Store Singleton Array for Use After App Re-Launches? - ios

Question

I need to share an NSMutabaleArray between multiple ViewControllers, so I created a singleton. But when I re-launch my app, the array seems to clear itself which causes problems within my app. This is my singleton:
.h:
#import <Foundation/Foundation.h>
#interface playlistArray : NSObject {
NSMutableArray *playlistSongsArray;
}
#property (nonatomic, retain) NSMutableArray *playlistSongsArray;
+ (id)sharedArray;
#end
.m
#import "playlistArray.h"
#implementation playlistArray
#synthesize playlistSongsArray;
#pragma mark Singleton Methods
+ (id)sharedArray {
static playlistArray *sharedArray = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedArray = [[self alloc] init];
});
return sharedArray;
}
- (id)init {
if (self = [super init]) {
self.playlistSongsArray = [[NSMutableArray alloc] init];
}
return self;
}
- (void)dealloc {
// Should never be called, but just here for clarity really.
}
#end
I thought about storing it in NSUserDefaults - is this the correct route? If so, how would I do this?
Any help would be much appreciated, thanks!

Answer 1

I am not sure creating a separate singleton class for just passing an array to different view controllers is a great idea from code structure point of view. Much more cleaner approach would be to pass playlistSongsArray directly whenever you create a new instance of a view controller that may need it.
However, if you still want to use singleton in your implementation for some reason, I'd change playlistArray class to something like PlaylistManager (notice that the common convention is to capitalise first letters of each word in class names) — by doing this, you don't constraint yourself with array-only implementation and you can use it to share other playlist information between your view controllers.
As for persistency between app launches, it really depends on what kind of data you store in your array. For example, you can use NSUserDefaults if your array stores relatively small number of NSStrings (or other <NSCoding>-compliant classes). Other most common options are NSKeyed​Archiver and Core Data. You can learn more about data persistency on iOS from Apple documentation or great online tutorials like this one on NSHipster.

Answer 2

I thought about storing it in NSUserDefaults - is this the correct route?
NSUserDefaults is certainly an option you have.
You can add the following two methods into your class and call it when appropriate (i.e., savePlayList when you modify the array;loadPlaylist` in the singleton init method):
- (void)savePlaylist {
NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
[defaults setObject:self.playlistSongsArray forKey:#"playlistSongs"];
[defaults synchronize];
}
- (void)loadPlaylist {
NSUserDefaults* defaults = [NSUserDefaults standardUserDefaults];
self.playlistSongsArray = [[defaults arrayForKey:#"playlistSongs"] mutableCopy];
}
Have a look at this post for a discussion about different approaches you have.

Answer 3

You get singleton array blank when you relaunch the app because all memory allocated to the array is removed. These singleton classes are allocated once during the lifecycle of the app.
So the best answer to fetch the data every time you open the app is to save the data into the disk. To do so there are few ways
Database using Sqlite
plist
NSUSerDefaults
You can go for userdefaults or plist if you want to save the app.
Below is the example of saving the array into userdefault
Saving the array into userdefaults:
// Get the standardUserDefaults object, store your UITableView data array against a key, synchronize the defaults
NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
[userDefaults setObject:array forKey:#"singletonArray"];
[userDefaults synchronize];
Retrieving the array:
NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
NSArray *singletonArray = [userDefaults objectForKey:#"singletonArray"];
Hope it will help you. Happy Coding :)