Method swizzling ALLOWS the implementation of an EXISTING selector to be switched at runtime for a different implementation in a classes DISPATCH table. Swizzling allows you to write code that can be executed before and/or after the ORIGINAL method. For example perhaps to track the time method execution took, or to insert log statements

#import "UIViewController+Log.h"
@implementation UIViewController (Log)
+ (void)load {
static dispatch_once_t once_token;
dispatch_once(&once_token, ^{
SEL viewWillAppearSelector = @selector(viewDidAppear:);
SEL viewWillAppearLoggerSelector = @selector(log_viewDidAppear:);
Method originalMethod = class_getInstanceMethod(self, viewWillAppearSelector);
Method extendedMethod = class_getInstanceMethod(self, viewWillAppearLoggerSelector);
method_exchangeImplementations(originalMethod, extendedMethod);
});
}
- (void) log_viewDidAppear:(BOOL)animated {
[self log_viewDidAppear:animated];
NSLog(@"viewDidAppear executed for %@", [self class]);
}
@end

Method swizzling allows the implementation of an existing selector to be switched at runtime for a different implementation in a classes dispatch table. Swizzling allows you to write code that can be executed before and/or after the original method. For example perhaps to track the time method execution took, or to insert log statements

#import "UIViewController+Log.h"
@implementation UIViewController (Log)
+ (void)load {
static dispatch_once_t once_token;
dispatch_once(&once_token, ^{
SEL viewWillAppearSelector = @selector(viewDidAppear:);
SEL viewWillAppearLoggerSelector = @selector(log_viewDidAppear:);
Method originalMethod = class_getInstanceMethod(self, viewWillAppearSelector);
Method extendedMethod = class_getInstanceMethod(self, viewWillAppearLoggerSelector);
method_exchangeImplementations(originalMethod, extendedMethod);
});
}
- (void) log_viewDidAppear:(BOOL)animated {
[self log_viewDidAppear:animated];
NSLog(@"viewDidAppear executed for %@", [self class]);
}
@end