Internal Storage with NSTimer

ISTimerStorage.h

#import <Foundation/Foundation.h>

@interface ISTimerStorage : NSObject

+ (ISTimerStorage *)sharedStorage;

- (id)valueForKeyInInternalStorage:(NSString *)key;
- (void)setValue:(id)value forKeyInInternalStorage:(NSString *)key;

@end

ISTimerStorage.m

#import "ISTimerStorage.h"

@interface ISTimerStorage () {
    NSMutableDictionary *_internalStorage;
    NSString *_filePath;
}

@property (nonatomic, getter=isDirty) BOOL dirty;

@end

@implementation ISTimerStorage

+ (ISTimerStorage *)sharedStorage {
    static dispatch_once_t onceToken;
    static ISTimerStorage *sharedStorage = nil;
    dispatch_once(&onceToken, ^{
        sharedStorage = [[self alloc] init];
    });
    return sharedStorage;
}

- (id)init {
    if (self = [super init]) {
        // File Path
        NSString *docDirectory = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
        _filePath = [docDirectory stringByAppendingPathComponent:@"internal_timer.plist"];
        
        // Create pool
        _internalStorage = [NSMutableDictionary dictionaryWithContentsOfFile:_filePath];
        if (!_internalStorage) {
            // No file. Create new one
            _internalStorage = [NSMutableDictionary dictionary];
        }
        _dirty = NO;
        
        // ************************************************************** //
        // Timer
        [NSTimer scheduledTimerWithTimeInterval:5.0f target:self selector:@selector(save) userInfo:nil repeats:YES];
        // ************************************************************** //
    }
    return self;
}

#pragma mark - Timer

- (void)save {
    if (self.dirty) {
        // Save in background
        dispatch_queue_t backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
        dispatch_async(backgroundQueue, ^{
            [_internalStorage writeToFile:_filePath atomically:YES];
            self.dirty = NO;
        });
    }
}

#pragma mark - Interface

- (id)valueForKeyInInternalStorage:(NSString *)key {
    return _internalStorage[key];
}

- (void)setValue:(id)value forKeyInInternalStorage:(NSString *)key {
    if (!value) return;
    _internalStorage[key] = value;
    self.dirty = YES;
}

@end

When using this implementation

If I change the content of internal storage frequently (about 100 times) in 20 seconds
===> It only does Disk I/O 4 times.

If I leave my app running in 1 minutes without changing the content of internal storage
===> The code for writing disk is called every 5 seconds. So it runs 20 times additional.

If my app crashes after changing the content of internal storage in "5" seconds
===> The change is not saved and lost.