PropertyKit provides tools for working with Objective-C Declared Properties.

Copyright © 2011, Jon Parise.

Simply include these source files in your project:

• PropertyKit.h
• PropertyKit.mm

The repository also includes a SenTestingKit-compatible unit test:

• PropertyKitTests.h
• PropertyKitTests.m

Objective-C properties have a name and a set of attributes. The Objective-C runtime represents these attributes as a property type string. Accessing individual attributes involves parsing this custom string format. PropertyKit handles this parsing for you and provides two convenient ways to access property attributes.

The C API wraps the low-level parser:

objc_property_t property = class_getProperty([UIDevice class], "name");
PKPropertyAttributes attributes = PKPropertyAttributesMake(property);
NSLog(@"Property %s is %s", property_getName(property), attributes.isReadOnly ? "readonly" : "readwrite");

And the higher-level PKProperty class provides more convenient access:

PKProperty *property = [PKProperty propertyWithName:@"name" forClass:[UIDevice class]];
NSLog(@"Property %@ is %@", property.name, property.isReadOnly ? @"readonly" : @"readwrite");

PropertyKit provides a mechanism for observing changes to property values. This is similar to Key-Value Observing but trades features for speed. It works by replacing synthesized property setters with custom implementations that call an object-level notification selector when a property is changed.

Objects can only observe their own properties; objects cannot directly observe the properties of other objects.

Observed properties need to be registered:

+ (void)initialize
{
    [self addObservedProperty:@"hidden"];
}

The object will then be notified of changes to observed properties:

- (void)observeValueForProperty:(NSString *)name value:(id)value
{
    NSLog(@"Property %@ has a new value: %@", name, value);
}

• Consider sending both the old and new values to the observer.
• Support structs (e.g. CGRect). This will require wrapping the new value in an NSValue box so that it could be passed back to the observer.
• Support atomic (synchronized) setter operations.
• Emit KVO notifications from our custom setter implementations.
• Offer the option of using method swizzling to call the original setter instead of completely replacing the setter implementation. This will provide greater end-user flexibility at the expense of some speed.