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Core Animation Extensions To Key-Value Coding

The CAAnimation and CALayer classes extend the NSKeyValueCoding protocol adding default values for keys, expanded wrapping conventions, and key path support for CGPoint, CGRect, CGSize, and CATransform3D.

Contents:

Key-Value Coding Compliant Container Classes
Default Value Support
Wrapping Conventions
Key Path Support for Structure Fields

Key-Value Coding Compliant Container Classes

Both CALayer and CAAnimation are key-value coding compliant container classes, allowing you to set values for arbitrary keys. That is, while the key “foo” is not a declared property of the CALayer class, however you can still set a value for the key “foo” as follows:

[theLayer setValue:[NSNumber numberWithInteger:50] forKey:@"foo"];

You retrieve the value for the key “foo” using the following code:

fooValue=[theLayer valueForKey:@"foo"];

Default Value Support

Core Animation adds a new convention to key value coding that allows a class to provide a default value that is used when a class has no value set for that key. Both CALayer or CAAnimation support this convention using the class method defaultValueForKey:.

To provide a default value for a key you create a subclass of the class and override defaultValueForKey:. The subclass implementation examines the key parameter and then returns the appropriate default value. Listing 1 shows an example implementation of defaultValueForKey: that provides a new default value for the layer property masksToBounds.

Listing 1  Example implementation of defaultValueForKey:

+ (id)defaultValueForKey:(NSString *)key

{

    if ([key isEqualToString:@"masksToBounds"])

         return [NSNumber numberWithBool:YES];

    return [super defaultValueForKey:key];

}

Wrapping Conventions

When using the key-value coding methods to access properties whose values are not objects the standard key-value coding wrapping conventions support, the following wrapping conventions are used:

Key Path Support for Structure Fields

CAAnimation provides support for accessing the fields of selected structures using key paths. This is useful for specifying these structure fields as the key paths for animations, as well as setting and getting values using setValue:forKeyPath: and valueForKeyPath:.

CATransform3D exposes the following fields:

CGPoint exposes the following fields:

CGSize exposes the following fields:

CGRect exposes the following fields:

You can not specify a structure field key path using Objective-C 2.0 properties. This will not work:

    myLayer.transform.rotation.x=0;

Instead you must use setValue:forKeyPath: or valueForKeyPath: as shown below:

    [myLayer setValue:[NSNumber numberWithInt:0] forKeyPath:@"transform.rotation.x"];

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