isKindOfClass的底层实现

538 字

3 分钟

isKindOfClass的底层实现

2021-06-27

2026-10-12

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BOOL result1 = [[NSObject class] isKindOfClass:[NSObject class]];
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BOOL result2 = [[NSObject class] isMemberOfClass:[NSObject class]];
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BOOL result3 = [[RYModel class] isKindOfClass:[RYModel class]];
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BOOL result4 = [[RYModel class] isMemberOfClass:[RYModel class]];
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NSLog(@"Class\n %hhd \n %hhd \n %hhd \n %hhd", result1, result2, result3, result4);
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BOOL result5 = [[NSObject alloc] isKindOfClass:[NSObject class]];
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BOOL result6 = [[NSObject alloc] isMemberOfClass:[NSObject class]];
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BOOL result7 = [[RYModel alloc] isKindOfClass:[RYModel class]];
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BOOL result8 = [[RYModel alloc] isMemberOfClass:[RYModel class]];
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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：

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

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

1.1 流程图-Class#

Class-isKindOfClass

[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#

Instance-isKindeOfClass

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

二、isMemberOfClass#

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

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。

IsKindofClass01

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

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

3.2 objc_opt_isKindOfClass#

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