objective c - How to get dynamicaly height of image in dispatch_async method in scrollview -

i using below code images server. want dynamicaly height of image , add image in scrollview.

from below code when height outside dispatch_async method shows zero.

how can dynamically height of image async image load.

- (void)viewdidload { [self loadviewpublicevents]; }  -(void) loadviewpublicevents {      (int i=0;i<arraypublicevents.count;i++)     {          uiimageview *img_vw1=[[uiimageview alloc] init];          dispatch_async(dispatch_get_global_queue(0, 0), ^{             nsdata *imagedata = [nsdata datawithcontentsofurl:[nsurl urlwithstring:[nsstring stringwithformat:@"http://abc.us/uploads/event/%@",[[arraypublicevents objectatindex:i] valueforkey:@"image"]]]];             uiimage *images = [[uiimage alloc]initwithdata:imagedata];             dispatch_async(dispatch_get_main_queue(), ^{                 img_vw1.image = images;                 scaledheight = images.size.height;             });         });          nslog(@"%f",scaledheight); // print 0         img_vw1.backgroundcolor=[uicolor clearcolor];         img_vw1.frame=cgrectmake(0,y+5,screen_width,197);         [img_vw1 setcontentmode:uiviewcontentmodescaleaspectfit];          img_vw1.backgroundcolor=[uicolor clearcolor];         [self.scrll_vw addsubview:img_vw1];     } } 

thanks in advance

your code:

nslog(@"%f",scaledheight); // print 0 img_vw1.backgroundcolor=[uicolor clearcolor]; img_vw1.frame=cgrectmake(0,y+5,screen_width,197); [img_vw1 setcontentmode:uiviewcontentmodescaleaspectfit];  img_vw1.backgroundcolor=[uicolor clearcolor]; [self.scrll_vw addsubview:img_vw1]; 

is being executed, before loaded image.

thus have either wait (herefore use semaphores until thread has finished) or place inside of block.

as want modify ui makes sense place main block:

    uiimageview *img_vw1=[[uiimageview alloc] init];      dispatch_async(dispatch_get_global_queue(0, 0), ^{         nsdata *imagedata = [nsdata datawithcontentsofurl:[nsurl urlwithstring:[nsstring stringwithformat:@"http://abc.us/uploads/event/%@",[[arraypublicevents objectatindex:i] valueforkey:@"image"]]]];         uiimage *images = [[uiimage alloc]initwithdata:imagedata];         dispatch_async(dispatch_get_main_queue(), ^{             img_vw1.image = images;             scaledheight = images.size.height;              nslog(@"%f",scaledheight); // print 0             img_vw1.backgroundcolor=[uicolor clearcolor];             img_vw1.frame=cgrectmake(0,y+5,screen_width,197);             [img_vw1 setcontentmode:uiviewcontentmodescaleaspectfit];              img_vw1.backgroundcolor=[uicolor clearcolor];             [self.scrll_vw addsubview:img_vw1];         });     }); 

for more information, here link apple's documentation: https://developer.apple.com/library/content/documentation/general/conceptual/concurrencyprogrammingguide/operationqueues/operationqueues.html