iOS：KVO原理分析

目录
一，基本知识
二，NSKVONotifying_Person
三，NSKVONotifying_Person实例方法
四，setAge:方法
五，class，dealloc，_isKVOA
六，手动发送通知
七，使用进阶
八，移除观察者

一，基本知识

1，概念

KVO是Key-Value-Observing的缩写，意思是键值观察，作用是监听对象某个属性的变化

2，使用

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person = [Person new];
    self.person.age = 1;
    // 添加监听
    [self.person addObserver:self
                  forKeyPath:@"age"
                     options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                     context:@"111"];
    self.person.age = 2;
}

// 监听到变化的回调
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {
    if (context == @"111") {
        NSLog(@"%@---%@---%@", object, keyPath, change);
    }
}

- (void)dealloc {
    // 移除监听
    [self.person removeObserver:self
                     forKeyPath:@"age"
                        context:@"111"];
}

// 打印
<Person: 0x600003ae8ea0>---age---{
    kind = 1;
    new = 2;
    old = 1;
}

二，NSKVONotifying_Person

当self.person添加监听后，系统会自动生成Person的子类NSKVONotifying_Person，并让self.person的isa指针指向它的类对象

self.person = [Person new];
NSLog(@"添加监听前：%@", object_getClass(self.person));
[self.person addObserver:self
              forKeyPath:@"age"
                 options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                 context:@"111"];
NSLog(@"添加监听后：%@", object_getClass(self.person));

// 打印
添加监听前：Person
添加监听后：NSKVONotifying_Person

添加监听前

添加监听后

三，NSKVONotifying_Person实例方法

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person = [Person new];
    [self logMethodNamesWithClass:[Person class]];
    [self.person addObserver:self
                  forKeyPath:@"age"
                     options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                     context:@"111"];
    [self logMethodNamesWithClass:[Person class]];
}

// 打印类对象或元类对象中的方法名
- (void)logMethodNamesWithClass:(Class)cls {
    unsigned int count;
    Method *methodList = class_copyMethodList(cls, &count);
    NSMutableString *methodNames = [NSMutableString string];
    for (int i = 0; i < count; i++) {
        Method method = methodList[i];
        SEL selector = method_getName(method);
        NSString *methodName = NSStringFromSelector(selector);
        [methodNames appendString:methodName];
        [methodNames appendString:@", "];
    }
    free(methodList);
    NSLog(@"%@---%@", cls, methodNames);
}

// 打印
Person---setAge:, age,
NSKVONotifying_Person---setAge:, class, dealloc, _isKVOA,

实例方法

四，setAge:方法

1，在NSKVONotifying_Person类中的实现

- (void)setAge:(NSInteger)age {
    [self willChangeValueForKey:@"age"];
    [super setAge:age];
    [self didChangeValueForKey:@"age"];
}

运行下面代码来验证一下

@implementation Person
- (void)setAge:(NSInteger)age {
    _age = age;
    NSLog(@"%@:setAge", object_getClass(self));
}
- (void)willChangeValueForKey:(NSString *)key {
    [super willChangeValueForKey:key];
    NSLog(@"%@:willChangeValueForKey", object_getClass(self));
}
- (void)didChangeValueForKey:(NSString *)key {
    [super didChangeValueForKey:key];
    NSLog(@"%@:didChangeValueForKey", object_getClass(self));
}
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person = [Person new];
    NSLog(@"添加监听前");
    self.person.age = 1;
    [self.person addObserver:self
                  forKeyPath:@"age"
                     options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                     context:@"111"];
    NSLog(@"添加监听后");
    self.person.age = 2;
}

// 打印
添加监听前
Person:setAge
添加监听后
NSKVONotifying_Person:willChangeValueForKey
NSKVONotifying_Person:setAge
NSKVONotifying_Person:didChangeValueForKey

2，didChangeValueForKey:方法

- (void)didChangeValueForKey:(NSString *)key {
    NSLog(@"%@:didChangeValueForKey - begin", object_getClass(self));
    [super didChangeValueForKey:key];
    NSLog(@"%@:didChangeValueForKey - end", object_getClass(self));
}

