运行时中函数调用黑魔法swizzle，交换两个方法或者改变一个方法的实现

类方法的实现的改变

+ (IMP)swizzleClassSelector:(SEL)origSelector
withIMP:(IMP)newIMP
{
Class class = [self class];
Method origMethod = class_getClassMethod(class,
origSelector);
IMP origIMP = method_getImplementation(origMethod);
if(!class_addMethod(self, origSelector, newIMP,
method_getTypeEncoding(origMethod)))
{
method_setImplementation(origMethod, newIMP);
}

return origIMP;
}

实例方法的实现的改变

+ (IMP)swizzleSelector:(SEL)origSelector
withIMP:(IMP)newIMP {
Class class = [self class];
Method origMethod = class_getInstanceMethod(class,
origSelector);
IMP origIMP = method_getImplementation(origMethod);
if(!class_addMethod(self, origSelector, newIMP,
method_getTypeEncoding(origMethod)))
{
method_setImplementation(origMethod, newIMP);
}

return origIMP;
}