How to save data in iOS

Question

I'm making a game and when I close the app (close at multitask manager), all my data is gone! So, My question is very simple: How do I save the data?

Answer 1

Let's say you want to save score and level, which are both properties of an object called dataHolder.

DataHolder can be created as a singleton, so you don't have to worry too much about from where you access it (its sharedInstance actually):

It's code would look a bit like this:

DataHolder.h

#import <Foundation/Foundation.h>  @interface DataHolder : NSObject   + (DataHolder *)sharedInstance;  @property (assign) int level; @property (assign) int score;  -(void) saveData; -(void) loadData;  @end 

DataHolder.m

NSString * const kLevel = @"kLevel"; NSString * const kScore = @"kScore";  @implementation DataHolder  - (id) init {     self = [super init];     if (self)     {         _level = 0;         _score = 0;     }     return self; }  + (DataHolder *)sharedInstance {     static MDataHolder *_sharedInstance = nil;     static dispatch_once_t onceSecurePredicate;     dispatch_once(&onceSecurePredicate,^                   {                       _sharedInstance = [[self alloc] init];                   });      return _sharedInstance; }  //in this example you are saving data to NSUserDefault's //you could save it also to a file or to some more complex //data structure: depends on what you need, really  -(void)saveData {     [[NSUserDefaults standardUserDefaults]          setObject:[NSNumber numberWithInt:self.score] forKey:kScore];      [[NSUserDefaults standardUserDefaults]          setObject:[NSNumber numberWithInt:self.level] forKey:kLevel];      [[NSUserDefaults standardUserDefaults] synchronize]; }  -(void)loadData {     if ([[NSUserDefaults standardUserDefaults] objectForKey:kScore])     {         self.score = [(NSNumber *)[[NSUserDefaults standardUserDefaults]              objectForKey:kScore] intValue];          self.level = [(NSNumber *)[[NSUserDefaults standardUserDefaults]              objectForKey:kLevel] intValue];     }     else     {         self.level = 0;         self.score = 0;     }  }  @end 

Don't forget to #import "DataHolder.h" where you need it, or simply put it in ...-Prefix.pch.

You could perform actual loading and saving in appDelegate methods:

- (void)applicationDidEnterBackground:(UIApplication *)application {     [[DataHolder sharedInstance] saveData]; }  - (void)applicationWillEnterForeground:(UIApplication *)application {     [[DataHolder sharedInstance] loadData]; } 

You can access your score and level data from anywhere with [DataHolder sharedInstance].score and [DataHolder sharedInstance].level.

This might seem like an overkill for a simple task but it sure helps to keep things tidy and it can help you to avoid keeping all the data in appDelegate (which is usually the quick & dirty path to solution).