http://blog.yakitara.com/2011/04/aliasmethodchain-for-objective-c.html

NSObject+AliasMethodChain.h

// -*- ObjC -*-
// Hiroshi Saito / Yakitara.com
/*
    You know OCMock does great, but (If I'm not get wrong with it)
    - it will supposed to be used in tests not a production code
    - it can't mock class methods
    - it requires an instace to be mocked accessible in your tests
    
    If you not familiar with alias_method_chain, see:
    https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/module/aliasing.rb
    
    Examples
    --------
    // Example 1 (using category):
    @implementation NSUserDefaults (Log)
    + (void)load {
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        [self aliasClassMethod:@selector(standardUserDefaults) chainingPrefix:@"log"];
        [pool release];
    }
    + (NSUserDefaults *)log_standardUserDefaults {
        NSLog(@"standardUserDefaults swizzled!");
        return objc_msgSend(self, @selector(without_log_standardUserDefaults));
    }
    @end
    
    // Example 2 (using block):
    [self aliasClassMethod:@selector(standardUserDefaults) chainingPrefix:@"log" withBlock:^(id _class) {
        NSLog(@"standardUserDefaults swizzled!");
        return objc_msgSend(_class, @selector(without_log_standardUserDefaults));
    }];
*/
@interface NSObject (AliasMethodChain)
+ (void)aliasClassMethod:(SEL)selector chainingPrefix:(NSString *)prefix;
+ (void)aliasClassMethod:(SEL)selector chainingPrefix:(NSString *)prefix withBlock:(void *)block;
+ (void)revertAliasClassMethod:(SEL)selector chainingPrefix:(NSString *)prefix;

+ (void)aliasInstanceMethod:(SEL)selector chainingPrefix:(NSString *)prefix;
+ (void)aliasInstanceMethod:(SEL)selector chainingPrefix:(NSString *)prefix withBlock:(void *)block;
+ (void)revertAliasInstanceMethod:(SEL)selector chainingPrefix:(NSString *)prefix;
@end

NSObject+AliasMethodChain.m

// Hiroshi Saito / Yakitara.com
#import "NSObject+AliasMethodChain.h"
#import <objc/runtime.h>

static
void aliasMethodChain(Class class, SEL selector, NSString *prefix, IMP newImp) {
    Method method = class_isMetaClass(class) ? class_getClassMethod(class, selector) : class_getInstanceMethod(class, selector);
    const char * types = method_getTypeEncoding(method);
    const char* name = sel_getName(selector);
    
    SEL newImpSelector = sel_registerName([[NSString stringWithFormat:@"%@_%s", prefix, name] UTF8String]);
    if (newImp) {
        class_addMethod(class, newImpSelector, newImp, types);
    } else {
        newImp = class_getMethodImplementation(class, newImpSelector);
    }
    IMP origImp = class_replaceMethod(class, selector, newImp, types);
    SEL origImpSelector = sel_registerName([[NSString stringWithFormat:@"without_%@_%s", prefix, name] UTF8String]);
    class_addMethod(class, origImpSelector, origImp, types);
}

static
void revertAliasMethodChain(Class class, SEL selector, NSString *prefix) {
    Method method = class_isMetaClass(class) ? class_getClassMethod(class, selector) : class_getInstanceMethod(class, selector);
    const char * types = method_getTypeEncoding(method);
    SEL origImpSelector = sel_registerName([[NSString stringWithFormat:@"without_%@_%s", prefix, sel_getName(selector)] UTF8String]);
    IMP origImp = class_getMethodImplementation(class, origImpSelector);
    class_replaceMethod(class, selector, origImp, types);
}

#define _WORKAROUND_FOR_IMP_IMPLEMENTATION_WITH_BLOCK 1
#if _WORKAROUND_FOR_IMP_IMPLEMENTATION_WITH_BLOCK
static
void doWorkaroundFor_imp_implementationWithBlock(void *block) {
    // NOTE: I don't know why, but in a certain condition, imp_implementationWithBlock() can cause a deadlock.
    // NOTE: Copying the block will be a workaround for the issue.
    // NOTE: Following lines are sample backtrace on a deadlock.
    // #00x999430d6 in semaphore_wait_trap ()
    // #10x012749e7 in _rwlock_read_nodebug ()
    // #20x0126f07c in lookUpMethod ()
    // #30x0126f1d6 in _class_lookupMethodAndLoadCache ()
    // #40x012820e3 in objc_msgSend ()
    // #50x0199cbf7 in _Block_object_assign ()
    // #60x04d7d3a2 in __copy_helper_block_7 ()
    // #70x0199c992 in _Block_copy_internal ()
    // #80x0126d221 in imp_implementationWithBlock ()
    [[(id)block copy] autorelease];
}
#else
#define doWorkaroundFor_imp_implementationWithBlock(block)
#endif

@implementation NSObject (AliasMethodChain)
+ (void)aliasClassMethod:(SEL)selector chainingPrefix:(NSString *)prefix {
    aliasMethodChain(object_getClass(self), selector, prefix, nil);
}

+ (void)aliasClassMethod:(SEL)selector chainingPrefix:(NSString *)prefix withBlock:(void *)block {
    doWorkaroundFor_imp_implementationWithBlock(block);
    // CREDIT: http://www.friday.com/bbum/2011/03/17/ios-4-3-imp_implementationwithblock/
    aliasMethodChain(object_getClass(self), selector, prefix, imp_implementationWithBlock(block));
}

+ (void)revertAliasClassMethod:(SEL)selector chainingPrefix:(NSString *)prefix {
    revertAliasMethodChain(object_getClass(self), selector, prefix);
}

+ (void)aliasInstanceMethod:(SEL)selector chainingPrefix:(NSString *)prefix {
    aliasMethodChain([self class], selector, prefix, nil);
}

+ (void)aliasInstanceMethod:(SEL)selector chainingPrefix:(NSString *)prefix withBlock:(void *)block {
    doWorkaroundFor_imp_implementationWithBlock(block);
    // CREDIT: http://www.friday.com/bbum/2011/03/17/ios-4-3-imp_implementationwithblock/
    aliasMethodChain([self class], selector, prefix, imp_implementationWithBlock(block));
}

+ (void)revertAliasInstanceMethod:(SEL)selector chainingPrefix:(NSString *)prefix {
    revertAliasMethodChain([self class], selector, prefix);
}
@end

you don't need to do "[self class]" inside a class method. you can just use "self" because "self" is already the class object, and "[self class]" just returns the same class object