5期_AVFoundation-AVAudioFile

AVAudioFile是音频的为读和写的一个类，不管文件的实际格式是什么，读写文件都是通过
AVAudioPCMBuffer对象，包含AVAudioCommonFormat的样本，称为文件的“处理格式”。执行to和from的转换文件的实际格式。 读和写始终是顺序的，但是可以通过设置 framePosition进行。

@interface AVAudioFile : NSObject {
@private
    void *_impl;
}

/*! @method initForReading:error:
    @abstract Open a file for reading.
    为读或写而打开的音频文件
    @param fileURL
        the file to open
    @param outError
        on exit, if an error occurs, a description of the error
    @discussion
        This opens the file for reading using the standard format (deinterleaved floating point).
*/
- (nullable instancetype)initForReading:(NSURL *)fileURL error:(NSError **)outError;

/*! @method readIntoBuffer:error:
    @abstract Read an entire buffer.
    @param buffer
        要从文件中读取的缓冲区。它的格式必须与文件的格式匹配 
        处理格式
    @param outError
        on exit, if an error occurs, a description of the error
    @return
        YES for success.
    @discussion
        Reading sequentially from framePosition, attempts to fill the buffer to its capacity. On
        return, the buffer's length indicates the number of sample frames successfully read.
*/
- (BOOL)readIntoBuffer:(AVAudioPCMBuffer *)buffer error:(NSError **)outError;

- (BOOL)readIntoBuffer:(AVAudioPCMBuffer *)buffer frameCount:(AVAudioFrameCount)frames error:(NSError **)outError;

/*! @property url
    @abstract The URL the file is reading or writing.
*/
@property (nonatomic, readonly) NSURL *url;

/*! @property fileFormat
    @abstract The on-disk format of the file.
    文件在磁盘的格式
*/
@property (nonatomic, readonly) AVAudioFormat *fileFormat;

/*! @property processingFormat
    @abstract The processing format of the file.
    文件的处理格式。不同机型加载不同音频文件处理格式不同
*/
@property (nonatomic, readonly) AVAudioFormat *processingFormat;

/*! @property length
    @abstract The number of sample frames in the file.
    文件中本帧的数量
    @discussion
         Note: this can be expensive to compute for the first time.
*/
@property (nonatomic, readonly) AVAudioFramePosition length;
@end