iOS底层探索：KVO 1.准备工作 2.属性观察 3.对可变数组的观察

使用称为isa swizzling的技术实现自动键值观察。顾名思义，isa指针指向维护分派表的对象类。这个分派表本质上包含指向类实现的方法的指针以及其他数据。
当一个观察者为一个对象的属性注册时，被观察对象的isa指针被修改，指向一个中间类而不是真正的类。因此，isa指针的值不一定反映实例的实际类。
决不能依赖isa指针来确定类成员身份。相反，您应该使用class方法来确定对象实例的类。

1.准备工作

先定义一个类NXBookclass，再定义好一个工具方法打印前后类的变化，

@interface NXBookclass : NSObject {
    @public NSInteger _priceValue;
}
@property (nonatomic, copy) NSString *name;
@property (nonatomic, strong) NSMutableArray *descriptions;
@end

@implementation NXBookclass
//+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
//    return YES;
//}
@end

@interface NXApi : NSObject
+ (NSMutableDictionary *)descriptionClass:(Class)cls;
@end

@implementation NXApi
+ (NSMutableDictionary *)descriptionClass:(Class)cls {
    NSMutableDictionary *dicValue = [NSMutableDictionary dictionaryWithCapacity:5];
    while (cls && cls != [NSObject class]) {
        NSMutableDictionary *dicSubvalue = [NSMutableDictionary dictionaryWithCapacity:5];
        dicSubvalue[@"varList"] = [NXApi varList:cls forward:false];
        dicSubvalue[@"methodList"] = [NXApi methodList:cls forward:false];
        dicSubvalue[@"metaClass"] = [NXApi metaClass:cls forward:false];
        [dicValue setObject:dicSubvalue forKey:[NSString stringWithCString:class_getName(cls) encoding:NSUTF8StringEncoding]];
        cls = class_getSuperclass(cls);
    }
    return dicValue;
}
@end

2.属性观察

@interface NXKVO : NSObject
- (void)KVO;
@end

@implementation NXKVO
- (void)KVO{
    NXBookclass *bookclass = [[NXBookclass alloc] init];
    NSLog(@"观察前:%@", [NXApi descriptionClass:object_getClass(bookclass)]);//打印观察之前的类信息

  {
        [bookclass addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:NULL];

        NSLog(@"观察中:%@", [NXApi descriptionClass:object_getClass(bookclass)]);//打印观察之后的类信息

        bookclass.name = @"Swift";
        [bookclass removeObserver:self forKeyPath:@"name"];
  }

  NSLog(@"取消观察后:%@", [NXApi descriptionClass:object_getClass(bookclass)]);//打印取消观察之后的类信息
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"%s, key=%@, change=%@", __func__, keyPath, change);
}
@end

运行如上代码，打印信息如下：

观察前或取消观察后:
{
    NXBookclass =     {
        metaClass =         (
            NXBookclass
        );
        methodList =         (
            name,
            ".cxx_destruct",
            "setName:",
            descriptions,
            "setDescriptions:"
        );
        propertyList =         (
            name,
            descriptions
        );
        varList =         (
            "_priceValue",
            "_name",
            "_descriptions"
        );
    };
}

观察中:
{
    "NSKVONotifying_NXBookclass" =     {
        metaClass =         (
            "NSKVONotifying_NXBookclass"
        );
        methodList =         (
            "setName:",
            class,
            dealloc,
            "_isKVOA"
        );
        propertyList =         (
        );
        varList =         (
        );
    };
    NXBookclass =     {
        metaClass =         (
            NXBookclass
        );
        methodList =         (
            name,
            ".cxx_destruct",
            "setName:",
            descriptions,
            "setDescriptions:"
        );
        propertyList =         (
            name,
            descriptions
        );
        varList =         (
            "_priceValue",
            "_name",
            "_descriptions"
        );
    };
}

