JRSwizzle.m

// JRSwizzle.m semver:1.0
//   Copyright (c) 2007-2011 Jonathan 'Wolf' Rentzsch: http://rentzsch.com
//   Some rights reserved: http://opensource.org/licenses/MIT
//   https://github.com/rentzsch/jrswizzle

#import "JRSwizzle.h"

#if TARGET_OS_IPHONE
    #import <objc/runtime.h>
    #import <objc/message.h>
#else
    #import <objc/objc-class.h>
#endif

#define SetNSErrorFor(FUNC, ERROR_VAR, FORMAT,...)  \
    if (ERROR_VAR) {    \
        NSString *errStr = [NSString stringWithFormat:@"%s: " FORMAT,FUNC,##__VA_ARGS__]; \
        *ERROR_VAR = [NSError errorWithDomain:@"NSCocoaErrorDomain" \
                                         code:-1    \
                                     userInfo:[NSDictionary dictionaryWithObject:errStr forKey:NSLocalizedDescriptionKey]]; \
    }
#define SetNSError(ERROR_VAR, FORMAT,...) SetNSErrorFor(__func__, ERROR_VAR, FORMAT, ##__VA_ARGS__)

#if OBJC_API_VERSION >= 2
#define GetClass(obj)   object_getClass(obj)
#else
#define GetClass(obj)   (obj ? obj->isa : Nil)
#endif

@implementation NSObject (JRSwizzle)

+ (BOOL)jr_swizzleMethod:(SEL)origSel_ withMethod:(SEL)altSel_ error:(NSError**)error_ {
#if OBJC_API_VERSION >= 2
    Method origMethod = class_getInstanceMethod(self, origSel_);
    if (!origMethod) {
#if TARGET_OS_IPHONE
        SetNSError(error_, @"original method %@ not found for class %@", NSStringFromSelector(origSel_), [self class]);
#else
        SetNSError(error_, @"original method %@ not found for class %@", NSStringFromSelector(origSel_), [self className]);
#endif
        return NO;
    }
    
    Method altMethod = class_getInstanceMethod(self, altSel_);
    if (!altMethod) {
#if TARGET_OS_IPHONE
        SetNSError(error_, @"alternate method %@ not found for class %@", NSStringFromSelector(altSel_), [self class]);
#else
        SetNSError(error_, @"alternate method %@ not found for class %@", NSStringFromSelector(altSel_), [self className]);
#endif
        return NO;
    }
    
    class_addMethod(self,
                    origSel_,
                    class_getMethodImplementation(self, origSel_),
                    method_getTypeEncoding(origMethod));
    class_addMethod(self,
                    altSel_,
                    class_getMethodImplementation(self, altSel_),
                    method_getTypeEncoding(altMethod));
    
    method_exchangeImplementations(class_getInstanceMethod(self, origSel_), class_getInstanceMethod(self, altSel_));
    return YES;
#else
    //  Scan for non-inherited methods.
    Method directOriginalMethod = NULL, directAlternateMethod = NULL;
    
    void *iterator = NULL;
    struct objc_method_list *mlist = class_nextMethodList(self, &iterator);
    while (mlist) {
        int method_index = 0;
        for (; method_index < mlist->method_count; method_index++) {
            if (mlist->method_list[method_index].method_name == origSel_) {
                assert(!directOriginalMethod);
                directOriginalMethod = &mlist->method_list[method_index];
            }
            if (mlist->method_list[method_index].method_name == altSel_) {
                assert(!directAlternateMethod);
                directAlternateMethod = &mlist->method_list[method_index];
            }
        }
        mlist = class_nextMethodList(self, &iterator);
    }
    
    //  If either method is inherited, copy it up to the target class to make it non-inherited.
    if (!directOriginalMethod || !directAlternateMethod) {
        Method inheritedOriginalMethod = NULL, inheritedAlternateMethod = NULL;
        if (!directOriginalMethod) {
            inheritedOriginalMethod = class_getInstanceMethod(self, origSel_);
            if (!inheritedOriginalMethod) {
                SetNSError(error_, @"original method %@ not found for class %@", NSStringFromSelector(origSel_), [self className]);
                return NO;
            }
        }
        if (!directAlternateMethod) {
            inheritedAlternateMethod = class_getInstanceMethod(self, altSel_);
            if (!inheritedAlternateMethod) {
                SetNSError(error_, @"alternate method %@ not found for class %@", NSStringFromSelector(altSel_), [self className]);
                return NO;
            }
        }
        
        int hoisted_method_count = !directOriginalMethod && !directAlternateMethod ? 2 : 1;
        struct objc_method_list *hoisted_method_list = malloc(sizeof(struct objc_method_list) + (sizeof(struct objc_method)*(hoisted_method_count-1)));
        hoisted_method_list->obsolete = NULL;   // soothe valgrind - apparently ObjC runtime accesses this value and it shows as uninitialized in valgrind
        hoisted_method_list->method_count = hoisted_method_count;
        Method hoisted_method = hoisted_method_list->method_list;
        
        if (!directOriginalMethod) {
            bcopy(inheritedOriginalMethod, hoisted_method, sizeof(struct objc_method));
            directOriginalMethod = hoisted_method++;
        }
        if (!directAlternateMethod) {
            bcopy(inheritedAlternateMethod, hoisted_method, sizeof(struct objc_method));
            directAlternateMethod = hoisted_method;
        }
        class_addMethods(self, hoisted_method_list);
    }
    
    //  Swizzle.
    IMP temp = directOriginalMethod->method_imp;
    directOriginalMethod->method_imp = directAlternateMethod->method_imp;
    directAlternateMethod->method_imp = temp;
    
    return YES;
#endif
}

+ (BOOL)jr_swizzleClassMethod:(SEL)origSel_ withClassMethod:(SEL)altSel_ error:(NSError**)error_ {
    return [GetClass((id)self) jr_swizzleMethod:origSel_ withMethod:altSel_ error:error_];
}

@end