On Dec 19, 2008, at 6:53 PM, Graham Cox wrote:
In my app, I have an interface and code that allows the user to set
up different values that can be set as properties of an object. The
user is presented with a list of properties that can be set
(suitably converted for display in the UI) and then they can set up
the values of these properties, which can be one of five distinct
types (a custom type, an image, colour, string or numeric value).
I'd like to be able to gently prevent the user from making an
inappropriate choice. For example if the given property expects a
string, they should not be able to choose an image object.
First of all, are you familiar with Key-Value Validation?
http://developer.apple.com/DOCUMENTATION/Cocoa/Conceptual/KeyValueCoding/Concepts/Validation.html
So I'm looking for a way to safely and reliably determine whether a
property will accept an object of a given type. I wrote this
category on NSObject as a simple wrapper on the runtime to attempt
this, but it doesn't work quite how I expected - any property that
returns an object simply returns '@' as its return type - I can't
tell what class it is. Can anyone point me in the right direction?
I don't believe this is possible. The "@" you're getting is the type
encoding for an object pointer.
http://developer.apple.com/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/chapter_7_section_1.html
Note how it says "whether statically typed or typed id".
Basically, in Objective-C static typing is purely a compile-time
thing. The compiler can use it to check the semantics of your
program. However, it's "lost" after that. It's not baked into the
resulting executable. Therefore, the Objective-C runtime can't access
it. It just isn't there.
I think your only avenue is to implement explicit KVV methods for your
properties which have the necessary knowledge to test their type. I
suppose you could file a feature request to extend declared properties
so they can synthesize a type-checking validation method. Or request
a compile-time directive to determine the static type of a method
return or a property. (Hmm. gcc has a "typeof" extension. I wonder
if it works in Objective-C.)
Good luck,
Ken
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