KeyValueObservation

Block based key-value observations in Objective-C

Installation

KeyValueObservation is available through CocoaPods and Carthage.

To install via CocoaPods, simply add the following line to your Podfile:

pod 'KeyValueObservation'

To install via Carthage, simply add the following line to your Cartfile:

github "SomeRandomiOSDev/KeyValueObservation"

Usage

First import KeyValueObservation at the top of your Objective-C source file:

@import KeyValueObservation;

Then, adding an observer block is as simple as:

NSString *keyPath = @"Some Key Path";
NSKeyValueObservingOptions options = ...

NSObject *observation = [object observeKeyPath:keyPath 
                                       options:options 
                                 changeHandler:^(id object, SRDKeyValueObservedChange *change) {
    
    NSLog(@"Old value = %@", [change.oldValue description]);
    NSLog(@"New value = %@", [change.newValue description]);
    ...
}];

The Key-Value Observation remains in affect as long as the returned observation object remains alive. Once this object is deallocated the observation block is released and no longer responds to changes. Optionally, you could also call the invalidate method of the returned object to stop observing changes before the object is deallocated.

Like regular Foundation's KVO APIs, NSArray objects aren't directly observable using KVO. However, like the Foundation provided APIs, you can observe the changes for objects within the array. For example:

NSString *keyPath = @"Some Key Path";
NSIndexSet *indexes = ...
NSKeyValueObservingOptions options = ...

NSObject *observation = [array observeKeyPath:keyPath
                          forObjectsAtIndexes:indexes
                                      options:options 
                                changeHandler:^(id object, SRDKeyValueObservedChange *change) {
 
    NSIndexSet *changedIndexes = change.indexes;

    // oldValue and newValue are arrays containing the before and after values of the
    // objects at the `changedIndexes`
    NSLog(@"Old value = %@", [change.oldValue description]);
    NSLog(@"New value = %@", [change.newValue description]);
    
    ...
}];

An additional capability of this library is the ability to easily ignore certain Key-Value Observations for the context of a handler block without any additional overhead. For example:

@implementation FooBar

...

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    if ([keyPath isEqualToString:@"foobar"] && object == self.foobarObject) {
        // Some action. Won't ever be called from the context of -[FooBar someMethod] given 
        // the use of -[NSObject performWhileIgnoringObservations:handler:]
    }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

...

- (void)someMethod {
    [self performWhileIgnoringObservations:@[[SRDKVOInfo infoWithObserved:self.foobarObject keyPath:@"foobar"]] handler:^{
        self.foobarObject.foobar = @"foobar";
    }];
}

@end

In lieu of having to add flags to your object to determine when or when not to ignore particular Key-Value Observations one can simply do it from the context of a handler block. Any observations that match any of the SRDKVOInfo objects passed in to the method will be automatically ignored while executing the block. After the block finishes executing, the observations are once again passed through as normal.

Note that -[NSObject performWhileIgnoringObservations:handler:] uses method implementation swizzling to be able to ignore the specified observations, therefore it is important that you do not do any swizzling for the method -[NSObject observeValueForKeyPath:ofObject:change:context:] for the receiving class from the context of the handler block. Doing so could lead to unexpected results.

Contributing

Whether it's submitting a feature request, reporting a bug, or writing code yourself, all contributions to this library are welcome! Please see CONTRIBUTING for more information on how you can contribute.

Author

Joe Newton, [email protected]

License

KeyValueObservation is available under the MIT license. See the LICENSE file for more info.