那么，既然是isa替換强法，那主角當(dāng)然就是isa啦。那么這個(gè)技術(shù)出現(xiàn)在什么場(chǎng)景呢？其實(shí)這個(gè)技術(shù)在官方文檔中關(guān)于KVO的文檔中有提到過, 里面說到了嫡纠，KVO是通過isa-swizzling來實(shí)現(xiàn)的烦租。

Automatic key-value observing is implemented using a technique called isa-swizzling.

The isa pointer, as the name suggests, points to the object’s class which maintains a dispatch table. This dispatch table essentially contains pointers to the methods the class implements, among other data.

When an observer is registered for an attribute of an object the isa pointer of the observed object is modified, pointing to an intermediate class rather than at the true class. As a result the value of the isa pointer does not necessarily reflect the actual class of the instance.

You should never rely on the isa pointer to determine class membership. Instead, you should use the class method to determine the class of an object instance.

最后一段中也說到了延赌，永遠(yuǎn)不要用用isa來判斷一個(gè)類的繼承關(guān)系，而是應(yīng)該用class方法來判斷類的實(shí)例叉橱。

看著好像很厲害的技術(shù)耶_(雙眼blingbling~發(fā)光)挫以。那么為了驗(yàn)證這個(gè)技術(shù)真正的實(shí)現(xiàn)了，也就是isa替換了窃祝，我們寫了下面一段代碼來驗(yàn)證掐松。大家一起來觀摩一下：

//這里用來說明printInfo方法都做了說明
@implementation Person
(void)printInfo { 
NSLog(@”isa:%@,supperclass:%@”,NSStringFromClass(object_getClass(self)),
class_getSuperclass(object_getClass(self))); 
NSLog(@”self:%@, [self superclass]:%@”, self, [self superclass]); 
NSLog(@”age setter function pointer:%p”, class_getMethodImplementation(object_getClass(self), @selector(setAge:)));
NSLog(@”name setter function pointer:%p”, class_getMethodImplementation(object_getClass(self), @selector(setName:)));
NSLog(@”printInfo function pointer:%p”, class_getMethodImplementation(object_getClass(self), @selector(printInfo)));
}

@end

// 既然是KVO嘛，那我們就來模擬一個(gè)虛假的KVO的情況粪小，并且打印對(duì)應(yīng)的信息

Person *person = [[Person alloc] init];
NSLog(@”Before add observer————————————————————————–“);
[person printInfo];
[person addObserver:self forKeyPath:@”age” options:NSKeyValueObservingOptionNew context:&PrivateKVOContext]; 
NSLog(@”After add observer————————————————————————–“);
[person printInfo];
[person removeObserver:self forKeyPath:@”age”]; NSLog(@”After remove observer————————————————————————–“);
[person printInfo];

根據(jù)之前一篇歷史冤案之isKindOfClass與isMemberOfClass中提到大磺，object_getClass其實(shí)打印的就是isa指針。

其中第一個(gè)Person的實(shí)現(xiàn)中探膊，我們打印了isa, supperclass, 兩個(gè)方法的setter的函數(shù)指針以及printInfo的函數(shù)指針杠愧。而運(yùn)行代碼中分別在添加Observer前，添加Observer以及刪除Observer后分別打印出該類的信息逞壁。那么輸出的結(jié)果是什么呢流济？

Before add observer--------------------------------------------------------------------------
isa:Person, supper class:NSObject
self:<Person: 0x7ffe5aeeb340>, [self superclass]:NSObject
age setter function pointer:0x10fe03c40
name setter function pointer:0x10fe03be0
printInfo function pointer:0x10fe03a30
After add observer--------------------------------------------------------------------------
isa:NSKVONotifying_Person, supper class:Person
self:<Person: 0x7ffe5aeeb340>, [self superclass]:NSObject
age setter function pointer:0x10ff04c7f
name setter function pointer:0x10fe03be0
printInfo function pointer:0x10fe03a30
After remove observer--------------------------------------------------------------------------
isa:Person, supper class:NSObject
self:<Person: 0x7ffe5aeeb340>, [self superclass]:NSObject
age setter function pointer:0x10fe03c40
name setter function pointer:0x10fe03be0
printInfo function pointer:0x10fe03a30

不對(duì)锐锣，看看文章的標(biāo)題，其實(shí)在添加KVO之后绳瘟，isa已經(jīng)替換成了NSKVONotifying_Person,而根據(jù)class_getSuperclass得到的結(jié)果竟然是Person, 然后age是使我們KVO需要觀察的屬性雕憔，它的setter函數(shù)指針變了。而我們也知道糖声，所謂的OC的消息機(jī)制是通過isa去查找實(shí)現(xiàn)的斤彼，那么我們現(xiàn)在可以進(jìn)行大膽的猜想：

其實(shí)KVO的實(shí)現(xiàn)可能是：

• 添加Observer

通過runtime偷偷實(shí)現(xiàn)了一個(gè)子類，并且以NSKVONotifying_+類名來命名

將之前那個(gè)對(duì)象的isa指針指向了這個(gè)子類姨丈。

重寫了觀察的對(duì)象setter方法畅卓，并且在重寫的中添加了willChangeValueForKey:以及didChangeValueForKey:

• 移除Observer

只是簡(jiǎn)單的將其的isa指向原來的類對(duì)象中

然后我們?cè)诜治鲆幌拢?在真正調(diào)用的setAge:的情況下, 根據(jù)消息機(jī)制我們知道它先通過isa找到對(duì)應(yīng)對(duì)象的類， 也就是現(xiàn)在NSKVONotifying_Person, 然后再去找setAge:蟋恬，由于NSKVONotifying_Person這個(gè)對(duì)象重寫了這個(gè)方法翁潘， 那么就會(huì)直接取當(dāng)前的實(shí)現(xiàn)， 也就是帶有willChangeValueForKey:以及didChangeValueForKey:歼争， 那么自然就實(shí)現(xiàn)了對(duì)KVO的實(shí)現(xiàn)了拜马。

最后在看看一幅圖吧，稍微總結(jié)一下isa-swizzling是搞什么東東：