What is Swizzling and what are its advantages and disadvantages?

Swizzling is a technique in Objective-C and Swift that allows dynamically changing method implementations at runtime. It is commonly used to replace the default behavior of system classes.

Advantages:

• Enables adding or modifying functionality without changing the source code.
• Convenient for logging, tracking, bug fixing, or adding cross-cutting functions.

Disadvantages:

• Can lead to unpredictable behavior if used carelessly.
• Difficult to debug and maintain.
• Violates encapsulation and may conflict with future SDK updates.

Example of Swizzling in Objective-C:

#import <objc/runtime.h>

@implementation UIViewController (Tracking)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Method original = class_getInstanceMethod(self, @selector(viewWillAppear:));
        Method swizzled = class_getInstanceMethod(self, @selector(xxx_viewWillAppear:));
        method_exchangeImplementations(original, swizzled);
    });
}

- (void)xxx_viewWillAppear:(BOOL)animated {
    [self xxx_viewWillAppear:animated]; // call the original method
    NSLog(@"%@ will appear", NSStringFromClass([self class]));
}

@end

Here, we replace the implementation of viewWillAppear: with our own, adding logging.