kvc

很久之前无论是工作还是面试都会有kvc 和kvo的问题.一直是一知半解
http://www.cnblogs.com/Free-Thinker/p/5919043.html
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/KeyValueCoding/index.html#//apple_ref/doc/uid/10000107-SW1

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Key-value coding is a mechanism enabled by the NSKeyValueCoding informal protocol that objects adopt to provide indirect access to their properties. When an object is key-value coding compliant, its properties are addressable via string parameters through a concise, uniform messaging interface. This indirect access mechanism supplements the direct access afforded by instance variables and their associated accessor methods.

You typically use accessor methods to gain access to an object’s properties. A get accessor (or getter) returns the value of a property. A set accessor (or setter) sets the value of a property. In Objective-C, you can also directly access a property’s underlying instance variable. Accessing an object property in any of these ways is straightforward, but requires calling on a property-specific method or variable name. As the list of properties grows or changes, so also must the code which accesses these properties. In contrast, a key-value coding compliant object provides a simple messaging interface that is consistent across all of its properties.

Key-value coding is a fundamental concept that underlies many other Cocoa technologies, such as key-value observing, Cocoa bindings, Core Data, and AppleScript-ability. Key-value coding can also help to simplify your code in some cases.

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官方文档大概介绍了kvc的原理和应用面
可知 kvc功能是NSKeyValueCoding 有关

- (nullable id)valueForKey:(NSString *)key;                          //直接通过Key来取值
- (void)setValue:(nullable id)value forKey:(NSString *)key;          //通过Key来设值
- (nullable id)valueForKeyPath:(NSString *)keyPath;                  //通过KeyPath来取值
- (void)setValue:(nullable id)value forKeyPath:(NSString *)keyPath;  //通过KeyPath来设值

当然NSKeyValueCoding类别中还有其他的一些方法，下面列举一些

+ (BOOL)accessInstanceVariablesDirectly;
//默认返回YES，表示如果没有找到Set方法的话，会按照_key，_iskey，key，iskey的顺序搜索成员，设置成NO就不这样搜索
- (BOOL)validateValue:(inout id __nullable * __nonnull)ioValue forKey:(NSString *)inKey error:(out NSError **)outError;
//KVC提供属性值确认的API，它可以用来检查set的值是否正确、为不正确的值做一个替换值或者拒绝设置新值并返回错误原因。
- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key;
//这是集合操作的API，里面还有一系列这样的API，如果属性是一个NSMutableArray，那么可以用这个方法来返回
- (nullable id)valueForUndefinedKey:(NSString *)key;
//如果Key不存在，且没有KVC无法搜索到任何和Key有关的字段或者属性，则会调用这个方法，默认是抛出异常
- (void)setValue:(nullable id)value forUndefinedKey:(NSString *)key;
//和上一个方法一样，只不过是设值。
- (void)setNilValueForKey:(NSString *)key;
//如果你在SetValue方法时面给Value传nil，则会调用这个方法
- (NSDictionary *)dictionaryWithValuesForKeys:(NSArray *)keys;
//输入一组key,返回该组key对应的Value，再转成字典返回，用于将Model转到字典。

当调用setValue：属性值 forKey：@”name“的代码时，底层的执行机制如下

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1. setValue:forKey:默认的搜索顺序

一 . KVC从receiver（[receiver massage]）的类中查询访问方法set<Key>
二 . 如果set<Key>未找到，且receiver的类方法accessInstanceVariablesDirectly返回YES，则receiver依次搜索名字为_<key>, _is<Key>, <key>, is<Key>,的实例变量，注意搜索顺序。
三 . 如果找到了访问方法或者实例变量，则利用其设置对应的value。
四 . 如果既没有搜索到访问方法，也没有找到实例变量，则receiver会调用 setValue:forUndefinedKey: 方法

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如果开发者想让这个类禁用KVC里，那么重写+ (BOOL)accessInstanceVariablesDirectly方法让其返回NO即可，这样的话如果KVC没有找到set:属性名时，会直接用setValue：forUNdefinedKey：方法。

取值时

图片.png 图片.png 图片.png

KVC里面还有两个关于NSDictionary的方法

- (NSDictionary<NSString *, id> *)dictionaryWithValuesForKeys:(NSArray<NSString *> *)keys;
- (void)setValuesForKeysWithDictionary:(NSDictionary<NSString *, id> *)keyedValues;

dictionaryWithValuesForKeys:是指输入一组key，返回这组key对应的属性，再组成一个字典。
setValuesForKeysWithDictionary是用来修改Model中对应key的属性。下面直接用代码会更直观一点

Address* add = [Address new];
add.country = @"China";
add.province = @"Guang Dong";
add.city = @"Shen Zhen";
add.district = @"Nan Shan";
NSArray* arr = @[@"country",@"province",@"city",@"district"];
NSDictionary* dict = [add dictionaryWithValuesForKeys:arr]; //把对应key所有的属性全部取出来
NSLog(@"%@",dict);

NSDictionary* modifyDict = @{@"country":@"USA",@"province":@"california",@"city":@"Los angle"};
[add setValuesForKeysWithDictionary:modifyDict];            //用key Value来修改Model的属性
NSLog(@"country:%@  province:%@ city:%@",add.country,add.province,add.city);

//打印结果
2016-04-19 11:54:30.846 KVCDemo[6607:198900] {
    city = "Shen Zhen";
    country = China;
    district = "Nan Shan";
    province = "Guang Dong";
}
2016-04-19 11:54:30.847 KVCDemo[6607:198900] country:USA  province:california city:Los angle