观察的回调信息：
key=name, change={
    kind = 1;
    new = Swift;
    old = "<null>";
}

通过打印的日志信息，我们可以看到，我们的bookclass的的name属性的观察是成功的，name的值由null变成了Swift。

通过打印的日志我们可以看到如下现象：观察中的类与观察前后的类信息不一样，默认的类是NXBookclass->NSObject;而观察中的类是NSKVONotifying_NXBookclass->NXBookclass->NSObject。并且在NSKVONotifying_NXBookclass这个类中出现了setName:,class,dealloc,_isKVOA4个新增的方法。

1.iOS对KVO底层的实现，其实是通过派生一个NSKVONotifying_XXXX的类。bookclass.is指向NSKVONotifying_NXBookclass, 我们通过访问[bookclass class]会发现返回的仍然是NXBookclass, 这是因为NSKVONotifying_NXBookclass重写了class类，返回了NXBookclass。

2.观察实际上是通过重写观察的属性的setter方法来实现的。重写setName:方法：

- (void)setName:(NSString *)name {
    if(_name != name){
        [self willChangeValueForKey:@"name"];
        _name = name;
        [self didChangeValueForKey:@"name"];
    }
}

3._isKVOA,用来标记是否是一个KVO的实例，通过调试发现_isKVOA == YES。在最后一个removeObserver:forKeyPath:之后变成NO，并将isa指回去。

4.通过观察前，观察后，取消观察后对NSClassFromString(@"NSKVONotifying_NXBookclass")访问，可以发现首次观察之前结果为null,观察中和取消观察后都存在，也就是取消观察后NSKVONotifying_NXBookclass不会消失。如果存在自定义的类NSKVONotifying_NXBookclass，那么在进行注册观察的时候会失败，也就是观察并不会生效。

5.默认情况下类的automaticallyNotifiesObserversForKey :返回YES，runtime会自动派生子类帮助我们做好监听。我们可以重写该方法 返回NO，并且自己在当前类中重写setName:方法，这样系统不会派生新的类。

3.对可变数组的观察

准备代码，对可变数组的观察需要借用KVC的mutableArrayValueForKey方法才能观察得到：

{
    [bookclass addObserver:self forKeyPath:@"descriptions" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:NULL];

        NSLog(@"观察之中:%@", [NXApi descriptionClass:object_getClass(bookclass)]);

        bookclass.descriptions = [NSMutableArray arrayWithCapacity:2];
        [[bookclass mutableArrayValueForKey:@"descriptions"] addObject:@"A"];
        [[bookclass mutableArrayValueForKey:@"descriptions"] setObject:@"B" atIndexedSubscript:0];
        [[bookclass mutableArrayValueForKey:@"descriptions"] removeObjectAtIndex:0];

        [bookclass removeObserver:self forKeyPath:@"descriptions"];
}

观察打印的日志发现有4次KVO的回调：
分别为：

key=descriptions, change={
    kind = 1; 
    new =     ();
    old = "<null>";
}

key=descriptions, change={
    indexes = "<_NSCachedIndexSet: 0x600000d53420>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
    kind = 2;
    new =     (A);
}

key=descriptions, change={
    indexes = "<_NSCachedIndexSet: 0x600000d53420>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
    kind = 4;
    new =     (B);
    old =     (A);
}

key=descriptions, change={
    indexes = "<_NSCachedIndexSet: 0x600000d53420>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
    kind = 3;
    old =     (B);
}

仔细看，你会发现对数组初始化的时候也收到了回调，再仔细看每次回调的kind都不一样，经过查阅文档，我们可以发现有一个NSKeyValueChange的枚举，NSKeyValueChangeSetting = 1设置，NSKeyValueChangeInsertion = 2插入，NSKeyValueChangeReplacement = 4替换，NSKeyValueChangeRemoval = 3移除，刚好跟我们对数组的初始化、插入对象、替换对象、删除对象的操作一致。