iOS设计模式 —— KVC

 

刨根问底KVC

KVC 全称 key valued coding 键值编码

反射机制是在运行状态中，对于任意一个类，都可以知道这个类的全部属性和方法；对于任意一个对象，都可以调用它的任意一个方法和属性.JAVA,C#都有这个机制。ObjC也有，因此你根部没必要进行任何操做就能够进行属性的动态读写，就是KVC。

KVC的操做方法由NSKeyValueCoding提供，而他是NSObject的类别，也就是说ObjC中几乎全部的对象都支持KVC操做。

Person.h

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@interface Person : NSObject { int weight; } @property(nonatomic,readonly,copy) NSString *name; @property(nonatomic,readonly, assign) int age; @property(nonatomic,strong) Dog * dog; @property(nonatomic,assign) id ID; -(instancetype)initWithDict:(NSDictionary *)dict; @end

Person.m

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@implementation Person { int _height; } -(instancetype)initWithDict:(NSDictionary *)dict { if (self=[super init]) { //字典转模型的经常使用语句 [self setValuesForKeysWithDictionary:dict]; } return self; } //当key的值是没有定义时，设置会执行的方法 - (void)setValue:(id)value forUndefinedKey:(NSString *)key { if ([key isEqualToString:@"id"]) { self.ID = value; } } //当key的值是没有定义时，取值时执行的方法 - (id)valueForUndefinedKey:(NSString *)key { if ([key isEqualToString:@"id"]) { return self.ID; } return [NSNull null]; }

main.m

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Person * p1 = [[Person alloc]init]; Dog * d1 = [[Dog alloc] init]; p1.dog = d1; //setValue:属性值 forKey:属性名（用于简单路径) //使用KVC间接修改对象属性时，系统会自动判断对象属性的类型，并完成转换。如该程序中的“170”. [p1 setValue:@"170" forKey:@"height"]; [p1 setValue:@"70" forKey:@"weight"]; [p1 setValue:@"1" forKey:@"id"]; //setValue:属性值 forKeyPath:属性路径（用于复合路径) //用KVC取一个嵌套层次很深的路径的时候，只要给它一个路径就能把想要的属性给拿出来。（.能够理解为路径。一直一直进入）。可以帮助咱们很方便的编码。 [p1 setValue:@"dahuan" forKeyPath:@"dog.name"]; //valueForKey:属性名 、valueForKeyPath:属性名（用于复合路径） NSLog(@"height = %d weight = %d id = %@ dog.name = %@" ,[[p1 valueForKey:@"height"] intValue],[[p1 valueForKey:@"weight"] intValue],[p1 valueForKey:@"id"],[p1 valueForKeyPath:@"dog.name"]); log日志：height = 170 weight = 70 id = 11111 dog.name = dahuan 全部的属性均可以赋值成功 NSDictionary * dict = @{@"height":@"160", @"weight":@"60", @"id":@"11101" }; Person * p2 = [[Person alloc] initWithDict:dict]; NSLog(@"height = %d weight = %d id = %@",[[p2 valueForKey:@"height"] intValue],[[p2 valueForKey:@"weight"] intValue],[p2 valueForKey:@"id"]); log日志：height = 160 weight = 60 id = 11101 全部的属性均可以赋值成功

KVC查找属性的顺序

1. 用@property定义的属性的key值
2. setter方法的key值
3. 直接访问成员变量，先找key，若是找不到，再找_key
4. 以上三种都未找到就会调用- (void)setValue:(id)value forUndefinedKey:(NSString *)key 方法。
5. 若是没有重写setValue:forUndefinedKey程序会立刻崩溃。

注意：KVC能够访问成员变量，不管是否提供getter/setter方法,不管可见性是怎样，是否有readonly修饰。

setValue:forUndefinedKey与valueForUndefinedKey的应用

KVC的主要用途无非是ORM映射,就是将dictionary转换成model,但有些服务器返回的字段有多是oc的关键字好比‘id’,’description’等。如上代码举得id的例子，咱们没法让@property后面key值为id,因而使用大写的ID代替，KVC是区分大小写的咱们不用担忧。这时咱们只需在setValue:forUndefinedKey把id的key值赋值给ID的key值，就能够避免关键字的尴尬。

KVC的逆向使用

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Person * p1 = [[Person alloc]init]; [p1 setValue:@"170" forKey:@"height"]; [p1 setValue:@"70" forKey:@"weight"]; [p1 setValue:@"11111" forKey:@"id"]; NSArray * arr = @[@"height",@"weight",@"id"]; NSDictionary * dict = [p1 dictionaryWithValuesForKeys:arr]; NSLog(@"%@",dict); log日志： { height = 170; id = 11111; weight = 70; }

NSArray／NSSet等都支持KVC

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Person * p1 = [[Person alloc]init]; Dog * d1 = [[Dog alloc] init]; d1.name = @"iPhone"; p1.dog = d1; Person * p2 = [[Person alloc]init]; Dog * d2 = [[Dog alloc] init]; d2.name = @"ios"; p2.dog = d2; NSArray *persons=@[p1,p2]; NSArray *arrayM=[persons valueForKeyPath:@"dog.name"]; NSLog(@"%@",arrayM); log日志： ( iPhone, ios )

KVC计算

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Person * person = [[Person alloc] init]; NSMutableArray * books = [NSMutableArray array]; for (int i = 0; i < 10; i++) { Book * book = [[Book alloc] init]; book.price = i; [books addObject:book]; } person.books = books; NSNumber *bookCount = [person valueForKeyPath:@"books.@count"]; NSLog(@"book count :%@", bookCount); NSNumber *sum = [person valueForKeyPath:@"books.@sum.price"]; NSLog(@"sum :%@", sum); NSNumber *avg = [person valueForKeyPath:@"books.@avg.price"]; NSLog(@"vag :%@", avg); NSNumber *min = [person valueForKeyPath:@"books.@min.price"]; NSLog(@"min :%@", min); NSNumber *max = [person valueForKeyPath:@"books.@max.price"]; NSLog(@"max :%@", max);

KVC底层实现

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好比说以下的一行KVC的代码： [person setValue:@"dahuan" forKey:@"name"]; 就会被编译器处理成： SEL sel = sel_get_uid ("setValue:forKey:"); IMP method = objc_msg_lookup (person->isa,sel); method(person, sel, @"dahuan", @"name");

KVC与runtime应用

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#import "MyModel.h" #import <objc/runtime.h> @implementation MyModel //解档 - (id)initWithCoder:(NSCoder *)decoder { if (self = [super init]) { unsigned int count = 0; //获取类中全部成员变量名 Ivar *ivar = class_copyIvarList([MyModel class], &count); for (int i = 0; i<count; i++) { Ivar iva = ivar[i]; const char *name = ivar_getName(iva); NSString *strName = [NSString stringWithUTF8String:name]; //进行解档取值 id value = [decoder decodeObjectForKey:strName]; //利用KVC对属性赋值 [self setValue:value forKey:strName]; } free(ivar); } return self; } //归档 - (void)encodeWithCoder:(NSCoder *)encoder { unsigned int count; Ivar *ivar = class_copyIvarList([MyModel class], &count); for (int i=0; i<count; i++) { Ivar iv = ivar[i]; const char *name = ivar_getName(iv); NSString *strName = [NSString stringWithUTF8String:name]; //利用KVC取值 id value = [self valueForKey:strName]; [encoder encodeObject:value forKey:strName]; } free(ivar); } @end

最后附苹果KVC官方文档：

https://developer.apple.com/library/mac/documentation/General/Conceptual/DevPedia-CocoaCore/KeyValueCoding.html

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