVVideoCodecKey, AVVideoWidthKey, and AVVideoHeightKey. To avoid specifying values for each of those keys, use the initWithMediaType:outputSettings:sourceFormatHint: method.

Discussion

Each new input accepts data for a new track of the asset writer’s output file. You add an input to an asset writer using the AVAssetWriter method addInput:.

Passing nil for outputSettings instructs the input to pass through appended samples, doing no processing before they are written to the output file. This is useful if, for example, you are appending buffers that are already in a desirable compressed format. However, passthrough is currently supported only when writing to QuickTime Movie files (i.e. the AVAssetWriter was initialized with AVFileTypeQuickTimeMovie). For other file types, you must specify non-nil output settings.

Discussion

If you are monitoring each input's expectsMediaDataInRealTime value to keep the output file well interleaved, it is important to call this method when you have finished adding buffers to a track. This is necessary to prevent other inputs from stalling, as they may otherwise wait forever for that input's media data, attempting to complete the ideal interleaving pattern.

Discussion

The block should append media data to the input either until the input’s readyForMoreMediaData property becomes NO or until there is no more media data to supply (at which point it may choose to mark the input as finished using markAsFinished). The block should then exit. After the block exits, if the input has not been marked as finished, once the input has processed the media data it has received and becomes ready for more media data again, it will invoke the block again in order to obtain more.

A typical use of this method, with a block that supplies media data to an input while respecting the input’s readyForMoreMediaData property, might look like this:

[myAVAssetWriterInput requestMediaDataWhenReadyOnQueue:myInputSerialQueue usingBlock:^{

        while ([myAVAssetWriterInput isReadyForMoreMediaData])

        {

            CMSampleBufferRef nextSampleBuffer = [self copyNextSampleBufferToWrite];

            if (nextSampleBuffer)

            {

                [myAVAssetWriterInput appendSampleBuffer:nextSampleBuffer];

                CFRelease(nextSampleBuffer);

            }

            else

            {

                [myAVAssetWriterInput markAsFinished];

                break;

            }

        }

    }];

You should not use this method with a push-style buffer source, such as AVCaptureAudioDataOutput or AVCaptureVideoDataOutput, because such a combination will typically require intermediate queueing of buffers. Instead, this method is better suited to a pull-style buffer source such as an AVAssetReaderOutput object.

When using a push-style buffer source, it is generally better to immediately append each buffer to the asset writer input, directly as it is received using appendSampleBuffer:. Using this strategy, it is often possible to avoid having to queue up buffers in between the buffer source and the asset writer input. Note that many of these push-style buffer sources also produce buffers in real-time, in which case you should set expectsMediaDataInRealTime to YES.