一、KVO的初體驗

KVO的步驟：

• 1.添加觀察
• 2.observe回調
• 3.在合適位置更改觀察屬性的值
• 4.在dealloc里移除觀察

- (void)viewDidLoad {
    [super viewDidLoad];
    self.person  = [LGPerson new];
    [self.person addObserver:self forKeyPath:@"nick" options:NSKeyValueObservingOptionNew context:NULL];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    self.person.nick = [NSString stringWithFormat:@"%@+",self.person.nick];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"%@",change);
}
- (void)dealloc{
    [self.person removeObserver:self forKeyPath:@"dateArray"];
}

二、KVO的其他用法

1、切換手動與自動

• 自動開關 (默認)

+ (BOOL) automaticallyNotifiesObserversForKey:(NSString *)key{
    return YES;
}

• 手動開關

+ (BOOL) automaticallyNotifiesObserversForKey:(NSString *)key{
    return NO;
}

- (void)setNick:(NSString *)nick{
    [self willChangeValueForKey:@"nick"];
    _nick = nick;
    [self didChangeValueForKey:@"nick"];
}

2、路徑的處理

@implementation LGPerson
// 下載進度 -- writtenData/totalData
+ (NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key{
    
    NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key];
    if ([key isEqualToString:@"downloadProgress"]) {
        NSArray *affectingKeys = @[@"totalData", @"writtenData"];
        keyPaths = [keyPaths setByAddingObjectsFromArray:affectingKeys];
    }
    return keyPaths;
}
- (NSString *)downloadProgress{
    if (self.writtenData == 0) {
        self.writtenData = 1.0;
    }
    if (self.totalData == 0) {
        self.totalData = 100;
    }
    return [[NSString alloc] initWithFormat:@"%f",1.0f*self.writtenData/self.totalData];
}
@end

@implementation LGViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.person  = [LGPerson new];
    // 4: 路徑處理
    // 下載的進度 = 已下載 / 總下載
   [self.person addObserver:self forKeyPath:@"downloadProgress" options:(NSKeyValueObservingOptionNew) context:NULL];
}
@end

3、數組屬性的監聽

@interface LGPerson : NSObject
@property (nonatomic, strong) NSMutableArray *dateArray;
@end

@implementation LGViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.person  = [LGPerson new];
    // 1: 數組觀察
    self.person.dateArray = [NSMutableArray arrayWithCapacity:1];
    [self.person addObserver:self forKeyPath:@"dateArray" options:(NSKeyValueObservingOptionNew) context:NULL];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    // KVC 集合 array
    [[self.person mutableArrayValueForKey:@"dateArray"] addObject:@"1"];
}

三、KVO的原理分析

首先我們斷點到self.person = [[LGPerson alloc] init]后面,然后打印self.person的isa是LFPerson。

接下來 我們添加如下代碼：

- (void)viewDidLoad {
    [super viewDidLoad];
    // KVO 底層原理
    self.person = [[LGPerson alloc] init];
   // 打印LGPerson的方法
    [self printClassAllMethod:[LGPerson class]];
    [self.person addObserver:self forKeyPath:@"nickName" options:(NSKeyValueObservingOptionNew) context:NULL];
    // 打印LGPerson的方法
    [self printClassAllMethod:[LGPerson class]];
    // 打印NSKVONotifying_LGPerson的方法
    Class cls = objc_getClass("NSKVONotifying_LGPerson");
    [self printClassAllMethod:cls];
}

// 打印cls的方法
- (void)printClassAllMethod:(Class)cls{
    unsigned int count = 0;
    NSLog(@"以下為%@的方法列表:",NSStringFromClass(cls));
    Method *methodList = class_copyMethodList(cls, &count);
    for (int i = 0; i<count; i++) {
        Method method = methodList[i];
        SEL sel = method_getName(method);
        IMP imp = class_getMethodImplementation(cls, sel);
        NSLog(@"%@-%p",NSStringFromSelector(sel),imp);
    }
    free(methodList);
}

打印的結果：

接下來我們在dealloc里面斷點，在未執行remove操作前，self.person的isa指向NSKVONotifying_LGPerson

綜合上述，我們推測：

• 動態的創建NSKVONotifying_LGPerson子類，并添加方法

  1. 添加class : class的指向是LGPerson

  2.添加setter(即: 重寫setter方法)

• 將對象的isa指向NSKVONotifying_LGPerson：object_setClass(self, newClass);

• 將觀察者信息保存在關聯屬性的NSKVO_AssiociateKey的可變數組中

• 在重寫的setter方法里面：

  • 將消息轉發給父類，所以我們看到LGPerson里面的setNickName也會調用
  • 消息發送(將觀察者作為消息的接受者, observeValueForKeyPath:ofObject:change:context: 作為 SEL)

• 在移除監聽時，將self.person的isa重新指向父類LGPerson