先來看看一道常見的面試題:

void isKindOfAndisMemeberOf() {
    BOOL b1 = [(id)[NSObject class] isKindOfClass:[NSObject class]];
    BOOL b2 = [(id)[NSObject class] isMemberOfClass:[NSObject class]];
    BOOL b3 = [(id)[Person class] isKindOfClass:[Person class]];
    BOOL b4 = [(id)[Person class] isMemberOfClass:[Person class]];
    
    BOOL b5 = [(id)[NSObject alloc] isKindOfClass:[NSObject class]];
    BOOL b6 = [(id)[NSObject alloc] isMemberOfClass:[NSObject class]];
    BOOL b7 = [(id)[Person alloc] isKindOfClass:[Person class]];
    BOOL b8 = [(id)[Person alloc] isKindOfClass:[Person class]];
    
    NSLog(@"\n%hhd\n%hhd\n%hhd\n%hhd\n%hhd\n%hhd\n%hhd\n%hhd\n",b1,b2,b3,b4,b5,b6,b7,b8);
}

要想知道輸出結(jié)果為什么,必須先知道isKindOfClass與isMemberOfClass的具體實現(xiàn)。

+ (BOOL)isMemberOfClass:(Class)cls {
    return self->ISA() == cls;
}

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

+ (BOOL)isKindOfClass:(Class)cls {
    for (Class tcls = self->ISA(); 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;
}

解析

1. + (BOOL)isKindOfClass:(Class)cls實現(xiàn)判斷元類是不是為當(dāng)前類,再判斷元類的父類是不是當(dāng)前類,依次判斷下去。所以:

• NSObject的元類為根元類不同于NSObject,根元類的父類為NSObject與NSObject相同,所以b1為true。
• Person的元類為Person(元類)不同,Person(元類)的父類為根元類不同,根元類的父類為NSObject不同,NSObject父類為nil不同,所以b3為false。

2. + (BOOL)isMemberOfClass:(Class)cls是去和元類做比較,元類與類不同,所以b2與b4為false。

3.- (BOOL)isKindOfClass:(Class)cls與- (BOOL)isMemberOfClass:(Class)cls,都是判斷實例對象的類是不是對應(yīng)的類,區(qū)別在于isKindOf會往父類找,所以b5到b8都為true。

運行結(jié)果如圖