Observatory

What does Observatory do?

We all love the Objective-C KVO system. However it is quite tedious in it’s usage. It is annoying to setup and if you observe multiple properties on different objectes there will be code duplication all over the place. Observatory aims to fix those annoyances.

Example

Lets say you run a pet store and we want to check the food supply of some animals. The PETSquirrel and PETRabbit. With observatory we can do it with just a few lines of code:

@interface PETStore : NSViewController

// Some models (they should extend OBYModel)
@property (nonatomic, strong) PETRabbit* rabbit;
@property (nonatomic, strong) PETSquirrel* squirrel;

@end

@implementation PETStore

- (void)viewDidLoad
{
    // Observe all properties on the rabbit model
    [self observeModel:self.rabbit];
    
    // Observe only the nuts property on the squirrel model
    [self observeModel:self.squirrel onKeys[@"nuts"]];
}

// Called when the <carrots> property is changed on the <PETRabbit> instance
- (void)rabbit:(PETRabbit *)rabbit valueChangedForCarrots:(NSNumber *)carrots
{
    NSLog(@"Nom, I now have %d carrots!", [carrots intValue]);
}

// Called when the <nuts> property is changed on the <PETSquirrel> instance
- (void)squirrel:(PETSquirrel *)squirrel valueChangedForNuts:(NSNumber *)nuts
{
    NSLog(@"Nom, I now have %d nuts!", [nuts intValue]);
}

@end

So, what happened here? Observatory determines what the name is of the class and property you are observing and will generate a unique selector based of that.

So if we have a model PETRabbit with a property carrots the following selector will be called on the observer when the property carrots changes:

- (void)rabbit:(PETRabbit *)valueChangedForCarrots:(id)carrots;

The pattern here is:

- (void)[className]:(Class *)valueChangedFor[value]:(id)value;

Much easier, is it not?

Installation

Observatory is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod "Observatory"

and run pod install

API

NSObject+OBYObserver

For the observer the following methods are available

Observe a given OBYModel on all it’s keys:

- (void)observeModel:(OBYModel*)model;

Add observation to multiple OBYModels at once:

- (void)observeModels:(NSArray*)models;

Stop observation on all OBYModels:

- (void)removeAsObserverForAllModels;

Remove the observation for a given OBYModel:

- (void)removeAsObserverForModel:(OBYModel *)model;

Usage

First install Observatory as explained in the installation section. Then add

#import <Observatory/NSObject+Observer>

to the objects who will act as the observer. This will make the API listed above available.

The objects you want to observe should extend OBYModel.

Final note: Your classes should have a three letter prefix to determine the correct class name.

More

Observatory “falls back” to more generic selectors if a given selector is not implemented in your observer. If for example you have not implemented the earlier rabbit: valueChangedForCarrots selector it will try the following more generic selector:

- (void)rabbit:(PETRabbit *) valueChangedForKey(NSString *)key;

Where the structure is:

- (void)[model]:(Class *) valueChangedForKey(NSString *)key;

Finally if this selector has not been implemented either it will fall back to the most generic selector:

- (void)model:(OBYModel*)model valueChangedForKey:(NSString*)key;

Thanks

Finally, thanks to Nils Wiersema for working on the original project with me where this idea first saw the light.