使用HTTP-FLV把iPhone摄像头的画面进行直播，局域网内的设备可以通过VLC进行观看，不通过服务器，实现局域网点对点直播。
实现步骤
1、采集iPhone摄像头画面
2、采集到的数据硬编码成H264数据
3、把编码的数据通过FFmpeg封装成FLV tag
4、搭建HTTP服务器监听HTTP连接，连接成功之后发送数据

代码实现

1、采集iPhone摄像头画面

    _captureSession = [[AVCaptureSession alloc] init];

    _captureSession.sessionPreset = AVCaptureSessionPreset1280x720;
    _captureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    NSError * error = nil;
    _captureDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:_captureDevice error:&error];
    if (_captureDeviceInput) {
        [_captureSession addInput:_captureDeviceInput];
    }
    _captureVideoDataOutput = [[AVCaptureVideoDataOutput alloc] init];
    [_captureVideoDataOutput setAlwaysDiscardsLateVideoFrames:YES];

    NSDictionary * settings = [[NSDictionary alloc] initWithObjectsAndKeys:@(kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange), kCVPixelBufferPixelFormatTypeKey, nil];
    _captureVideoDataOutput.videoSettings = settings;

    dispatch_queue_t queue = dispatch_queue_create("CaptureQueue", NULL);

    [_captureVideoDataOutput setSampleBufferDelegate:self queue:queue];
    [_captureSession addOutput:_captureVideoDataOutput];

    _previewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession];
    _previewLayer.frame = CGRectMake(0, 100, self.view.bounds.size.width, self.view.bounds.size.height - 100);
    [self.view.layer addSublayer:_previewLayer];
    [_captureSession startRunning];

2、采集到的数据硬编码成H264数据

    //初始化硬编码器
    OSStatus status = VTCompressionSessionCreate(NULL, 1280, 720, kCMVideoCodecType_H264, NULL, NULL, NULL, didCompressH264, (__bridge void *)self, &_compressionSession);
    if (status != noErr) {
        NSLog(@"Create compressionSession error");
        return;
    }

    VTSessionSetProperty(_compressionSession, kVTCompressionPropertyKey_RealTime, kCFBooleanTrue);
    VTSessionSetProperty(_compressionSession, kVTCompressionPropertyKey_ProfileLevel, kVTProfileLevel_H264_High_AutoLevel);
    VTSessionSetProperty(_compressionSession, kVTCompressionPropertyKey_AllowFrameReordering, kCFBooleanTrue);
    VTSessionSetProperty(_compressionSession, kVTCompressionPropertyKey_MaxKeyFrameInterval, (__bridge CFTypeRef)(@(30)));
    VTSessionSetProperty(_compressionSession, kVTCompressionPropertyKey_ExpectedFrameRate, (__bridge CFTypeRef)(@(30)));
    VTSessionSetProperty(_compressionSession, kVTCompressionPropertyKey_AverageBitRate, (__bridge CFTypeRef)(@(800 * 1024)));
    status = VTCompressionSessionPrepareToEncodeFrames(_compressionSession);
    status = VTCompressionSessionCompleteFrames(_compressionSession, kCMTimeInvalid);
    if (status != noErr) {
        NSLog(@"Prepare error");
    }

//编码采集到的数据
- (void)captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
    CVImageBufferRef imageBuffer = (CVImageBufferRef)CMSampleBufferGetImageBuffer(sampleBuffer);
    CMTime pts = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
    CMTime dur = CMSampleBufferGetDuration(sampleBuffer);
    VTEncodeInfoFlags flags;
    OSStatus status = VTCompressionSessionEncodeFrame(_compressionSession, imageBuffer, pts, dur, NULL, NULL, &flags);
    if (status != noErr) {
        NSLog(@"Encode fail");
    } 
    //此处编码也可以使用FFmpeg进行软编码CVImageBufferRef是采集出的像素数据，和CVPixelBufferRef一样，可以取出yuv数据传入FFmpeg中进行编码
}

3、把编码的数据通过FFmpeg封装成FLV tag

int ret = avformat_alloc_output_context2(&ofmt_ctx, NULL, "flv", NULL);
if (ret < 0) {
  NSLog(@"Could not allocate output format context!");
}
//这里我们通过write_buffer方法把数据写入内存通过HTTP发送出去，而不是写入文件或服务器地址，需要创建AVIOContext然后赋值给AVFormatContext
//这里申请的AVIOContext要通过avio_context_free()释放
unsigned char * outBuffer = (unsigned char *)av_malloc(32768);
AVIOContext * avio_out = avio_alloc_context(outBuffer, 32768, 1, NULL, NULL, write_buffer, NULL);
ofmt_ctx->pb = avio_out;
ofmt_ctx->flags = AVFMT_FLAG_CUSTOM_IO;

AVCodec * codec = avcodec_find_encoder(AV_CODEC_ID_H264);
out_stream = avformat_new_stream(ofmt_ctx, codec);
codec_ctx = avcodec_alloc_context3(codec);

