iphone - Displaying the time duration the app was in background -

i new ios programming, , trying make simple app know more lifecycle , different states. app store current timestamp when app running, , whenever goes background , comes up, show message "good see after xx minutes".

now, approach this, @ beginning there timestamp stored in variable, , when viewdidload() method called, check timestamp current timestamp , display subtracted value.

when app going background, change value of local timestamp variable in viewdidunload() method , when coming compare timestamps , display message.

any pointers regarding correct approach of doing this?

use below code in appdelegate.m file

put below method

-(void)mincalculation_backgroundtime:(nsstring *)backgroundtime forgroundtime:(nsstring *)foregroundtime {     nsdateformatter *dateformat = [[nsdateformatter alloc]init];     [dateformat setdateformat:@"mm/dd/yyyy hh:mm:ss"];      nsdate *lastdate = [dateformat datefromstring:foregroundtime];     nsdate *todaysdate = [dateformat datefromstring:backgroundtime];     nstimeinterval lastdiff = [lastdate timeintervalsincenow];     nstimeinterval todaysdiff = [todaysdate timeintervalsincenow];     nstimeinterval datediff = lastdiff - todaysdiff;     int min = datediff/60;     nslog(@"good see after %i minutes",min); } 

and replace below 2 methods

- (void)applicationdidenterbackground:(uiapplication *)application {     nsdateformatter *dateformat = [[nsdateformatter alloc]init];     [dateformat setdateformat:@"mm/dd/yyyy hh:mm:ss"];     nsstring *backgroundtime = [dateformat stringfromdate:[nsdate date]];     [[nsuserdefaults standarduserdefaults]setvalue:backgroundtime forkey:@"backgroundtime"];     // use method release shared resources, save user data, invalidate timers, , store enough application state information restore application current state in case terminated later.      // if application supports background execution, method called instead of applicationwillterminate: when user quits. }  - (void)applicationwillenterforeground:(uiapplication *)application {     nsdateformatter *dateformat = [[nsdateformatter alloc]init];     [dateformat setdateformat:@"mm/dd/yyyy hh:mm:ss"];     nsstring *foregroundtime = [dateformat stringfromdate:[nsdate date]];     nsstring *backgroundtime = [[nsuserdefaults standarduserdefaults]valueforkey:@"backgroundtime"];     [self mincalculation_backgroundtime:backgroundtime forgroundtime:foregroundtime];     // called part of transition background inactive state; here can undo many of changes made on entering background. } 

try problem solve