Method Swizzling in Objective-C

There are many reasons for you to use method swizzling, even if it's considered harmful and dangerous.

Despite it's flaws, this little technique can be so powerful and let you reveal secrets of iOS like status bar is drawn by the current app, and replace methods in class you don't own like customizing UINavigationBar background in old days to achieve a higher level of UI customisation.

So here's the base snippet:

/*
 * This file is part of the http://ioscodesnippet.com
 * http://ioscodesnippet.com/2013/02/17/method-swizzling-in-objective-c/
 * (c) Jamz Tang <jamz@jamztang.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

void SwizzleInstanceMethod(Class c, SEL orig, SEL new);
void SwizzleClassMethod(Class c, SEL orig, SEL new);

/*
 * This file is part of the http://ioscodesnippet.com
 * http://ioscodesnippet.com/2013/02/17/method-swizzling-in-objective-c/
 * (c) Jamz Tang <jamz@jamztang.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

#import <objc/runtime.h>

void SwizzleInstanceMethod(Class c, SEL orig, SEL new)
{
    Method origMethod = class_getInstanceMethod(c, orig);
    Method newMethod = class_getInstanceMethod(c, new);
    if(class_addMethod(c, orig, method_getImplementation(newMethod), method_getTypeEncoding(newMethod)))
        class_replaceMethod(c, new, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    else
        method_exchangeImplementations(origMethod, newMethod);
}

void SwizzleClassMethod(Class c, SEL orig, SEL new) {

    Method origMethod = class_getClassMethod(c, orig);
    Method newMethod = class_getClassMethod(c, new);

    c = object_getClass((id)c);

    if(class_addMethod(c, orig, method_getImplementation(newMethod), method_getTypeEncoding(newMethod)))
        class_replaceMethod(c, new, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    else
        method_exchangeImplementations(origMethod, newMethod);
}

Example usage:

@implementation NSParagraphStyle (JTParagraphStyleDebug)

+ (void)load {
    SwizzleInstanceMethod([self class], @selector(initWithCoder:), @selector(initWithCoderSwizzled:));
}

// -[NSParagraphStyle initWithCoder:] is called when an NSAttributedString is specified in an UILabel in Interface Builder.
- (id)initWithCoderSwizzled:(NSCoder *)aDecoder {
    self = [self initWithCoderSwizzled:(id)[[UINibDecoderProxy alloc] initWithTarget:aDecoder]];
    return self;
}

@end

Above code is extracted from JTAttributedLabel to use UINibDecoderProxy to diagnose NSAttributedString in Interface Builder.

Enjoy and have a good use with it!

Installation

Using CocoaPods: [?]
pod 'MethodSwizzle', '~> 0.0.1'

Clone this repository:
git clone git://gist.github.com/4970366.git MethodSwizzle

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