How to add observer on NSMutableArray?

Question

I have searched a lot but didn't find useful code or tutorial.

In my application, I have an mutable array which update in every 60 seconds.

The objects in array is being displayed by table view in multiple view controllers.

I want to reload table view automatically when only when values in array changes or updated.

For this, I want to add observer on mutable array i.e when values in array changes then it should call a particular method for e.g

-(void)ArrayUpdatedNotification:(NSMutableArray*)array
{
    //Reload table or do something
} 

Thanks in advance.

Answer 1

You can abstract the array into a data container class with accessor methods, and then use key-value observing to observe when the array that backs the container object is changed (you cannot use KVO on an NSArray directly).

A simple example of a class used as an abstraction on top of an array follows. You use its insertObject:inDataAtIndex: and removeObjectFromDataAtIndex: methods instead of directly accessing the with addObject: and removeObject:.

// DataContainer.h
@interface DataContainer : NSObject

// Convenience accessor
- (NSArray *)currentData;

// For KVC compliance, publicly declared for readability
- (void)insertObject:(id)object inDataAtIndex:(NSUInteger)index;
- (void)removeObjectFromDataAtIndex:(NSUInteger)index;
- (id)objectInDataAtIndex:(NSUInteger)index;
- (NSArray *)dataAtIndexes:(NSIndexSet *)indexes;
- (NSUInteger)countOfData;

@end

// DataContainer.m

@interface DataContainer ()

@property (nonatomic, strong) NSMutableArray *data;

@end

@implementation DataContainer

//  We'll use automatic notifications for this example
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
    if ([key isEqualToString:@"data"]) {
        return YES;
    }
    return [super automaticallyNotifiesObserversForKey:key];
}

- (id)init
{
    self = [super init];
    if (self) {
        // This is the ivar which provides storage
        _data = [NSMutableArray array];
    }
    return self;
}

//  Just a convenience method
- (NSArray *)currentData
{
    return [self dataAtIndexes:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, [self countOfData])]];
}

//  These methods enable KVC compliance
- (void)insertObject:(id)object inDataAtIndex:(NSUInteger)index
{
    self.data[index] = object;
}

- (void)removeObjectFromDataAtIndex:(NSUInteger)index
{
    [self.data removeObjectAtIndex:index];
}

- (id)objectInDataAtIndex:(NSUInteger)index
{
    return self.data[index];
}

- (NSArray *)dataAtIndexes:(NSIndexSet *)indexes
{
    return [self.data objectsAtIndexes:indexes];
}

- (NSUInteger)countOfData
{
    return [self.data count];
}

@end

The reason that we do this is so we can now observe changes made to the underlying array. This is done through Key Value Observing. A simple view controller that instantiates and observes a data controller is shown:

// ViewController.h
@interface ViewController : UIViewController

@end

// ViewController.m

@interface ViewController ()

@property (nonatomic,strong) DataContainer *dataContainer;

@end

@implementation ViewController

static char MyObservationContext;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        //  Instantiate a DataContainer and store it in our property
        _dataContainer = [[DataContainer alloc] init];
        //  Add self as an observer. The context is used to verify that code from this class (and not its superclass) started observing.
        [_dataContainer addObserver:self
                         forKeyPath:@"data"
                            options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew)
                            context:&MyObservationContext];
    }

    return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    //  Check if our class, rather than superclass or someone else, added as observer
    if (context == &MyObservationContext) {
        //  Check that the key path is what we want
        if ([keyPath isEqualToString:@"data"]) {
            //  Verify we're observing the correct object
            if (object == self.dataContainer) {
                NSLog(@"KVO for our container property, change dictionary is %@", change);
            }
        }
    }
    else {
        //  Otherwise, call up to superclass implementation
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    //  Insert and remove some objects. Console messages should be logged.
    [self.dataContainer insertObject:[NSObject new] inDataAtIndex:0];
    [self.dataContainer insertObject:[NSObject new] inDataAtIndex:1];
    [self.dataContainer removeObjectFromDataAtIndex:0];
}

- (void)dealloc
{
    [_dataContainer removeObserver:self forKeyPath:@"data" context:&MyObservationContext];
}

@end

When this code runs, three changes to the data are observed by the view controller and logged to the console:

KVO for our container property, change dictionary is {
        indexes = "<NSIndexSet: 0x8557d40>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
        kind = 2;
        new =     (
            "<NSObject: 0x8557d10>"
        );
    }
KVO for our container property, change dictionary is {
        indexes = "<NSIndexSet: 0x715d2b0>[number of indexes: 1 (in 1 ranges), indexes: (1)]";
        kind = 2;
        new =     (
            "<NSObject: 0x71900c0>"
        );
    }
KVO for our container property, change dictionary is {
        indexes = "<NSIndexSet: 0x8557d40>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
        kind = 3;
        old =     (
            "<NSObject: 0x8557d10>"
        );
    }

While this is somewhat complex (and can get much more involved), this is the only way to be notified automatically that a mutable array's contents were changed.