ios - Crash and Received memory warning -

i have below loop download files server ipad 3 device. running hundred of files, got error , app crash. console shown me "received memory warning. same logic running on ipad air passed. can adivse how resolve problem.

ipad 3 -> ios 9.3.5 ipad air -> ios 10.3.3

func download() {         (index, subjson): (string, json) in serverjson! {              (_, subjson): (string, json) in subjson {                     let filepath = subjson["path"].stringvalue                     let nupdated = subjson["updated"].stringvalue                      if let oupdated = localjson?[index].array?.filter({ $0["path"].string == filepath}).first?["updated"].stringvalue {                         if (oupdated == nupdated)                         {                             dispatchqueue.main.async { () -> void in                                 self.progressview.progress = float(self.count) / float(self.totalcount)                             }                              count += 1                              continue                         }                     }                       var abspath = filepath                      let stridx = abspath.index(abspath.startindex, offsetby: 2)                       if (abspath.hasprefix("./"))                     {                         abspath = abspath.substring(from: stridx)                     }                       let sourceurl = url(string: self.sourceurl.appending(abspath))                       {                         let filedata = try nsdata(contentsof: sourceurl!, options: nsdata.readingoptions())                         try writefiletolocal(abspath, filedata)                     } catch {                         print(error)                     }                      dispatchqueue.main.async { () -> void in                         self.progressview.progress = float(self.count) / float(self.totalcount)                     }                      //print("path: \(abspath)")                     //print("%: \(float(count) / float(totalcount))")                      count += 1                  }          }           {             // remove temp json file first if exists.             if filemanager.fileexists(atpath: oldmanifestpath) {                 try? filemanager.removeitem(atpath: oldmanifestpath)             }              // write temp json file local.             try data?.write(to: oldmanifesturl)              self.defaults.set(hashes, forkey: "lasthash")         } catch {             print(error)         }          dispatchqueue.main.async {             self.progressview.ishidden = true             self.changeviewproperties(2)         }      }  } private func writefiletolocal(_ url:string, _ data:nsdata) throws {          let url = url(string: url)         let path = url?.deletinglastpathcomponent().relativepath         let file = url?.lastpathcomponent          let documentspath = nsurl(fileurlwithpath: nssearchpathfordirectoriesindomains(.documentdirectory, .userdomainmask, true)[0])         let filepath = documentspath.appendingpathcomponent(path!)               var isdir: objcbool = false             if !filemanager.default.fileexists(atpath: (filepath?.path)!, isdirectory:&isdir) {                 try filemanager.default.createdirectory(atpath: filepath!.path, withintermediatedirectories: true, attributes: nil)             }             filemanager.default.changecurrentdirectorypath((filepath?.path)!)             try data.write(tofile: file!, options: .atomicwrite)             print("update: \(filepath!), \(file!)")          filemanager.default.changecurrentdirectorypath(documentspath.path!)     } 

then call function "download" in dispatchqueue.

dispatchqueue.global().async {             self.downloadfiles() }