Method swizzling implementation (see http://defagos.github.io/yet_another_article_about_method_swizzling/ for details)

example_block_swizzling.m

@implementation UILabel (SwizzlingExamples)

+ (void)load
{
    SwizzleSelectorWithBlock_Begin(self, @selector(initWithFrame:))
    ^(UILabel *self, CGRect frame) {
        if ((self = ((id (*)(id, SEL, CGRect))_imp)(self, _cmd, frame))) {
            // ...
        }
        return self;
    }
    SwizzleSelectorWithBlock_End;
    
    SwizzleSelectorWithBlock_Begin(self, @selector(awakeFromNib))
    ^(UILabel *self) {
        ((void (*)(id, SEL))_imp)(self, _cmd);
        
        // ...
    }
    SwizzleSelectorWithBlock_End;

    SwizzleSelectorWithBlock_Begin(self, sel_getUid("dealloc"))
    ^(__unsafe_unretained UILabel *self) {
        // ...
        
        ((void (*)(id, SEL))_imp)(self, _cmd);
    }
    SwizzleSelectorWithBlock_End;
}

@end

example_imp_swizzling.m

static id (*initWithFrame)(id, SEL, CGRect) = NULL;
static void (*awakeFromNib)(id, SEL) = NULL;
static void (*dealloc)(__unsafe_unretained id, SEL) = NULL;

static id swizzle_initWithFrame(UILabel *self, SEL _cmd, CGRect frame)
{
    if ((self = initWithFrame(self, _cmd, frame))) {
        // ...
    }
    return self;
}

static void swizzle_awakeFromNib(UILabel *self, SEL _cmd)
{
    awakeFromNib(self, _cmd);
    
    // ...
}

static void swizzle_dealloc(__unsafe_unretained UILabel *self, SEL _cmd)
{
    // ...
    
    dealloc(self, _cmd);
}

@implementation UILabel (SwizzlingExamples)

+ (void)load
{
    SwizzleSelector(self, @selector(initWithFrame:), swizzle_initWithFrame, &initWithFrame);
    SwizzleSelector(self, @selector(awakeFromNib), &swizzle_awakeFromNib, &awakeFromNib);
    SwizzleSelector(self, sel_getUid("dealloc"), &swizzle_dealloc, &dealloc);
}

@end

swizzling.m

#import <objc/runtime.h>
#import <objc/message.h>

#define SwizzleSelector(clazz, selector, newImplementation, pPreviousImplementation) \
    (*pPreviousImplementation) = (__typeof((*pPreviousImplementation)))class_swizzleSelector((clazz), (selector), (IMP)(newImplementation))

#define SwizzleClassSelector(clazz, selector, newImplementation, pPreviousImplementation) \
    (*pPreviousImplementation) = (__typeof((*pPreviousImplementation)))class_swizzleClassSelector((clazz), (selector), (IMP)(newImplementation))

#define SwizzleSelectorWithBlock_Begin(clazz, selector) { \
    SEL _cmd = selector; \
    __block IMP _imp = class_swizzleSelectorWithBlock((clazz), (selector),
#define SwizzleSelectorWithBlock_End );}

#define SwizzleClassSelectorWithBlock_Begin(clazz, selector) { \
    SEL _cmd = selector; \
    __block IMP _imp = class_swizzleClassSelectorWithBlock((clazz), (selector),
#define SwizzleClassSelectorWithBlock_End );}

IMP class_swizzleSelector(Class clazz, SEL selector, IMP newImplementation)
{
    // If the method does not exist for this class, do nothing
    Method method = class_getInstanceMethod(clazz, selector);
    if (! method) {
        // Cannot swizzle methods which are not implemented by the class or one of its parents
        return NULL;
    }
    
    // Make sure the class implements the method. If this is not the case, inject an implementation, only calling 'super'
    const char *types = method_getTypeEncoding(method);
    
#if !defined(__arm64__)
    NSUInteger returnSize = 0;
    NSGetSizeAndAlignment(types, &returnSize, NULL);
    
    // Large structs on 32-bit architectures
    if (sizeof(void *) == 4 && types[0] == _C_STRUCT_B && returnSize != 1 && returnSize != 2 && returnSize != 4 && returnSize != 8) {
        class_addMethod(clazz, selector, imp_implementationWithBlock(^(__unsafe_unretained id self, va_list argp) {
            struct objc_super super = {
                .receiver = self,
                .super_class = class_getSuperclass(clazz)
            };
            
            // Sufficiently large struct
            typedef struct LargeStruct_ {
                char dummy[16];
            } LargeStruct;
            
            // Cast the call to objc_msgSendSuper_stret appropriately
            LargeStruct (*objc_msgSendSuper_stret_typed)(struct objc_super *, SEL, va_list) = (void *)&objc_msgSendSuper_stret;
            return objc_msgSendSuper_stret_typed(&super, selector, argp);
        }), types);
    }
    // All other cases
    else {
#endif
        class_addMethod(clazz, selector, imp_implementationWithBlock(^(__unsafe_unretained id self, va_list argp) {
            struct objc_super super = {
                .receiver = self,
                .super_class = class_getSuperclass(clazz)
            };
            
            // Cast the call to objc_msgSendSuper appropriately
            id (*objc_msgSendSuper_typed)(struct objc_super *, SEL, va_list) = (void *)&objc_msgSendSuper;
            return objc_msgSendSuper_typed(&super, selector, argp);
        }), types);
#if !defined(__arm64__)
    }
#endif
    
    // Swizzling
    return class_replaceMethod(clazz, selector, newImplementation, types);
}

IMP class_swizzleClassSelector(Class clazz, SEL selector, IMP newImplementation)
{
    return class_swizzleSelector(object_getClass(clazz), selector, newImplementation);
}

IMP class_swizzleSelectorWithBlock(Class clazz, SEL selector, id newImplementationBlock)
{
    IMP newImplementation = imp_implementationWithBlock(newImplementationBlock);
    return class_swizzleSelector(clazz, selector, newImplementation);
}

IMP class_swizzleClassSelectorWithBlock(Class clazz, SEL selector, id newImplementationBlock)
{
    IMP newImplementation = imp_implementationWithBlock(newImplementationBlock);
    return class_swizzleClassSelector(clazz, selector, newImplementation);
}

How does having va_list as an argument instead of ... work? And how does calling objc_msgSendSuper with a va_list as the last argument expand into the full arguments within? Doesn't that just pass the va_list as an argument to the final Objective-C function?