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {
    if (context == @"111") {
        NSLog(@"%@---%@---%@", object_getClass(object), keyPath, change);
    }
}

// 打印
NSKVONotifying_Person:didChangeValueForKey - begin
NSKVONotifying_Person---age---{
    kind = 1;
    new = 2;
    old = 1;
}
NSKVONotifying_Person:didChangeValueForKey - end

由上可以看出是在[super didChangeValueForKey:key]中触发observeValueForKeyPath:方法的，所以在NSObject中此方法的实现如下

- (void)didChangeValueForKey:(NSString *)key {
    // 参数是在添加监听时保存的
    [observer observeValueForKeyPath:keyPath
                            ofObject:self
                              change:change
                             context:context];
}

五，class，dealloc，_isKVOA

1，class

问题：self.person本质是NSKVONotifying_Person，为什么class方法返回Person？
答：因为NSKVONotifying_Person重写了class方法，返回了父类Person，这么做是为了隐藏底层实现细节，如果返回NSKVONotifying_Person，class方法的调用者会感到莫名其妙

self.person = [Person new];
[self.person addObserver:self
              forKeyPath:@"age"
                 options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                 context:@"111"];
NSLog(@"%@", [self.person class]);
NSLog(@"%@", object_getClass(self.person));

// 打印
Person
NSKVONotifying_Person

2，dealloc

因为系统会自动生成NSKVONotifying_Person，所以需要重写dealloc方法，以便在销毁时清理系统产生的垃圾

3，_isKVOA

标识NSKVONotifying_Person是KVO产生的类

- (BOOL)_isKVOA {
    return YES;
}

六，手动发送通知

1，automaticallyNotifiesObserversForKey:方法是一个开关，如果返回YES，表示允许自动发送通知，修改属性值会收到回调；如果返回NO，表示不允许自动发送通知，修改属性值不会收到回调，必须手动发送通知才会收到回调；此方法默认返回YES
2，手动发送通知必须同时调用willChangeValueForKey:方法和didChangeValueForKey:方法

@implementation Person
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
    return NO;
}
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person = [Person new];
    self.person.age = 1;
    [self.person addObserver:self
                  forKeyPath:@"age"
                     options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                     context:@"111"];
    // 手动通知
    [self.person willChangeValueForKey:@"age"];
    self.person.age = 2;
    [self.person didChangeValueForKey:@"age"];
}

// 打印
<Person: 0x6000020d72f0>---age---{
    kind = 1;
    new = 2;
    old = 1;
}

七，使用进阶

1，属性依赖

一个对象的值取决于它内部所有属性的值，这种依赖关系可以通过keyPathsForValuesAffectingValueForKey:或keyPathsForValuesAffecting<#DependentKey#>方法进行设置，一旦给该对象添加了监听，它内部任何属性的改变都会通知观察者

// Dog
@interface Dog : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) NSInteger age;
@end

@implementation Dog
- (NSString *)description {
    return [NSString stringWithFormat:@"name: %@, age: %zd", _name, _age];
}
@end

// Person
@interface Person : NSObject
@property (nonatomic, strong) Dog *dog;
@end

@implementation Person
- (instancetype)init {
    if (self = [super init]) {
        _dog = [Dog new];
    }
    return self;
}
//+ (NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key {
//    if ([key isEqualToString:@"dog"]) {
//        return [NSSet setWithObjects:@"_dog.name", @"_dog.age", nil];
//    } else {
//        return [super keyPathsForValuesAffectingValueForKey:key];
//    }
//}
+ (NSSet<NSString *> *)keyPathsForValuesAffectingDog {
    return [NSSet setWithObjects:@"_dog.name", @"_dog.age", nil];
}
@end

// 使用Person
- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person = [Person new];
    self.person.dog.name = @"xiaoHuang";
    self.person.dog.age = 1;
    [self.person addObserver:self
                  forKeyPath:@"dog"
                     options:NSKeyValueObservingOptionNew
                     context:@"111"];
    self.person.dog.name = @"xiaoHei";
}

// 打印
<Person: 0x6000004783d0>---dog---{
    kind = 1;
    new = "name: xiaoHei, age: 1";
}

