im using following code populate collection view cells images.
- (uicollectionviewcell *)collectionview:(uicollectionview *)collectionview cellforitematindexpath:(nsindexpath *)indexpath { static nsstring *identifier = @"cell"; uicollectionviewcell *cell = [collectionview dequeuereusablecellwithreuseidentifier:identifier forindexpath:indexpath]; uiimageview *recipeimageview = (uiimageview *)[cell viewwithtag:100]; recipeimageview.image = nil; if ([imagearray count] >0){ dispatch_async(dispatch_get_global_queue(dispatch_queue_priority_background, 0), ^(void) { nsdata *data0 = [nsdata datawithcontentsofurl: [nsurl urlwithstring:[imagearray objectatindex:indexpath.row]]]; uiimage *image = [uiimage imagewithdata: data0]; dispatch_sync(dispatch_get_main_queue(), ^(void) { recipeimageview.image = image; }); }); } [spinnershow stopanimating]; return cell; } the problem that, when im scrolling images flickering , flashing. why so? how can make images stable without flickering?
just short overview, answer
uicollectionview highly optimized, , keep on-screen visible rows in memory. now, rows cells cached in pool , reused , not regenerated. whenever, user scrolls uicollectionview, adds just-hidden rows in pool , reuses them next visible rows.
so, now, coming answer
when scroll collectionview, collectionview datasource method gets called again every indexpath, coming in visible range , image gets downloaded again
- (uicollectionviewcell *)collectionview:(uicollectionview *)collectionview cellforitematindexpath:(nsindexpath *)indexpath solution
instantiate instance nsmutabledictionary, outside of method.
now in code
@implementation classname{ nsmutabledictionary *cachedimage; } -(void)viewdidload(){ [super viewdidload]; cachedimage = [nsmutabledictionary new]; } /*old code*/ uiimageview *recipeimageview = (uiimageview *)[cell viewwithtag:100]; recipeimageview.image = nil; if ([imagearray count] >0){ //if image downloaded, use , don't download it. if(cachedimage[[imagearray objectatindex:indexpath.row]] != nil){ recipeimageview.image = cachedimage[[imagearray objectatindex:indexpath.row]]; } else{ dispatch_async(dispatch_get_global_queue(dispatch_queue_priority_background, 0), ^(void) { nsdata *data0 = [nsdata datawithcontentsofurl: [nsurl urlwithstring:[imagearray objectatindex:indexpath.row]]]; uiimage *image = [uiimage imagewithdata: data0]; dispatch_sync(dispatch_get_main_queue(), ^(void) { recipeimageview.image = image; //****save downloaded image cachedimage[[imagearray objectatindex:indexpath.row]] = image; //****save downloaded image }); }); } } /*old code*/ 