objective-c - respondsToSelector 和 class_getInstanceMethod

Question

在内部，respondsToSelector 是否不仅仅是找出 class_getInstanceMethod 是否返回非零值？

respondsToSelector 本质上是 class_getInstanceMethod 的包装器吗？像这样:

- (BOOL)respondsToSelector:(SEL)sel {
  return class_getInstanceMethod(self, sel) != nil;
}

Answer 1

在苹果开源NSObject implementation , respondsToSelector: 在第 1472 行，看起来像这样:

- (BOOL)respondsToSelector:(SEL)sel { 
    if (!sel) return NO; 
    return class_respondsToSelector([self class], sel); 
}

class_respondsToSelector() 依次位于 objc-class.mm 的第 729 行:

BOOL class_respondsToSelector(Class cls, SEL sel)
{
    IMP imp;

    if (!sel  ||  !cls) return NO;

    // Avoids +initialize because it historically did so.
    // We're not returning a callable IMP anyway.
    imp = lookUpMethod(cls, sel, NO/*initialize*/, YES/*cache*/, nil);
    return (imp != (IMP)_objc_msgForward_internal) ? YES : NO;
}