uiscrollview - iOS Delay even when using dispatch -


i have wallpaper app has paged scrollview on main view. each page of scrollview displays 9 images. when view scrolled i'm loading next 10 pages of images , set previous 10 pages uiimages i've loaded nil prevent memory warnings.

the problem when view scrolls , following method of scrollview gets called, there few seconds delay before view can scroll though have put block code loads new images in dispatch_async. , when comment out whole section of code dispatch stuff, there no delay.

if has idea on why happens. please please let me know.

thank much

- (void)scrollviewdidscroll:(uiscrollview *)scrollv { float fractionalpage = scrollview.contentoffset.x / self.view.frame.size.width;  if (curpagenum != lround(fractionalpage)) {      curpagenum = lround(fractionalpage);              //display page number     pagenumlabel.text = [nsstring stringwithformat:@"%d",curpagenum+1];     pagenumlabel.alpha = 1.0;     [nstimer scheduledtimerwithtimeinterval:1.0 target:self selector:@selector(dismisspagenum) userinfo:nil repeats:no];       //loading more pages 9 image in each     lastloadedpagenum = min(lastloadedpagenum + 10, numpages - 1);      dispatch_queue_t getimages = dispatch_queue_create("getting images", nil);     dispatch_async(getimages, ^{          (int = curpagenum + 4 ; <= lastloadedpagenum ; i++) {              int numpicsperpage = 9;             if (picsnames.count%9 && == numpages-1) {                  numpicsperpage = picsnames.count%9;             }              (int j = 0 ; j < numpicsperpage ; j++) {                  uiimage *image = [brain imagewith:[picsnames objectatindex:(i*9) + j]];                  dispatch_async(dispatch_get_main_queue(), ^{                      //loading image imageview                     uiimageview *imageview = (uiimageview *)[scrollview viewwithtag:image_views_tag + (i*9) + j];                     imageview.image = image;                 });             }         }     });      //setting old imageview images nil release memory     int oldfirstloadedpagenum = firtloadedpagenum;     firtloadedpagenum = max(curpagenum - 4, 0);     (int = oldfirstloadedpagenum ; < firtloadedpagenum ; i++) {          int numpicsperpage = 9;         if (picsnames.count%9 && == numpages-1) {              numpicsperpage = picsnames.count%9;         }          (int j = 0 ; j < numpicsperpage ; j++) {              uiimageview *imageview = (uiimageview *)[scrollview viewwithtag:image_views_tag + (i*9) + j];             imageview.image = nil;             [((uiactivityindicatorview *)[imageview viewwithtag:activity_indicator_tag]) startanimating];         }                 } } }

brain method imagewith: 

-(uiimage *)imagewith:(nsstring *)imagename { nsstring *imagepath = [[nssearchpathfordirectoriesindomains(nsdocumentdirectory, nsuserdomainmask, yes) objectatindex:0] stringbyappendingpathcomponent:imagename]; uiimage *image = [uiimage imagewithcontentsoffile:imagepath];  if (!image && [self connected]) {      image = [uiimage imagewithdata:[nsdata datawithcontentsofurl:[nsurl urlwithstring:[nsstring stringwithformat:@"%@/%@", picsurl, imagename]]]];      if (image) {          [uiimagepngrepresentation(image) writetofile:imagepath atomically:yes];     }  }  return image; }

clearly code looping causes delay. think since dispatch inside loop gets called after bit of iterations there no real gain in using multi threading here.


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