ObjC: 使用KVC

昨天介绍了KVO的简单使用,今天想起来在某个大神没落的博客中有篇详细介绍...http://marshal.easymorse.com/tech/objc使用kvc

转过来给刚看完KVO的童鞋们方便的了解KVC

KVC是什么?即:Key-Value Coding,直译是:键值编码。

还是没明白什么意思?先看看下面的代码。

Book类的代码,头文件:

1 #import <Foundation/Foundation.h>

2 

3 @interface Book : NSObject { 

4     NSString *name;

5 

6 }

7 

8 @end

实现文件:

1 #import "Book.h"

2 

3 @implementation Book

4 

5 @end

这个Book类太简单了,只有一个实例变量name。而且,按照以前掌握的技术,没有办法给这个变量赋值了。

下面KVC登场,在main方法中给Book实例的name属性赋值并获取该属性的值:

int main (int argc, const char * argv[]) { 

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];



    Book *book=[[Book alloc] init]; 

    [book setValue:@"《Objective C入门》" forKey:@"name"]; 

    NSString *name=[book valueForKey:@"name"]; 

    NSLog(@"book name: %@",name); 

    

    [pool drain]; 

    return 0; 

}

这里会发现ObjC的KVC很类似Java中通过反射得到类实例变量的方式。比如valueForKey方法先尝试在Book实例上找getName方法,如果找到就调用。如果没有找到,则查找实例是否有name变量或者_name变量。如果还没找到,会抛出类似下面的异常:

Terminating app due to uncaught exception ‘NSUnknownKeyException’, reason: ‘[<Book 0x10010c730> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key name1.’

下面把代码做一点修改,首先创建了个新类Author,图书的作者,头文件:

#import <Cocoa/Cocoa.h>



@interface Author : NSObject { 

    NSString *name; 

}



@end

也有个name属性,表示作者的姓名。实现文件什么也没写:

#import "Author.h"



@implementation Author



@end

然后,将author属性添加到Book类中,即每个Book实例都有一个author属性。头文件:

#import <Foundation/Foundation.h>



@class Author;



@interface Book : NSObject { 

    NSString *name; 

    Author *author; 

}



@end

实现文件还是什么都没有:

#import "Book.h"



@implementation Book



@end

在main方法中,通过kvc方式获取book的author的name属性:

int main (int argc, const char * argv[]) { 

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];



    Book *book=[[[Book alloc] init] autorelease]; 

    [book setValue:@"《Objective C入门》" forKey:@"name"]; 

    NSString *name=[book valueForKey:@"name"]; 

    NSLog(@"book name: %@",name); 

    

    Author *author=[[[Author alloc] init] autorelease]; 

    [author setValue:@"Marshal Wu" forKey:@"name"]; 

    [book setValue:author forKey:@"author"]; 

    NSString *authorName=[book valueForKeyPath:@"author.name"]; 

    NSLog(@"author name: %@",authorName); 

    

    [pool drain]; 

    return 0; 

}

可以看到,写法很类似JSP的EL表达式:${book.author.name}

在ObjC的世界里叫Path,路径。当然,你也可以:

[book setValue:@"zhangsan" forKeyPath:@"author.name"];

通过路径设置属性。

KVC还有一个很重要的特点,自动装箱拆箱功能。这在ObjC中是仅有的,其他情况下均需要使用比如NSNumber来手动拆装箱的。

比如Book类头文件做了下面的增加:

#import <Foundation/Foundation.h>



@class Author;



@interface Book : NSObject { 

    NSString *name; 

    Author *author; 

    float price; 

}



@end

实现文件还是没有动,不提了。main方法增加了对price赋值和获取值的调用,使用KVC方式:

int main (int argc, const char * argv[]) { 

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];



    Book *book=[[[Book alloc] init] autorelease]; 

    [book setValue:@"《Objective C入门》" forKey:@"name"]; 

    NSString *name=[book valueForKey:@"name"]; 

    NSLog(@"book name: %@",name); 

    

    Author *author=[[[Author alloc] init] autorelease]; 

    [author setValue:@"Marshal Wu" forKey:@"name"]; 

    [book setValue:author forKey:@"author"]; 

