ios - CVPixelBufferUnlockBaseAddress - Block UI -

i'm struggling debug weird problem. in captureoutput:didoutputsamplebuffer:fromconnection: right after cvpixelbufferunlockbaseaddress(imagebuffer,0); entire ui stops responding touches. camera preview works buttons stop responding , added uitapgesture , not work. tried putting dispatch still no success.

- (void)captureoutput:(avcaptureoutput *)captureoutput didoutputsamplebuffer:(cmsamplebufferref)samplebuffer        fromconnection:(avcaptureconnection *)connection {     if (state != camera) {         return;     }      if (self.state != camera_decoding)     {         self.state = camera_decoding;     }       cvimagebufferref imagebuffer = cmsamplebuffergetimagebuffer(samplebuffer);     //lock image buffer     cvpixelbufferlockbaseaddress(imagebuffer,0);     //get information image     baseaddress = (uint8_t *)cvpixelbuffergetbaseaddressofplane(imagebuffer,0);      int pixelformat = cvpixelbuffergetpixelformattype(imagebuffer);     switch (pixelformat) {         case kcvpixelformattype_420ypcbcr8biplanarvideorange:              //nslog(@"capture pixel format=nv12");             bytesperrow = cvpixelbuffergetbytesperrowofplane(imagebuffer,0);             width = bytesperrow;//cvpixelbuffergetwidthofplane(imagebuffer,0);             height = cvpixelbuffergetheightofplane(imagebuffer,0);             break;         case kcvpixelformattype_422ypcbcr8:               //nslog(@"capture pixel format=uyuy422");             bytesperrow = cvpixelbuffergetbytesperrowofplane(imagebuffer,0);             width = cvpixelbuffergetwidth(imagebuffer);             height = cvpixelbuffergetheight(imagebuffer);             int len = width*height;             int dstpos=1;             (int i=0;i<len;i++){                 baseaddress[i]=baseaddress[dstpos];                 dstpos+=2;             }              break;         default:             //  nslog(@"capture pixel format=rgb32");             break;     }      unsigned char *presult=null;      int reslength = mwb_scangrayscaleimage(baseaddress,width,height, &presult);      cvpixelbufferunlockbaseaddress(imagebuffer,0); 

probably happening because running operations on main thread.
creating output session have opportunity run callback on queue:

avcapturevideodataoutput * dataoutput = [[avcapturevideodataoutput alloc] init];             [dataoutput setalwaysdiscardslatevideoframes:yes];             [dataoutput setvideosettings:[nsdictionary dictionarywithobject:[nsnumber numberwithint:kcvpixelformattype_32bgra] forkey:(id)kcvpixelbufferpixelformattypekey]]; dispatch_queue_t queue = dispatch_queue_create("it.cloudintouchlabs.avsession", dispatch_queue_serial);             [dataoutput setsamplebufferdelegate:(id)self queue:queue];             if ([capturesession_ canaddoutput:dataoutput]) {                 [capturesession_ addoutput:dataoutput];             } 

in sample create serial queue.