2，监听容器对象中数据的变化

不能直接拿容器对象进行操作，必须通过mutableArrayValueForKey:方法来获取容器对象，这样才能触发回调

@interface Person : NSObject
@property (nonatomic, strong) NSMutableArray *friends;
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person = [Person new];
    self.person.friends = @[@"zhangSan"].mutableCopy;
    [self.person addObserver:self
                  forKeyPath:@"friends"
                     options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                     context:@"111"];
//    [self.person.friends addObject:@"liSi"];
    [[self.person mutableArrayValueForKey:@"friends"] addObject:@"liSi"];
}

// 打印
<Person: 0x600002d30040>---friends---{
    indexes = "<_NSCachedIndexSet: 0x600002f669c0>[number of indexes: 1 (in 1 ranges), indexes: (1)]";
    kind = 2;
    new =     (
        liSi
    );
}

3，监听容器对象中元素（对象）的属性变化

添加监听和移除监听用另外的方法

@interface ViewController ()
@property (nonatomic, strong) NSArray *pArray;
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    
    Person *p1 = [Person new];
    p1.age = 1;
    Person *p2 = [Person new];
    p2.age = 2;
    self.pArray = @[p1, p2];

    NSIndexSet *indexSet = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, self.pArray.count)];
    [self.pArray addObserver:self
          toObjectsAtIndexes:indexSet
                  forKeyPath:@"age"
                     options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                     context:@"111"];
    p1.age = 3;
}
- (void)dealloc {
    NSIndexSet *indexSet = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, self.pArray.count)];
    [self.pArray removeObserver:self
           fromObjectsAtIndexes:indexSet
                     forKeyPath:@"age"
                        context:@"111"];
}
@end

// 打印
<Person: 0x6000036abc90>---age---{
    kind = 1;
    new = 3;
    old = 1;
}

八，移除观察者

1，在观察者销毁时必须把它移除掉，否则在再次改变属性值时会crash

// Person
@implementation Person
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {
    if (context == @"111") {
        NSLog(@"%@---%@---%@", object, keyPath, change);
    }
}
- (void)dealloc {
    NSLog(@"Person dealloc");
}
@end

// Dog
@interface Dog : NSObject
@property (nonatomic, assign) NSInteger age;
@end

// 使用
self.person = [Person new];
self.dog = [Dog new];
self.dog.age = 1;
[self.dog addObserver:self.person
           forKeyPath:@"age"
              options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
              context:@"111"];
self.dog.age = 2;
self.person = nil;
self.dog.age = 3; // crash

// 打印
<Dog: 0x60000203c420>---age---{
    kind = 1;
    new = 2;
    old = 1;
}
Person dealloc

2，在移除观察者时必须保证观察者没有销毁，否则会crash

// 使用
self.person = [Person new];
self.dog = [Dog new];
self.dog.age = 1;
[self.dog addObserver:self.person
           forKeyPath:@"age"
              options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
              context:@"111"];
self.dog.age = 2;
self.person = nil;
[self.dog removeObserver:self.person
              forKeyPath:@"age"
                 context:@"111"];

// 打印
<Dog: 0x600001310830>---age---{
    kind = 1;
    new = 2;
    old = 1;
}
Person dealloc
*** Terminating app due to uncaught exception 'NSRangeException', 
reason: 'Cannot remove an observer <(null) 0x0> for the key path "age" from 
<Dog 0x600001310830> because it is not registered as an observer.'

3，移除观察者与添加观察者的次数必须保持一致，否则会crash

// 使用
self.person = [Person new];
self.dog = [Dog new];
self.dog.age = 1;
BOOL isAdd = NO;
if (isAdd) {
    [self.dog addObserver:self.person
               forKeyPath:@"age"
                  options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
                  context:@"111"];
}
self.dog.age = 2;
[self.dog removeObserver:self.person
              forKeyPath:@"age"
                 context:@"111"];

// 打印
*** Terminating app due to uncaught exception 'NSRangeException', 
reason: 'Cannot remove an observer <Person 0x600003173560> for the key path "age" from 
<Dog 0x600003173570> because it is not registered as an observer.'