[iOS] KVC 和 KVO

開發iOS經常會看見KVO和KVC這兩個概念，特地瞭解了一下。
我的新博客wossoneri.com link

KVC Key Value Coding

KVC是一種用間接方式訪問類的屬性的機制。比如你要給一個類中的屬性賦值或者取值，可以直接通過類和點運算符實現，當然也可以使用KVC。不過對於私有屬性，點運算符就不起作用，因為私有屬性不暴露給調用者，不過使用KVC卻依然可以實現對私有屬性的讀寫。

先看一下KVC的一部分源碼，當然只能看到頭文件：

// NSKeyValueCoding.h

@interface NSObject(NSKeyValueCoding)

+ (BOOL)accessInstanceVariablesDirectly;

- (nullable id)valueForKey:(NSString *)key;
- (void)setValue:(nullable id)value forKey:(NSString *)key;
- (BOOL)validateValue:(inout id __nullable * __nonnull)ioValue forKey:(NSString *)inKey error:(out NSError **)outError;

- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key;

- (NSMutableOrderedSet *)mutableOrderedSetValueForKey:(NSString *)key NS_AVAILABLE(10_7, 5_0);

- (NSMutableSet *)mutableSetValueForKey:(NSString *)key;

- (nullable id)valueForKeyPath:(NSString *)keyPath;
- (void)setValue:(nullable id)value forKeyPath:(NSString *)keyPath;
- (BOOL)validateValue:(inout id __nullable * __nonnull)ioValue forKeyPath:(NSString *)inKeyPath error:(out NSError **)outError;
- (NSMutableArray *)mutableArrayValueForKeyPath:(NSString *)keyPath;
- (NSMutableOrderedSet *)mutableOrderedSetValueForKeyPath:(NSString *)keyPath NS_AVAILABLE(10_7, 5_0);
- (NSMutableSet *)mutableSetValueForKeyPath:(NSString *)keyPath;

- (nullable id)valueForUndefinedKey:(NSString *)key;
- (void)setValue:(nullable id)value forUndefinedKey:(NSString *)key;
- (void)setNilValueForKey:(NSString *)key;
- (NSDictionary<NSString *, id> *)dictionaryWithValuesForKeys:(NSArray<NSString *> *)keys;
- (void)setValuesForKeysWithDictionary:(NSDictionary<NSString *, id> *)keyedValues;

@end


@interface NSArray<ObjectType>(NSKeyValueCoding)

- (id)valueForKey:(NSString *)key;
- (void)setValue:(nullable id)value forKey:(NSString *)key;

@end


@interface NSDictionary<KeyType, ObjectType>(NSKeyValueCoding)

- (nullable ObjectType)valueForKey:(NSString *)key;

@end


@interface NSMutableDictionary<KeyType, ObjectType>(NSKeyValueCoding)

- (void)setValue:(nullable ObjectType)value forKey:(NSString *)key;

@end


@interface NSOrderedSet<ObjectType>(NSKeyValueCoding)

- (id)valueForKey:(NSString *)key NS_AVAILABLE(10_7, 5_0);
- (void)setValue:(nullable id)value forKey:(NSString *)key NS_AVAILABLE(10_7, 5_0);

@end


@interface NSSet<ObjectType>(NSKeyValueCoding)

- (id)valueForKey:(NSString *)key;
- (void)setValue:(nullable id)value forKey:(NSString *)key;

@end

可以看到這個類裡面包含了對類NSObject,NSArray,NSDictionary,NSMutableDictionary,NSOrderedSet,NSSet的拓展。拓展的方法基本上為

- (id)valueForKey:(NSString *)key;
- (void)setValue:(nullable id)value forKey:(NSString *)key;

也就是說，基本上Objective-C里所有的對象都支持KVC操作，操作包含如上兩類方法，動態讀取和動態設值。

好多地方說是NSObject實現了NSKeyValueCoding協議。而代碼里是類的拓展。這兩種說法是相通的嘛？

舉個