iphone - how to access any method only once while launching the app for first time -

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i have web service fetch data in json form. calling service everytime application launching (inside appdidfinishlaunching method) , storing in local file. want call service first time app launching , next time wanted use file in have stored json data. 

nsarray *paths = nssearchpathfordirectoriesindomains(nsdocumentdirectory, nsuserdomainmask, yes);     nsstring *documentsdirectory = [paths objectatindex:0];      //live json data url      nsurl *url = [nsurl urlwithstring:@"my url"];     nsdata *urldata = [nsdata datawithcontentsofurl:url];      //attempt download live data     if (urldata)     {         nsstring *filepath = [nsstring stringwithformat:@"%@/%@", documentsdirectory,@"data.json"];         [urldata writetofile:filepath atomically:yes];      }     //copy data initial package applications documents folder     else     {         //file write         nsstring *filepath = [nsstring stringwithformat:@"%@/%@", documentsdirectory,@"data.json"];          //file copy         nsstring *json = [ [nsbundle mainbundle] pathforresource:@"data" oftype:@"json" indirectory:@"html/data" ];         nsdata *jsondata = [nsdata datawithcontentsoffile:json options:kniloptions error:nil];          //write file device         [jsondata writetofile:filepath atomically:yes];     }      nserror *jsonerror = nil;      nsstring *jsonfilepath = [nsstring stringwithformat:@"%@/%@", documentsdirectory,@"data.json"];     nsdata *jsondata = [nsdata datawithcontentsoffile:jsonfilepath options:kniloptions error:&jsonerror ];     nsmutablearray *jsonarray = [nsjsonserialization jsonobjectwithdata:jsondata options:nsjsonreadingmutablecontainers error:&jsonerror];     self.serverresponsearray = jsonarray;     nslog(@"%@",self.serverresponsearray);

by code writing json response local file , storing in array use in whole app.

   static nsstring * const somekey = @"havechecked"; - (void)applicationdidfinishlaunching:(nsnotification *)anotification {      nsuserdefaults * stduserdefaults = [nsuserdefaults standarduserdefaults];     if(![[stduserdefaults objectforkey:somekey] boolvalue])     {         [self trysomething];     } } -(void)gotjson {     //async callback     nsuserdefaults * stduserdefaults = [nsuserdefaults standarduserdefaults];     [stduserdefaults setbool:yes forkey:somekey];     [stduserdefaults synchronize];  }

it sounds need learn dealing files, here stack overflow reading:

what documents directory (nsdocumentdirectory)?
write file on ios


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