Objective-C block based Key-Value Observing

DNKVObservation.h

// *********************************************************************************************************************
//   Module Name:  DNKVObservation.h
//   Description:  Include file.
//   Author Name:  David F. Negrete, 2013
// *********************************************************************************************************************


#pragma mark - Type Definitions
@class DNKVObservation;
typedef void (^DNObservationBlock) (id observer, DNKVObservation *observation);
typedef NSNumber DNKVObservationID;


#pragma mark -
/** Class represents a Key Value Observation between two objects.
 *
 * This object encapsulates the key value observation details, such as the observed and observer objects, the key path
 * being observed and the observed values.
 *
 * In addition, it privately stores any data required by the observation, such as whether it's currently skipping
 * observations, and the block to execute.
 */
@interface DNKVObservation : NSObject


#pragma mark - Properties
/// @name Properties
/// ----------------

/// The object taking responsibility for the observation.
@property (readonly, weak, nonatomic) id observer;

/// The object being observed.
@property (readonly, weak, nonatomic) id target;

/// The key being observed.
@property (readonly, copy, nonatomic) NSString *key;

/// An identifier object.
@property (readonly, strong, nonatomic) DNKVObservationID *idNumber;


#pragma mark - Class Methods
/// @name Creation
/// --------------

/** Sets up the observation between two objects and executes a block of code when a key is updated.
 * @param observer The object setting up the observation.
 * @param target The object to observe.
 * @param keyPath The key that will be monitored.
 * @param block A block to execute when an observation notification is fired.
 */
+ (DNKVObservation*) observationByObject:(id)observer
                                  target:(id)target
                                     key:(NSString*)keyPath
                                  action:(DNObservationBlock)block;


#pragma mark - Interface
/// @name Observation Change
/// ------------------------

/** Convenience method to get the new value of a key from the NSKeyValueObserving change dictionary.
 * @return The new value assigned to the observed key.
 */
- (id) newValue;

/** Convenience method to get the old value of a key from the NSKeyValueObserving change dictionary.
 * @return The previous value assigned to the observed key.
 */
- (id) oldValue;

/** Convenience method to get the change type of a key from the NSKeyValueObserving change dictionary.
 * @return An enum value of the change type.
 */
- (NSKeyValueChange) changeType;

/** Convenience method to get the index set of a key from the NSKeyValueObserving change dictionary.
 * @return The index set containing the location of updated objects within a collection.
 */
- (NSIndexSet*) changeIndexes;


@end

DNKVObservation.m

// *********************************************************************************************************************
//   Module Name:  DNKVObservation.m
//   Description:  Object representation of an observation KVC/KVO.
//   Author Name:  David F. Negrete, 2013
// *********************************************************************************************************************


#import "DNKVObservation.h"

#pragma mark Imports
#import "NSObject+DNKeyValueObserving.h"


@interface DNKVObservation ()
@property (copy, nonatomic) DNObservationBlock actionBlock;
@property (weak, nonatomic) NSDictionary *observationChange;
@property (nonatomic, getter=isObserving) BOOL observing;
@property (nonatomic, getter=isSuspended) BOOL suspended;
@end


@interface NSObject (KVOHelper)
- (DNKVObservation*) registeredObservationForTarget:(id)target key:(NSString*)keyPath;
@end


static int DNKVObservationCurrentIdentifier = 1000;


#pragma mark -
@implementation DNKVObservation


#pragma mark - Class Lifecycle
// *********************************************************************************************************************
// + observationByObject: target: key: action:
// *********************************************************************************************************************
+ (DNKVObservation*) observationByObject:(id)observer
                                  target:(id)target
                                     key:(NSString*)keyPath
                                  action:(DNObservationBlock)block
{
  DNKVObservation *observation = [[self alloc] initWithObject:observer
                                                       target:target
                                                          key:keyPath
                                                       action:block];
  
