ios - SWIFT 2 - UICollectionView - slow scrolling -

i have setup uicollectionview in project data json file. works however, scrolling slow , when view scrolling coming cell, few moments shows content of cell before.

i have tried using dispatch_async still slow , jumpy.

any idea doing wrong?

override func collectionview(collectionview: uicollectionview, cellforitematindexpath indexpath: nsindexpath) -> uicollectionviewcell {          let videocell = collectionview.dequeuereusablecellwithreuseidentifier("videocell", forindexpath: indexpath) uicollectionviewcell         let communityviewcontroller = storyboard?.instantiateviewcontrollerwithidentifier("community_id")          videocell.frame.size.width = (communityviewcontroller?.view.frame.size.width)!         videocell.center.x = (communityviewcontroller?.view.center.x)!          videocell.layer.bordercolor = uicolor.lightgraycolor().cgcolor         videocell.layer.borderwidth = 2           let fileurl = nsurl(string:self.uservideosinfo[indexpath.row][2])         let asset = avasset(url: fileurl!)         let assetimggenerate = avassetimagegenerator(asset: asset)         assetimggenerate.appliespreferredtracktransform = true         let time = cmtimemake(asset.duration.value / 3, asset.duration.timescale)          dispatch_async(dispatch_get_global_queue(dispatch_queue_priority_default, 0)) {              //self.showindicator()              let namelabelstring = self.uservideosinfo[indexpath.row][0]             let commentlabelstring = self.uservideosinfo[indexpath.row][1]             let datelabelstring = self.uservideosinfo[indexpath.row][3]              let buttonplayuservideo = videocell.viewwithtag(1) as! uibutton             let namelabel = videocell.viewwithtag(2) as! uilabel             let commentuservideo = videocell.viewwithtag(3) as! uilabel             let dateuservideo = videocell.viewwithtag(4) as! uilabel             let thumbuservideo = videocell.viewwithtag(5) as! uiimageview             let deleteuservideo = videocell.viewwithtag(6) as! uibutton              buttonplayuservideo.layer.setvalue(indexpath.row, forkey: "indexplaybtn")             deleteuservideo.layer.setvalue(indexpath.row, forkey: "indexdeletebtn")              dispatch_async(dispatch_get_main_queue()) {                    namelabel.text = namelabelstring                 commentuservideo.text = commentlabelstring                 dateuservideo.text = datelabelstring                  self.shadowtext(namelabel)                 self.shadowtext(commentuservideo)                 self.shadowtext(dateuservideo)                  if let cgimage = try? assetimggenerate.copycgimageattime(time, actualtime: nil) {                     thumbuservideo.image = uiimage(cgimage: cgimage)                 }               }          }          //this important         videocell.layer.shouldrasterize = true         videocell.layer.rasterizationscale = uiscreen.mainscreen().scale           return videocell     } 

at first - working ui objects global queue , seems without purpose. forbidden - or behavior undefined.

secondary, heavy operation creation of thumbnail perform on main queue. consider using of avassetimagegenerator's method

public func generatecgimagesasynchronouslyfortimes(requestedtimes: [nsvalue], completionhandler handler: avassetimagegeneratorcompletionhandler) 

instead of own asyncs.

at third, viewwithtag pretty heavy operation causing enumeration on subviews. consider declare properties in cell views need.

upd: declare properties in cell, create subclass of uicollectionviewcell appropriate properties iboutlets. then, in view controller viewdidload implementation, call

collecionview.registerclass(<yourcellsubclass>.dynamictype,  forcellwithreuseidentifier:"videocell")  

or, if collection view cell configured in storyboard, specify class of cell , connect subviews class' outlets directly in cell's settings window in interface builder.

at fourth, cells being reused collection view. each time cell going out of visible area, removed collection view , put reuse queue. when scroll cell, view controller asked again provide cell. , you're fetching thumbnail video again each newly appeared cell. consider caching of fetched thumbnails storing them in array collectionview's indexpath.item index.