image - iOS - How to get thumbnail from video without play? -

i'm trying thumbnail video , show in tableview. here code:

- (uiimage *)imagefromvideourl:(nsurl *)contenturl {     avasset *asset = [avasset assetwithurl:contenturl];      //  thumbnail @ start of video     cmtime thumbnailtime = [asset duration];     thumbnailtime.value = 25;      //  image video @ given time     avassetimagegenerator *imagegenerator = [[avassetimagegenerator alloc] initwithasset:asset];      cgimageref imageref = [imagegenerator copycgimageattime:thumbnailtime actualtime:null error:null];     uiimage *thumbnail = [uiimage imagewithcgimage:imageref];     cgimagerelease(imageref);      return thumbnail; } 

but image allways return black. what's wrong?

nsurl *videourl = [nsurl fileurlwithpath:filepath];// filepath video file path     avurlasset *asset = [[avurlasset alloc] initwithurl:videourl options:nil]; avassetimagegenerator *generateimg = [[avassetimagegenerator alloc] initwithasset:asset]; nserror *error = null; cmtime time = cmtimemake(1, 1); cgimageref refimg = [generateimg copycgimageattime:time actualtime:null error:&error]; nslog(@"error==%@, refimage==%@", error, refimg);  uiimage *frameimage= [[uiimage alloc] initwithcgimage:refimg]; return frameimage;