AVCodecContext类型的pCodecCtx ，每个AVStream存储一个视频/音频流的相关数据；每个AVStream对应一个AVCodecContext，存储该视频/音频流使用解码方式的相关数据；每个AVCodecContext中对应一个AVCodec，包含该视频/音频对应的解码器。每种解码器都对应一个AVCodec结构。

AVFrame类型的pFrame，视频的话，每个结构一般是存一帧，音频可能有好几帧。解码前数据是AVPacket，解码后数据是AVFrame。

FMPEG中结构体很多。最关键的结构体他们之间的对应关系如下所示：

图片来自:FFMPEG中最关键的结构体之间的关系

下面就是初始化的代码

-(id)initWithVideo:(NSString *)moviePath {
    if (!(self=[super init])) return nil;
 
    AVCodec         *pCodec;
        
    // Register all formats and codecs
    avcodec_register_all();
    av_register_all();
    
    // Open video file
    if(avformat_open_input(&pFormatCtx, [moviePath cStringUsingEncoding:NSASCIIStringEncoding], NULL, NULL) != 0) {
        av_log(NULL, AV_LOG_ERROR, "Couldn't open file\n");
        goto initError;
    }
    
    // Retrieve stream information
    if(avformat_find_stream_info(pFormatCtx,NULL) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Couldn't find stream information\n");
        goto initError;
    }
    
    // Find the first video stream
    if ((videoStream =  av_find_best_stream(pFormatCtx, AVMEDIA_TYPE_VIDEO, -1, -1, &pCodec, 0)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot find a video stream in the input file\n");
        goto initError;
    }
    
    // Get a pointer to the codec context for the video stream
    pCodecCtx = pFormatCtx->streams[videoStream]->codec;
    
    // Find the decoder for the video stream
    pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
    if(pCodec == NULL) {
        av_log(NULL, AV_LOG_ERROR, "Unsupported codec!\n");
        goto initError;
    }
    
    // Open codec
    if(avcodec_open2(pCodecCtx, pCodec, NULL) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot open video decoder\n");
        goto initError;
    }
    
    // Allocate video frame
    pFrame = avcodec_alloc_frame();
            
    outputWidth = pCodecCtx->width;
    self.outputHeight = pCodecCtx->height;
            
    return self;
    
initError:
    [self release];
    return nil;
}

sourceWidth和sourceHeight方法

获取屏幕的宽和高

-(int)sourceWidth {
    return pCodecCtx->width;
}

-(int)sourceHeight {
    return pCodecCtx->height;
}

setupScaler方法

设置视频播放视图的尺寸

-(void)setupScaler {

    // Release old picture and scaler
    avpicture_free(&picture);
    sws_freeContext(img_convert_ctx);   
    
    // Allocate RGB picture
    avpicture_alloc(&picture, PIX_FMT_RGB24, outputWidth, outputHeight);
    
    // Setup scaler
    static int sws_flags =  SWS_FAST_BILINEAR;
    img_convert_ctx = sws_getContext(pCodecCtx->width, 
                                     pCodecCtx->height,
                                     pCodecCtx->pix_fmt,
                                     outputWidth, 
                                     outputHeight,
                                     PIX_FMT_RGB24,
                                     sws_flags, NULL, NULL, NULL);
    
}

duration方法

获取音视频文件的总时间

-(double)duration {
    return (double)pFormatCtx->duration / AV_TIME_BASE;
}

currentTime方法

显示音视频当前播放的时间

-(double)currentTime {
    AVRational timeBase = pFormatCtx->streams[videoStream]->time_base;
    return packet.pts * (double)timeBase.num / timeBase.den;
}

seekTime:(double)seconds方法

直接跳到音视频的第seconds秒进行播放，默认从第0.0秒开始

-(void)seekTime:(double)seconds {
    AVRational timeBase = pFormatCtx->streams[videoStream]->time_base;
    int64_t targetFrame = (int64_t)((double)timeBase.den / timeBase.num * seconds);
    avformat_seek_file(pFormatCtx, videoStream, targetFrame, targetFrame, targetFrame, AVSEEK_FLAG_FRAME);
    avcodec_flush_buffers(pCodecCtx);
}

stepFrame方法

解码视频得到帧

-(BOOL)stepFrame {
    // AVPacket packet;
    int frameFinished=0;

    while(!frameFinished && av_read_frame(pFormatCtx, &packet)>=0) {
        // Is this a packet from the video stream?
        if(packet.stream_index==videoStream) {
            // Decode video frame
            avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
        }
        
    }
    return frameFinished!=0;
}

currentImage方法

获取当前的UIImage对象，以呈现当前播放的画面

-(UIImage *)currentImage {
    if (!pFrame->data[0]) return nil;
    [self convertFrameToRGB];
    return [self imageFromAVPicture:picture width:outputWidth height:outputHeight];
}

convertFrameToRGB

转换音视频帧到RGB

-(void)convertFrameToRGB {  
    sws_scale (img_convert_ctx, pFrame->data, pFrame->linesize,
               0, pCodecCtx->height,
               picture.data, picture.linesize); 
}

(UIImage *)imageFromAVPicture:(AVPicture)pict width:(int)width height:(int)height方法

把AVPicture转换成UIImage把音视频画面显示出来

-(UIImage *)imageFromAVPicture:(AVPicture)pict width:(int)width height:(int)height {
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
    CFDataRef data = CFDataCreateWithBytesNoCopy(kCFAllocatorDefault, pict.data[0], pict.linesize[0]*height,kCFAllocatorNull);
    CGDataProviderRef provider = CGDataProviderCreateWithCFData(data);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGImageRef cgImage = CGImageCreate(width, 
                                       height, 
                                       8, 
                                       24, 
                                       pict.linesize[0], 
                                       colorSpace, 
                                       bitmapInfo, 
                                       provider, 
                                       NULL, 
                                       NO, 
                                       kCGRenderingIntentDefault);
    CGColorSpaceRelease(colorSpace);
    UIImage *image = [UIImage imageWithCGImage:cgImage];
    CGImageRelease(cgImage);
    CGDataProviderRelease(provider);
    CFRelease(data);
    
    return image;
}