  [observation startObservation];
  
  return observation;
}


#pragma mark - Lifecycle
// *********************************************************************************************************************
// - initWithObject: target: key: action:
// *********************************************************************************************************************
- (id) initWithObject:(id)observer target:(id)target key:(NSString*)keyPath action:(DNObservationBlock)block
{
  self = [super init];
  if (self) {
    _observer = observer;
    _target = target;
    _key = [keyPath copy];
    _actionBlock = [block copy];
    _observing = NO;
    _suspended = NO;
    _idNumber = @(DNKVObservationCurrentIdentifier++);
  }
  
  return self;
}

// *********************************************************************************************************************
// - dealloc
// *********************************************************************************************************************
- (void) dealloc
{
  [self stopObservation];
}


#pragma mark - Configuration
// *********************************************************************************************************************
// - observingOptions
// *********************************************************************************************************************
- (NSKeyValueObservingOptions) observingOptions
{
  return NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew;
}

// *********************************************************************************************************************
// - observingContext
// *********************************************************************************************************************
- (void*) observingContext
{
  return &_idNumber;
}


#pragma mark - Interface
// *********************************************************************************************************************
// - newValue
// *********************************************************************************************************************
- (id) newValue
{
  return [self observationChange][NSKeyValueChangeNewKey];
}

// *********************************************************************************************************************
// - oldValue
// *********************************************************************************************************************
- (id) oldValue
{
  return [self observationChange][NSKeyValueChangeOldKey];
}

// *********************************************************************************************************************
// - changeIndexes
// *********************************************************************************************************************
- (NSIndexSet*) changeIndexes
{
  return [self observationChange][NSKeyValueChangeIndexesKey];
}

// *********************************************************************************************************************
// - changeType
// *********************************************************************************************************************
- (NSKeyValueChange) changeType
{
  return [[self observationChange][NSKeyValueChangeKindKey] integerValue];
}


#pragma mark - Key Value Observing
// *********************************************************************************************************************
// - observeValueForKeyPath: ofObject: change: context:
// *********************************************************************************************************************
- (void) observeValueForKeyPath:(NSString*)keyPath
                       ofObject:(id)object
                         change:(NSDictionary*)change
                        context:(void*)context
{
  if (self.isSuspended) {
    return;
  }
  
  if (context != [self observingContext]) {
    DNLog(@"Observation for unknown context, skipping action");
    return;
  }
  
  if (![self.target isEqual:object] || ![self.key isEqual:keyPath]) {
    DNLog(@"Observation for unknown object or key path, skipping action");
    return;
  }
  
  if (!self.observer) {
    DNLog(@"Observer has been deallocated, skipping action");
    return;
  }

  if (! [[change valueForKey:NSKeyValueChangeNewKey] isEqual:[change valueForKey:NSKeyValueChangeOldKey]]) {
    if (self.actionBlock) {
      self.observationChange = change;
      self.actionBlock( self.observer, self );
    }
    else {
      DNLog(@"WARNING: No action block specified when observing %@ key %@",object, keyPath);
    }
  }
}

// *********************************************************************************************************************
// - startObservation
// *********************************************************************************************************************
- (void) startObservation
{
  if (!self.isObserving) {
    [self.target addObserver:self forKeyPath:self.key options:[self observingOptions] context:[self observingContext]];
    self.observing = YES;
  }
}

// *********************************************************************************************************************
// - stopObservation
// *********************************************************************************************************************
- (void) stopObservation
{
  if (self.isObserving) {
    [self.target removeObserver:self forKeyPath:self.key context:[self observingContext]];
    self.observing = NO;
  }
}

// *********************************************************************************************************************
// - suspendObservation
// *********************************************************************************************************************
- (void) suspendObservation
{
  self.suspended = YES;
}

// *********************************************************************************************************************
// - resumeObservation
// *********************************************************************************************************************
- (void) resumeObservation
{
  self.suspended = NO;
}


@end