+ (Class)class {
    return self;
}

- (Class)class {
    return object_getClass(self);
}

Class object_getClass(id obj)
{
    if (obj) return obj->getIsa();
    else return Nil;
}

inline Class 
objc_object::getIsa() 
{
    if (isTaggedPointer()) {
        uintptr_t slot = ((uintptr_t)this >> TAG_SLOT_SHIFT) & TAG_SLOT_MASK;
        return objc_tag_classes[slot];
    }
    return ISA();
}

inline Class 
objc_object::ISA() 
{
    assert(!isTaggedPointer()); 
    return (Class)(isa.bits & ISA_MASK);
}

+ (BOOL)isKindOfClass:(Class)cls {
    for (Class tcls = object_getClass((id)self); tcls; tcls = tcls->superclass) {
        if (tcls == cls) return YES;
    }
    return NO;
}

- (BOOL)isKindOfClass:(Class)cls {
    for (Class tcls = [self class]; tcls; tcls = tcls->superclass) {
        if (tcls == cls) return YES;
    }
    return NO;
}

+ (BOOL)isMemberOfClass:(Class)cls {
    return object_getClass((id)self) == cls;
}

- (BOOL)isMemberOfClass:(Class)cls {
    return [self class] == cls;
}

總結(jié)

  + isKindOfClass 會拿到當(dāng)前類對象的 isA(metaclass ) 遞歸(superclass)和當(dāng)前比較是否相等 
  +  isMemberOfClass的源碼實現(xiàn)是拿到自己的isa指針和自己比較，是否相等
  - isKindOfClass 會拿到當(dāng)前實例對象的 class 遞歸(superclass)的和當(dāng)前類對象進(jìn)行比較 
  -  isMemberOfClass 會拿到當(dāng)前實例對象的類對象 和當(dāng)前 實例對象比較