runtime.h 
/** 
 * Returns a specified instance method for a given class.
 * 
 * @param cls The class you want to inspect.
 * @param name The selector of the method you want to retrieve.
 * 
 * @return The method that corresponds to the implementation of the selector specified by 
 *  \e name for the class specified by \e cls, or \c NULL if the specified class or its 
 *  superclasses do not contain an instance method with the specified selector.
 *
 * @note This function searches superclasses for implementations, whereas \c class_copyMethodList does not.
 */
OBJC_EXPORT Method _Nullable
class_getInstanceMethod(Class _Nullable cls, SEL _Nonnull name)
    OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0);

/***********************************************************************
* class_getInstanceMethod.  Return the instance method for the
* specified class and selector.
**********************************************************************/
Method class_getInstanceMethod(Class cls, SEL sel)
{
    if (!cls  ||  !sel) return nil;
    // This deliberately avoids +initialize because it historically did so.
    // This implementation is a bit weird because it's the only place that 
    // wants a Method instead of an IMP.
    Method meth;
    meth = _cache_getMethod(cls, sel, _objc_msgForward_impcache);
    if (meth == (Method)1) {
        // Cache contains forward:: . Stop searching.
        return nil;
    } else if (meth) {
        return meth;
    }
    // Search method lists, try method resolver, etc.
    lookUpImpOrForward(nil, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER);
    meth = _cache_getMethod(cls, sel, _objc_msgForward_impcache);
    if (meth == (Method)1) {
        // Cache contains forward:: . Stop searching.
        return nil;
    } else if (meth) {
        return meth;
    }
    return _class_getMethod(cls, sel);
}