1. Multi-level key-value observing allows objects to observe changes to properties across multiple levels of nested objects. 2. To implement multi-level KVO, the model must support dynamic property access using valueForUndefinedKey to traverse nested object hierarchies. 3. Observers must add and remove observation for each nested level, and handle change notifications separately for each level.

1. Multi-Level KVO
Jakub Hladík

2. Jakub Hladík
jakub@hippotaps.com
@ku33ing, @hippotaps
iOS app developer
hippotaps co-founder
iOS teacher at FIT CTU Prague

9. Key-Value Observing
What?
• Key-value observing is a mechanism that
allows objects to be notified of changes to
specified properties of other objects.
• alternatives you should already know:
• delegate/protocol
• NSNotification
• callback using @selector or block
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10. Key-Value Coding First
• mechanism that allows accessing object
properties indirectly by a name/key (string)
• NSKeyValueCoding protocol
• NSObject provides default implementation
(getters/setters, …)
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11. KVC How?
- (id)tableView:(NSTableView *)tableview
objectValueForTableColumn:(id)column row:(NSInteger)row
{
ChildObject *child = [childrenArray objectAtIndex:row];
if ([[column identifier] isEqualToString:@"name"]) {
return [child name];
}
if ([[column identifier] isEqualToString:@"age"]) {
return [child age];
}
if ([[column identifier] isEqualToString:@"favoriteColor"]) {
return [child favoriteColor];
}
}
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12. KVC How?
- (id)tableView:(NSTableView *)tableview
objectValueForTableColumn:(id)column row:(NSInteger)row
{
ChildObject *child = [childrenArray objectAtIndex:row];
return [child valueForKey:[column identifier]];
}
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13. KVC Terminology
• properties:
• attributes (scalar, string, NSNumber, …)
• to-one relationships (self.otherObject, self.superView, …)
• to-many relationships (collection of objects – NSArray,
NSSet, …)
• key (@”age”) – identifies object property
• path (@”superView.frame”, @”address.street”)
– dot.separated keys specifying a sequence of objects to
traverse
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14. Getting/Setting
Attributes
– valueForKey:
– valueForKeyPath:
– dictionaryWithValuesForKeys:
– valueForUndefinedKey:
…
– setValue:forKeyPath:
– setValuesForKeysWithDictionary:
– setNilValueForKey:
– setValue:forKey:
– setValue:forUndefinedKey:
…
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15. Getting/Setting
Attributes
@interface MyClass
@property NSString *stringProperty;
@property NSInteger integerProperty;
@property MyClass *linkedInstance;
@end
MyClass *myInstance = [[MyClass alloc] init];
NSString *string = [myInstance valueForKey:@"stringProperty"];
[myInstance setValue:@2 forKey:@"integerProperty"];
MyClass *anotherInstance = [[MyClass alloc] init];
myInstance.linkedInstance = anotherInstance;
[myInstance setValue:@2 forKeyPath:@"linkedInstance.integerProperty"];
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16. Finally KVO
– observeValueForKeyPath:ofObject:change:context:
– addObserver:forKeyPath:options:context:
– removeObserver:forKeyPath:
– removeObserver:forKeyPath:context:
…
– willChangeValueForKey:
– didChangeValueForKey:
– willChange:valuesAtIndexes:forKey:
– didChange:valuesAtIndexes:forKey:
…
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17. KVO
17

