Key-Value Coding Compliant Container Classes

The CAAnimation and CALayer classes are key-value coding compliant container classes, which means that you can set values for arbitrary keys. Even if the key someKey is not a declared property of the CALayer class, you can still set a value for it as follows:

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

You can also retrieve the value for arbitrary keys like you would retrieve the value for other key paths. For example, to retrieve the value of the someKey path set previously, you would use the following code:

someKeyValue=[theLayer valueForKey:@"someKey"];

Default Value Support

Core Animation adds a convention to key value coding whereby a class can provide a default value for a key that has no set value. The CAAnimation and CALayer classes support this convention using the defaultValueForKey: class method.

To provide a default value for a key, create a subclass of the desired class and override its defaultValueForKey: method. Your implementation of this method should examine the key parameter and return the appropriate default value. Listing C-1 shows a sample implementation of the defaultValueForKey: method for a layer object that provides a default value for the masksToBounds property.

Listing C-1  Example implementation of defaultValueForKey:

+ (id)defaultValueForKey:(NSString *)key

{

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

         return [NSNumber numberWithBool:YES];

    return [super defaultValueForKey:key];

}

Wrapping Conventions

When the data for a key consists of a scalar value or C data structure, you must wrap that type in an object before assigning it to the layer. Similarly, when accessing that type, you must retrieve an object and then unwrap the appropriate values using the extensions to the appropriate class. Table C-1 lists the C types commonly used and the Objective-C class you use to wrap them.

Key Path Support for Structures

The CAAnimation and CALayer classes lets you access the fields of selected data structures using key paths. This feature is a convenient way to specify the field of a data structure that you want to animate. You can also use these conventions in conjunction with the setValue:forKeyPath: and valueForKeyPath: methods to set and get those fields.

[myLayer setValue:[NSNumber numberWithFloat:10.0] forKeyPath:@"transform.translation.x"];