参考链接

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// NSObject.h
@interface NSObject
...
- (IMP)methodForSelector:(SEL)aSelector;
+ (IMP)instanceMethodForSelector:(SEL)aSelector;
...
@end
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// NSObject.mm
...
+ (IMP)instanceMethodForSelector:(SEL)sel {
    if (!sel) [self doesNotRecognizeSelector:sel];
    return class_getMethodImplementation(self, sel);
}

- (IMP)methodForSelector:(SEL)sel {
    if (!sel) [self doesNotRecognizeSelector:sel];
    return object_getMethodImplementation(self, sel);
}
...
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// objc-class.mm
...
IMP class_getMethodImplementation(Class cls, SEL sel)
{
    IMP imp;

    if (!cls  ||  !sel) return nil;

    imp = lookUpImpOrNil(cls, sel, nil, 
                         YES/*initialize*/, YES/*cache*/, YES/*resolver*/);

    // Translate forwarding function to C-callable external version
    if (!imp) {
        return _objc_msgForward;
    }

    return imp;
}
...