iOS --- 关于KVC使用的一些小技巧

KVC是iOS开发中经常会用到的技巧, 主要包括valueForKey:/setValue:ForKey:, valueForKeyPath:/setValue:forKeyPath:两队组合方法.
valueForKey:会首先查找以参数名命名的getter方法, 如果没有找到, 则在对象内寻找名称格式为_key或key的实例变量.
另外还要dictionaryWithValuesForKeys:/setValuesForKeysWithDictionary方法用于获取或设置指定的内容.

这里有两个类:

#pragma mark - Dog

@interface Dog : NSObject

@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) NSInteger age;
@property (nonatomic, copy) NSString *city;

@end

@implementation Dog

@end



#pragma mark - Person

@interface Person : NSObject

@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) NSInteger age;

@property (nonatomic, strong) Dog *dog;

@property (nonatomic, strong) NSArray<Dog *> *dogs;

@end

@implementation Person

@end

其中, Person类有一个dogs属性. 下面将在这两个类的基础上展示一些小的demo.

基本使用

Dog *aDog = [[Dog alloc] init];
aDog.name = @"My Dog";
aDog.age = 2;
NSString *dogName = [aDog valueForKey:@"name"];
NSInteger dogAge = [[aDog valueForKey:@"age"] integerValue];
NSLog(@"dogName : %@, dogAge : %ld", dogName, dogAge);

键路径

Person *aPerson = [[Person alloc] init];
aPerson.name = @"Chris";
aPerson.age = 18;
aPerson.dog = aDog;

NSString *name = [aPerson valueForKeyPath:@"name"];
NSInteger age = [[aPerson valueForKeyPath:@"age"] integerValue];
dogName = [aPerson valueForKeyPath:@"dog.name"];
dogAge = [[aPerson valueForKeyPath:@"dog.age"] integerValue];
NSLog(@"testKeyPath: name : %@, age : %ld", name, age);
NSLog(@"testKeyPath: dogName : %@, dogAge : %ld", dogName, dogAge);

注意, 键路径的写法遵循点语法.

批量操作

使用KVC, 有时候可以非常方便地进行一些批量的操作.

Dog *dog1 = [[Dog alloc] init];
dog1.name = @"Dog 1";
dog1.age = 1;
dog1.city = @"Shanghai";

Dog *dog2 = [[Dog alloc] init];
dog2.name = @"Dog 2";
dog2.age = 2;
dog2.city = @"Shanghai";

Dog *dog3 = [[Dog alloc] init];
dog3.name = @"Dog 3";
dog3.age = 3;
dog3.city = @"Beijing";

Person *aPerson = [[Person alloc] init];
aPerson.name = @"Chris";
aPerson.age = 18;
aPerson.dogs = @[dog1, dog2, dog3];

测试代码如下:

NSArray *dogNames = [aPerson valueForKeyPath:@"dogs.name"];
NSArray *dogAges = [aPerson valueForKeyPath:@"dogs.age"];
NSMutableString *str = [[NSMutableString alloc] init];
[str appendString:@"dogNames :"];
for (NSInteger i = 0; i < dogNames.count; i++) {
    [str appendFormat:@" %@, %ld years old. ", dogNames[i], [dogAges[i] integerValue]];
}

NSArray实现valueForKeyPath:的方法是循环遍历它的内容并向每个对象发送消息.

批量修改如下:

[aPerson setValue:@"Xiamen" forKeyPath:@"dogs.city"];

cities = [aPerson valueForKeyPath:@"dogs.@distinctUnionOfObjects.city"];
NSLog(@"cities : %@", cities);

快速运算

使用KVC进行快速运算的键路径语法类似于 *** dogs.@count *** , *** dogs.@sum.age *** 等.

// dogs表示取出内容, @count即进行计算, 通知KVC机制进行键路径左侧值的对象总数
NSNumber *countOfDogs = [aPerson valueForKeyPath:@"dogs.@count"];
NSLog(@"count of dogs : %@", countOfDogs);

// 获取@sum左侧的集合, 对集合中的每个对象执行右侧操作age, 将结果组成一个集合并返回.
NSNumber *ageCountOfDogs = [aPerson valueForKeyPath:@"dogs.@sum.age"];
NSLog(@"ageCountOfDogs of dogs : %@", ageCountOfDogs);

NSNumber *ageAvgOfDogs = [aPerson valueForKeyPath:@"dogs.@avg.age"];
NSLog(@"age avg of dogs : %@", ageAvgOfDogs);
NSLog(@"age min : %@, max : %@", [aPerson valueForKeyPath:@"dogs.@min.age"], [aPerson valueForKeyPath:@"dogs.@max.age"]);

// 获取唯一值
NSArray *cities = [aPerson valueForKeyPath:@"dogs.@distinctUnionOfObjects.city"];
NSLog(@"cities : %@", cities);

这里就不作过多描述.

dictionaryWithValuesForKeys:/setValuesForKeysWithDictionary:

直接看代码更直观:

// 根据设置的key, 来进行组合结果.
Dog *lastDog = [[aPerson valueForKeyPath:@"dogs"] lastObject];
NSArray *keys = @[@"name"];
NSDictionary *values = [lastDog dictionaryWithValuesForKeys:keys];
NSLog(@"values : %@", values);

// 使用setValuesForKeysWithDictionary根据字典对Dog进行修改
NSDictionary *newValues = @{@"name": @"My Dog"};
[lastDog setValuesForKeysWithDictionary:newValues];
values = [lastDog dictionaryWithValuesForKeys:keys];
NSLog(@"values : %@", values);

Demo

Demo请参考:
DemoKVC
需要注意的是:
*** 不要滥用KVC, KVC需要解析字符串来计算需要的结果, 因此速度较慢. 且无法进行错误检查. ***

iOS --- 关于KVC使用的一些小技巧