18. KVO
18

19. Simple Observing
[self setValue:[[self randomPhoneArray] mutableCopy]
forKey:@"dataArray"];
// self.dataArray = [[self randomPhoneArray] mutableCopy];
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
if ([keyPath isEqualToString:@"dataArray"]) {
self.dataArray = change[NSKeyValueChangeNewKey];
[self.tableView reloadData];
}
}
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20. Demo
Simple Observing
HPT-Multi-Level-KVO
branch stage1
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21. NSMutableArray
Observing
- (void)insertDataObject:(id)object
atIndex:(NSUInteger)index
{
[self willChange:NSKeyValueChangeInsertion
valuesAtIndexes:[NSIndexSet indexSetWithIndex:index]
forKey:@"dataArray"];
[self.dataArray insertObject:object
atIndex:index];
[self didChange:NSKeyValueChangeInsertion
valuesAtIndexes:[NSIndexSet indexSetWithIndex:index]
forKey:@"dataArray"];
}
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22. NSMutableArray
Observing
- (void)removeDataObjectAtIndex:(NSUInteger)index
{
[self willChange:NSKeyValueChangeRemoval
valuesAtIndexes:[NSIndexSet indexSetWithIndex:index]
forKey:@"dataArray"];
[self.dataArray removeObjectAtIndex:index];
[self didChange:NSKeyValueChangeRemoval
valuesAtIndexes:[NSIndexSet indexSetWithIndex:index]
forKey:@"dataArray"];
}
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23. NSMutableArray
Observing
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context
{
if ([keyPath isEqualToString:@"dataArray"]) {
NSIndexSet *set = change[NSKeyValueChangeIndexesKey];
NSKeyValueChange valueChange = [change[NSKeyValueChangeKindKey] unsignedIntegerValue];
NSArray *new = change[NSKeyValueChangeNewKey];
switch (valueChange) {
case NSKeyValueChangeInsertion:
[self addObject:new.lastObject atIndex:set.lastIndex];
break;
case NSKeyValueChangeRemoval:
[self removeObjectAtIndex:set.lastIndex];
break;
case NSKeyValueChangeSetting:
self.dataArray = [new mutableCopy];
[self.tableView reloadData];
break;
default:
break;
}
}
}
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24. Demo
NSMutableArray Change
Observing
HPT-Multi-Level-KVO
branch stage2
24

25. Array Observing
Simpler
- (void)insertDataObject:(id)object
atIndex:(NSUInteger)index
{
NSMutableArray *array = [self
mutableArrayValueForKey:@"dataArray"];
[array insertObject:object atIndex:index];
}
- (void)removeDataObjectAtIndex:(NSUInteger)index
{
NSMutableArray *array = [self
mutableArrayValueForKey:@"dataArray"];
[array removeObjectAtIndex:index];
}
25

26. Demo
NSMutableArray Change
Observing Done Better
HPT-Multi-Level-KVO
branch stage3
26

27. Nested Object
Observing
• NSMutableArray in NSMutableArray ):
• Not possible without ugly hacks…
• NSNotifications… (:
[[NSNotificationCenter defaultCenter]
postNotificationName:@"objectInsertedAtIndexPath"
object:self
userInfo:@{ @"object" : object,
@"indexPath" : indexPath
}];
27

28. Multi-Level
Demo
Nested NSMutableArray Change
Observing using NSNotification
HPT-Multi-Level-KVO
branch stage4
28

29. Is That It?
29

30. No Way!
30
- (id)valueForUndefinedKey:(NSString *)key
{
NSUInteger i = [key integerValue];
return self.dataArray[i];
}

31. Model Change
- (id)valueForUndefinedKey:(NSString *)key
{
NSUInteger i = [key integerValue];
return self.dataArray[i];
}
- (void)insertDataObject:(id)object atIndexPath:(NSIndexPath *)indexPath
{
if (indexPath.section == self.dataArray.count) {
[self insertDataObject:[NSMutableArray array] atIndex:indexPath.section];
}
[self willChange:NSKeyValueChangeInsertion
valuesAtIndexes:[NSIndexSet indexSetWithIndex:indexPath.row]
forKey:[@(indexPath.section) description]];
NSMutableArray *array = self.dataArray[indexPath.section];
[array insertObject:object atIndex:indexPath.row];
[self didChange:NSKeyValueChangeInsertion
valuesAtIndexes:[NSIndexSet indexSetWithIndex:indexPath.row]
forKey:[@(indexPath.section) description]];
}
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32. Observer Change 1
- (void)addObserverForKey:(NSString *)key
{
[[HPTDataService sharedService] addObserver:self
forKeyPath:key
options:NSKeyValueObservingOptionNew
context:NULL];
}
- (void)removeObserverForLastKey
{
[[HPTDataService sharedService] removeObserver:self
forKeyPath:[@(self.dataArray.count-1) description]
context:NULL];
}
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33. Observer Change 2
- (void)addObject:(id)object atIndex:(NSUInteger)index
{
[self.dataArray insertObject:object atIndex:index];
[self addObserverForKey:[@(self.dataArray.count-1) description]];
[self.tableView beginUpdates];
[self.tableView insertSections:[NSIndexSet indexSetWithIndex:index]
withRowAnimation:UITableViewRowAnimationRight];
[self.tableView endUpdates];
}
33

34. Final Working
Multi-Level
KVO Demo
YEAH!
HPT-Multi-Level-KVO
branch stage5
34

35. The End
• Key-Value Observing Programming Guide
• Key-Value Coding Programming Guide
• https://github.com/kubbing/HPT-Multi-Level-KVO
• http://www.slideshare.net/JakubHladk/for-mobile-513
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