Question

In Objective-C, is there a way to set a default hander to avoid unrecognizedselector exception ? I want to make the NSNULL and NSNumber to response all the methods in NSString.

Thanks!

Answer 1

To handle the “unrecognized selector” exception, we should override two methods:

- (void)forwardInvocation:(NSInvocation *)anInvocation;
- (NSMethodSignature*)methodSignatureForSelector:(SEL)selector;

In this case, if we want NSNull to perform the NSSString method if “unrecognized selector” exception occurred, we should do this:

@interface NSNull (InternalNullExtention)
@end



@implementation NSNull (InternalNullExtention)

- (NSMethodSignature*)methodSignatureForSelector:(SEL)selector
{
    NSMethodSignature* signature = [super methodSignatureForSelector:selector];
    if (!signature) {
        signature = [@"" methodSignatureForSelector:selector];
    }
    return signature;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    SEL aSelector = [anInvocation selector];

    if ([@"" respondsToSelector:aSelector])
        [anInvocation invokeWithTarget:@""];
    else
        [self doesNotRecognizeSelector:aSelector];
}
@end