Re: performSelector:withObject fails with class object
Re: performSelector:withObject fails with class object
- Subject: Re: performSelector:withObject fails with class object
- From: Tron Thomas <email@hidden>
- Date: Sat, 07 Feb 2009 08:44:30 -0800
Greg Parker wrote:
On Feb 5, 2009, at 9:50 PM, Tron Thomas wrote:
I have a couple of classes that are delcared like this:
#import <objc/objc.h>
#import <objc/Object.h>
@interface SomeClass : Object
{
@private
// Instance data members ...
}
+ (SomeClass*)instanceFromData:(id)data;
// Other methods ...
@end
Class Object is dead. Don't use it. Use NSObject instead.
In yet another class, I have this function:
+ (id)createInstanceForClass:
(const char*)className
withData:(id)data
{
id metaClass = ::objc_getMetaClass(className);
if(nil == metaClass){
return nil;
}
unsigned int count;
Method* method = ::class_copyMethodList(metaClass, &count);
for(unsigned int index = 0; index < count; ++index){
std::clog << ::sel_getName(::method_getName(method[index])) <<
std::endl;
}
return [metaClass performSelector:@selector(instanceFromData:)
withObject:data];
}
When the middle section of the function containing the
class_copyMethodList is simply meant for verification, and prints:
instanceFromData
when the class name for either of the first two classes is provided.
This seems to indicate the proper class object is being used and the
expectation is that the call to performSelector:withObject: should
succeed.
Your introspection of the metaclass's method list shows what messages
the class object will respond to. But then you send instanceFromData:
to the metaclass object, not the class object. The metaclass doesn't
respond to instanceFromData:, so it blows up.
(Instances respond to messages from their class's method list. Classes
respond to messages from their metaclass's method list. Metaclasses
respond to messages from the metaclass's metaclass's method list,
which is the root metaclass's method list. If you don't understand
that third sentence, don't worry; in practice you only care about the
first two.)
If you want to call the class method +[SomeClass instanceFromData:],
do this instead:
const char *className = "SomeClass";
id cls = ::objc_getClass(className);
return [cls performSelector:@selector(instanceFromData:)
withObject:data];
Or you can skip performSelector:withData: entirely. It's intended for
code where the selector is not known at compile time. Since you know
the selector, just use it directly:
const char *className = "SomeClass";
id cls = ::objc_getClass(className);
return [cls instanceFromData:data];
However, when the performSelector:withObject: is executed, output
like the following is logged, and the program traps in the debugger:
*** NSInvocation: warning: object 0x1b0dc of class 'Object' does not
implement methodSignatureForSelector: -- trouble ahead
*** NSInvocation: warning: object 0x1b0dc of class 'Object' does not
implement doesNotRecognizeSelector: -- abort
These ugly errors arise because you're using class Object instead of
class NSObject. Class Object is dead.
I thought about trying NSObject after I posted the mailing list. The
main reason I went with Object originally was because I just wanted to
write something quick and simple. I did not want to worry about linking
to the entire Cocoa framework and have to create an auto-release pool.
However, I just configured everything to use garbage collection and that
helped to keep things simple enough to still use NSObject.
I originally was trying to use objc_getClass instead of
objc_getMetaClass. It did not seem to be working. Maybe this was
because I was hitting the NSInvocation warnings which would have
happened anyway, since I was using Object instead of NSObject.
Anyway, I switched to using NSClassFromString as suggested by Ken and
everything is working now.
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