ios - Saving image from a URL asynchronously -

so have gone through questions on so, , have put them create method accepts 2 paramaters, first url of image download , display in uiimageview , second placeholder image uiimageview. want save image won't downloaded every time. have used sdwebimage download image, had confusion when came saving image in documents directory using sdwebimage, decided not use it. used dispatch_sync(dispatch_get_main_queue() , method looks :

- (void)saveimagefromurl:(uiimage*)image:(nsurl*)imageurl {  nsstring *url = [imageurl absolutestring]; nsarray *parts = [url componentsseparatedbystring:@"/"]; nsstring *filename = [parts lastobject];  nsarray *paths = nssearchpathfordirectoriesindomains(nsdocumentdirectory, nsuserdomainmask, yes); nsstring *documentsdirectory = [paths objectatindex:0];  nsstring *fullpath = [documentsdirectory stringbyappendingpathcomponent:[nsstring stringwithformat:@"%@.png", filename]]; bool fileexists = [[nsfilemanager defaultmanager] fileexistsatpath:fullpath];  if (fileexists) {     nslog(@"file exists");     _myuiimage.image = [uiimage imagenamed:fullpath];     return; }  else {     dispatch_async(dispatch_get_main_queue(), ^{         [self.myuiimage sd_setimagewithurl:imageurl placeholderimage:image];         uiimage *imagefromurl = [uiimage imagewithdata:[nsdata datawithcontentsofurl:imageurl]];         nsdata *imagedatanew = uiimagepngrepresentation(imagefromurl);         nsfilemanager *filemanager = [nsfilemanager defaultmanager];         [filemanager createfileatpath:fullpath contents:imagedatanew attributes:nil];     });   } } 

i have couple of questions, implementation enough since working on app on app store ? downloading of image url done asynchronously ? (i know using dispatch_async need confirm). if yes, wont block ui, right ?

dispatch_async(dispatch_get_global_queue( dispatch_queue_priority_default, 0), ^(void){  // *** load image url asynchronously without blocking main thread ***     uiimage *imagefromurl = [uiimage imagewithdata:[nsdata datawithcontentsofurl:imageurl]];      dispatch_async(dispatch_get_main_queue(), ^(void){          // *** create file path documentdirectory ***         nsstring * docdirpath = nssearchpathfordirectoriesindomains(nsdocumentdirectory, nsuserdomainmask, yes)[0];         nsstring *filepath = [docdirpath stringbyappendingpathcomponent:@"myimage.jpg"]          // *** write image disk ***          bool issaved = [uiimagejpegrepresentation(image, 1.0f) writetofile:dirpath atomically:yes];         if(issaved)         {             nslog(@"image write disc successfully.");         }     }); });