AVRational dst_fps = {30, 1};
codec_ctx->codec_tag = 0;
codec_ctx->codec_id = AV_CODEC_ID_H264;
codec_ctx->codec_type = AVMEDIA_TYPE_VIDEO;
codec_ctx->width = 1280;
codec_ctx->height = 720;
codec_ctx->gop_size = 12;
codec_ctx->pix_fmt = AV_PIX_FMT_NV12;
codec_ctx->framerate = dst_fps;
codec_ctx->time_base = av_inv_q(dst_fps);
if(ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER) {
  codec_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
}
ret = avcodec_parameters_from_context(out_stream->codecpar, codec_ctx);
if (ret < 0) {
  NSLog(@"Could not initialize stream codec parameters!");
}
AVDictionary * codec_options = NULL;
av_dict_set(&codec_options, "profile", "high", 0);
av_dict_set(&codec_options, "preset", "superfast", 0);
av_dict_set(&codec_options, "tune", "zerolatency", 0);

ret = avcodec_open2(codec_ctx, codec, &codec_options);
if (ret < 0) {
  NSLog(@"Could not open video encoder!");
}

out_stream->codecpar->extradata = codec_ctx->extradata;
out_stream->codecpar->extradata_size = codec_ctx->extradata_size;

ret = avformat_write_header(ofmt_ctx, NULL);
if (ret < 0) {
  NSLog(@"Could not write header!");
}

static int write_buffer(void * opaque, uint8_t * buf, int buf_size) {
  //在avformat_write_header的时候这里得到的数据是FLV文件的头部，在av_write_frame的时候这里得到的是FLV tag数据，可以通过HTTP发送出去，我使用的GCDWebServer库
  //这里buf_size上限是我们创建时的32768，如果每个tag数据小于32768就会得到完整的tag数据，如果tag数据大于32768就会得到部分tag数据，要自行处理
  return 0;
}

static void didCompressH264(void *outputCallbackRefCon, void *sourceFrameRefCon, OSStatus status, VTEncodeInfoFlags infoFlags, CMSampleBufferRef sampleBuffer ) {
  //iOS硬编码的H264数据是AVCC格式的，每个NALU的前4个字节是数据长度，AVCC格式的数据直接写入文件是不能播放的，需要转换成0x00000001开始码开头的Annex B格式
  if (status != noErr) {
    NSLog(@"Compress H264 failed");
    return;
  }
  if (!CMSampleBufferDataIsReady(sampleBuffer)) {
    return;
  }
  CMBlockBufferRef dataBuffer = CMSampleBufferGetDataBuffer(sampleBuffer);
  size_t length, totalLength;
  char * dataPointer;
  const char bytesHeader[] = "\x00\x00\x00\x01";
  OSStatus statusCodeRet = CMBlockBufferGetDataPointer(dataBuffer, 0, &length, &totalLength, &dataPointer);
  bool keyFrame = !CFDictionaryContainsKey((CFDictionaryRef)(CFArrayGetValueAtIndex(CMSampleBufferGetSampleAttachmentsArray(sampleBuffer, true), 0)), (const void)kCMSampleAttachmentKey_NotSync);
  NSMutableData * pktData = [NSMutableData data];
  if (keyFrame) {
    CMFormatDescriptionRef format = CMSampleBufferGetFormatDescription(sample);
    size_t sparameterSetSize, sparameterSetCount;
    const uint8_t * sparameterSet;
    status = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(format, 0, &sparameterSet, &sparameterSetSize, &sparameterSetCount, 0);
    if (status == noErr) {
      size_t pparameterSetSize, pparameterSetCount;
      const uint8_t * pparameterSet;
      status = CMVideoFormatDescriptionGetH264ParameterSetAtIndex(format, 1, &pparameterSet, &pparameterSetSize, &pparameterSetCount, 0);
      if (status == noErr) {
        size_t headerLength = 4;
        size length = 2 * headerLength + sparameterSetSize + pparameterSetSize;
        unsigned char * buffer = (unsigned char *)malloc(sizeof(unsigned char) * length);
        memcpy(buffer, bytesHeader, headerLength);
        memcpy(buffer + headerLength, sparameterSet, sparameterSetSize);
        memcpy(buffer + headerLength + sparameterSetSize, bytesHeader, headerLength);
        memcpy(buffer + headerLength + sparameterSetSize + headerLength, pparameterSet, pparameterSetSize);
        [pktData appendBytes:buffer length:length];
      }
    }
  }
  size_t bufferOffset = 0;
  int AVCCHeaderLength = 4;
  while(bufferOffset < totalLength - AVCCHeaderLength) {
    uint32_t NALUintLength = 0;
    memcpy(&NALUnitLength, dataPointer + bufferOffset, AVCCHeaderLength);
    NALUnitLength = CFSwapInt32BigToHost(NALUnitLength);
    unsigned char * buffer = (unsigned char *)malloc(sizeof(unsigned char) * (NALUnitLength + AVCCHeaderLength));
    memcpy(buffer, bytesHeader, AVCCHeaderLength);
    memcpy(buffer + AVCCHeaderLength, dataPointer + bufferOffset + AVCCHeaderLength, NALUnitLength);
    [pktData appendBytes:buffer length:NALUnitLength + AVCCHeaderLength];
    bufferOffset += AVCCHeaderLength + NALUnitLength;
  }
  AVPacket pkt = {0};
  av_init_packet(&pkt);
  pkt.data = (uint8_t *)[pktData bytes];
  pkt.size = (int)[pktData length];
  //pkt_pts从0开始递增
  pkt.pts = pkt_pts;
  pkt.dts = pkt.pts;
  if (keyFrame) {
    pkt.flags = AV_PKT_FLAG_KEY;
  } else {
    pkt.flags = 0;
  }
  pkt.stream_index = 0;
  av_packet_rescale_ts(&pkt, codec_ctx->time_base, out_stream->time_base);
  av_write_frame(ofmt_ctx, &pkt);
  pkt_pts++;
}

通过VLC可以观看直播，通过ffplay播放黑屏，原因还未发现。