Internal Storage with KVO

ISStorage.h

#import <Foundation/Foundation.h>

@interface ISStorage : NSObject

+ (ISStorage *)sharedStorage;

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

@end

ISStorage.m

#import "ISStorage.h"

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

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

@end

@implementation ISStorage

+ (ISStorage *)sharedStorage {
    static dispatch_once_t onceToken;
    static ISStorage *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.plist"];
        
        // Create pool
        _internalStorage = [NSMutableDictionary dictionaryWithContentsOfFile:_filePath];
        if (!_internalStorage) {
            // No file. Create new one
            _internalStorage = [NSMutableDictionary dictionary];
        }
        _dirty = NO;
        
        // ************************************************************** //
        // KVO
        [self addObserver:self forKeyPath:@"dirty" options:NSKeyValueObservingOptionNew context:NULL];
        // ************************************************************** //
    }
    return self;
}

- (void)dealloc {
    [self removeObserver:self forKeyPath:@"dirty"];
}

#pragma mark - KVO

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {
    if ([keyPath isEqualToString:@"dirty"]) {
        BOOL newDirtyValue = [change[NSKeyValueChangeNewKey] boolValue];
        if (newDirtyValue) {
            // Update after 5 sec
            int64_t delayInSeconds = 5.0f;
            dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
            // In background thread
            dispatch_queue_t globalBackgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
            // Go!
            dispatch_after(popTime, globalBackgroundQueue, ^(void){
                [_internalStorage writeToFile:_filePath atomically:YES];
                _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

1. If I change the content of internal storage frequently (about 100 times) in 20 seconds
   ===> It only does Disk I/O 4 times.
2. If I leave my app running in 1 minutes without changing the content of internal storage
   ===> The code for writing disk is never called.
3. If my app crashes after changing the content of internal storage in "5" seconds
   ===> The change is not saved and lost.