08.isKindOfClass的底层实现

BOOL result1 = [[NSObject class] isKindOfClass:[NSObject class]];
BOOL result2 = [[NSObject class] isMemberOfClass:[NSObject class]];
BOOL result3 = [[RYModel class] isKindOfClass:[RYModel class]];
BOOL result4 = [[RYModel class] isMemberOfClass:[RYModel class]];

NSLog(@"Class\n %hhd \n %hhd \n %hhd \n %hhd", result1, result2, result3, result4);

BOOL result5 = [[NSObject alloc] isKindOfClass:[NSObject class]];
BOOL result6 = [[NSObject alloc] isMemberOfClass:[NSObject class]];
BOOL result7 = [[RYModel alloc] isKindOfClass:[RYModel class]];
BOOL result8 = [[RYModel alloc] isMemberOfClass:[RYModel class]];

NSLog(@"Instacne\n %hhd \n %hhd \n %hhd \n %hhd", result5, result6, result7, result8);

思考一下，会输出什么？

结果：

2021-06-27 08:29:10.567002+0800 IsKindOf[62355:691675] Class
 1 
 0 
 0 
 0
2021-06-27 08:29:10.567836+0800 IsKindOf[62355:691675] Instacne
 1 
 1 
 1 
 1

如果仅从字面上去理解代码，那么你的答案应该会是不一样的的。我们从源码层面来理解一下吧，源码不长，很容易理解。

一、isKindOfClass

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

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

1.1 流程图-Class

[NSObject class] 即 RootClass
- RootClass -(ISA)> RootMetaClass -(superClass)> RootClass
- true
[RYModel class]
- [RYModel class] -(ISA)> RYModelMetaClass -(superClass)> RootMetaClass -(superClass)> RootClass -(superClass)> nil
- false

1.2 流程图-Instance

NSObjectInstance
- NSObjectInstance -(class)> RootClass
- true
RYModelInstance
- RYModelInstance -(class)> RYModelClass
- true

二、isMemberOfClass

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

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

这里就很简单了，就不展开说了。

三、你以为这就完了？

如果你看到这里觉得没啥问题？那就有问题了。。。

我们运行下OC源码，两个断点。

运行之后发现 +/- (Bool)isKindOfClass:(Class)cls 居然根本没有调用。

3.1 汇编

通过断点汇编，我们在调用 isKindOfClass 的地方发现了这样的指令 objc_opt_isKindOfClass

->  0x10000381c <+332>: mov    rdi, qword ptr [rip + 0x4f7d] ; (void *)0x0000000100008808: RYModel
    0x100003823 <+339>: call   0x100003b42               ; symbol stub for: objc_opt_class
    0x100003828 <+344>: mov    rbx, rax
    0x10000382b <+347>: mov    rdi, qword ptr [rip + 0x4f6e] ; (void *)0x0000000100008808: RYModel
    0x100003832 <+354>: call   0x100003b42               ; symbol stub for: objc_opt_class
    0x100003837 <+359>: mov    rdi, rbx
    0x10000383a <+362>: mov    rsi, rax
    0x10000383d <+365>: call   0x100003b48               ; symbol stub for: objc_opt_isKindOfClass

3.2 objc_opt_isKindOfClass

// Calls [obj isKindOfClass]
BOOL
objc_opt_isKindOfClass(id obj, Class otherClass)
{
#if __OBJC2__
    // 判空
    if (slowpath(!obj)) return NO;
    Class cls = obj->getIsa();
    if (fastpath(!cls->hasCustomCore())) {
        for (Class tcls = cls; tcls; tcls = tcls->getSuperclass()) {
            if (tcls == otherClass) return YES;
        }
        return NO;
    }
#endif
    // 这里可以忽略
    return ((BOOL(*)(id, SEL, Class))objc_msgSend)(obj, @selector(isKindOfClass:), otherClass);
}

在这里下断点，我们发现的确进到了这里。并且 Class 和 Instance 的方法都走到了这里。

编译器对这里进行的优化，用更高效的 objc_opt_isKindOfClass 代替了原有的方法。

四、参考

slowpath & fastpath

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