What is the difference when swizzling between these 2 mechanisms - ios

Question

There is a slight variation between these 2 ways of swizzling. I just want clarification if there is something fundamentally different or wrong between them
Assuming we are swizzling viewDidLoad on UIView
First way (using class_addMethod):
#implementation UIView (SwizzleFirstWay)
+ (void)load {
SEL originalSelector = #selector(viewDidLoad);
SEL swizzledSelector = #selector(swizzled_viewDidLoad);
Method originalMethod = class_getInstanceMethod(self, originalSelector);
Method swizzledMethod = class_getInstanceMethod(self, swizzledSelector);
class_addMethod(self,
originalSelector,
class_getMethodImplementation(self, originalSelector),
method_getTypeEncoding(originalMethod));
// Adding the method
class_addMethod(self,
swizzledSelector,
class_getMethodImplementation(self, swizzledSelector),
method_getTypeEncoding(swizzledMethod));
method_exchangeImplementations(originalMethod, swizzledMethod);
}
+ (void)swizzled_viewDidLoad {
// ... the swizzled implementation
// ...
// ...
[self swizzled_viewDidLoad]; // calling back to the original implementation
}
#end
Second way (without using class_addMethod):
+ (void)load {
SEL originalSelector = #selector(viewDidLoad);
SEL swizzledSelector = #selector(swizzled_viewDidLoad);
Method originalMethod = class_getInstanceMethod(self, originalSelector);
Method swizzledMethod = class_getInstanceMethod(self, swizzledSelector);
// NOT USING class_addMethod
method_exchangeImplementations(originalMethod, swizzledMethod);
}
+ (void)swizzled_viewDidLoad {
// ... the swizzled implementation
// ...
// ...
[self swizzled_viewDidLoad]; // calling back to the original implementation
}
#end

Answer 1

Very good question, actually. If you use method_exchangeImplementations without calling class_addMethod first, you can accidentally swizzle your superclass implementation. The reason for that is class_getInstanceMethod searches superclasses for inherited implementation in a case when a method is not implemented in the class itself. And it is, obviously, not something you want to achieve.
A quick example. Using following code
#interface UIWebView (SwizzlingTest)
- (void)swizzled_removeFromSuperview {
[self swizzled_removeFromSuperview];
}
+ (void)load {
SEL originalSelector = #selector(removeFromSuperview);
SEL swizzledSelector = #selector(swizzled_removeFromSuperview);
Method originalMethod = class_getInstanceMethod(self, originalSelector);
Method swizzledMethod = class_getInstanceMethod(self, swizzledSelector);
method_exchangeImplementations(originalMethod, swizzledMethod);
}
#end
Results in
-[UICheckeredPatternView swizzled_removeFromSuperview]: unrecognized selector sent to instance 0x101b24250
because removeFromSuperview is inherited from UIView, but not implemented in UIWebView.