On Nov 3, 2008, at 10:53 AM, Gordon Apple wrote:
The following example is one of several I have run into recently.
Inheritance is as follows: CAKeyframeAnimation:
CAPropertyAnimation :
CAAnimation. CAKeyframeAnimation has no explicit initializer or
factory
method. Dudney's book uses [CAKeyframeAnimation animation] to
create an
object. However, CAKeyframeAnimation has no such factory method.
CAAnimation does. Why should I assume that the "animation" factory
method
will return an object of class CAKeyframeAnimation instead of
CAAnimation
when it is not re-defined in CAKeyframeAnimation.h?
The docs say that CAKeyframeAnimation shoud be created with the
inherited (from CAPropertyAnimation) animationWithKeyPath factory
method.
That raises the same issue.
Is there a basic assumption that such factory methods are
implicitly
redefined for all subclasses?
In Objective-C, classes are objects with inheritance. What you'll get
back from a class convenience method is going to depend on how its
implemented (and not necessarily whether it is redefined by a subclass.)
Consider
@interface Person : NSObject
+ (id)person
@end
@interface Child : Base
@end
Case #1
@implementation Person
+ (id)person
{
return [[[self alloc] init] autorelease];
}
@end
Case #2
@implementation Person
+ (id)person
{
return [[[Person alloc] init] autorelease];
}
@end
In case #1, [Child person] will return a Child instance to us.
In case #2, [Child person] will return a Person instance to us.
I think the CoreAnimation authors intended for case #1, but that's not
what the documentation says.
Creates and returns a new CAAnimation instance.
+ (id)animation
Return Value
An CAAnimation object whose input values are initialized.
I'd recommend filing a bug against the documentation for clarification/
correction.
Jim
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