    NSString *authorName=[book valueForKeyPath:@"author.name"];

    NSLog(@"author name: %@",authorName); 

    

    [book setValue:@"zhangsan" forKeyPath:@"author.name"]; 

    

    [book setValue:@"10.4" forKey:@"price"]; 

    NSLog(@"book price is %@",[book valueForKey:@"price"]); 

    

    

    [pool drain]; 

    return 0; 

}

可以看到给price输入的是NSString类型,但是没有问题,因为KVC方式会根据字符串自动转型为适当的数值。再看打印price属性,%@是打印对象,而price属性是float基本型,这里KVC肯定做了自动装箱的处理,将基本型转为NSNumber对象。

KVC还具备对集合的操作能力。比如,图书可以有相关图书,这是个1对多的关系。可以用集合来表示,这里用NSArray表示,在Book类的头文件中改动:

#import <Foundation/Foundation.h>



@class Author;



@interface Book : NSObject { 

    NSString *name; 

    Author *author; 

    float price; 

    NSArray *relativeBooks; 

}



@end

如果想得到相关图书的价格NSArray,可以使用KVC方式,见main方法:

int main (int argc, const char * argv[]) { 

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];



    Book *book=[[[Book alloc] init] autorelease]; 

    [book setValue:@"《Objective C入门》" forKey:@"name"]; 

    NSString *name=[book valueForKey:@"name"]; 

    NSLog(@"book name: %@",name); 

    

    Author *author=[[[Author alloc] init] autorelease]; 

    [author setValue:@"Marshal Wu" forKey:@"name"]; 

    [book setValue:author forKey:@"author"]; 

    NSString *authorName=[book valueForKeyPath:@"author.name"];

    NSLog(@"author name: %@",authorName); 

    

    [book setValue:@"zhangsan" forKeyPath:@"author.name"]; 

    

    [book setValue:@"10.4" forKey:@"price"]; 

    NSLog(@"book price is %@",[book valueForKey:@"price"]); 

    

   Book *book1=[[[Book alloc] init] autorelease]; 

    [book1 setValue:@"5.0" forKey:@"price"]; 

    

    Book *book2=[[[Book alloc] init] autorelease]; 

    [book2 setValue:@"4.0" forKey:@"price"]; 

    

    NSArray *books=[NSArray arrayWithObjects:book1,book2,nil]; 

    [book setValue:books forKey:@"relativeBooks"]; 

    NSLog(@"relative books price: %@",[book valueForKeyPath:@"relativeBooks.price"]); 

    

    [pool drain]; 

    return 0; 

}

日志将打印出相关图书的价格列表:

2011-05-26 19:27:57.456 ReleaseMemoDemo[10042:a0f] book name: 《Objective C入门》 

2011-05-26 19:27:57.461 ReleaseMemoDemo[10042:a0f] author name: Marshal Wu 

2011-05-26 19:27:57.462 ReleaseMemoDemo[10042:a0f] book price is 10.4 

2011-05-26 19:27:57.463 ReleaseMemoDemo[10042:a0f] relative books price: ( 

    5, 

    4 

)

KVC还能对集合做运算,比如想得到相关图书的个数、相关图书的价格总和、相关图书的平均价格、价格的最大值和价格的最小值,见下面的代码:

NSArray *books=[NSArray arrayWithObjects:book1,book2,nil]; 

[book setValue:books forKey:@"relativeBooks"]; 

NSLog(@"relative books price: %@",[book valueForKeyPath:@"relativeBooks.price"]); 

NSLog(@"relative books count: %@",[book valueForKeyPath:@"relativeBooks.@count"]); 

NSLog(@"relative books price sum: %@",[book valueForKeyPath:@"[email protected]"]); 

NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"[email protected]"]); 

NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"[email protected]"]); 

NSLog(@"relative books price avg: %@",[book valueForKeyPath:@[email protected]]);
2011-05-26 19:45:27.786 ReleaseMemoDemo[10289:a0f] relative books price: ( 

    5, 

    4 

) 

2011-05-26 19:45:27.787 ReleaseMemoDemo[10289:a0f] relative books count: 2 

2011-05-26 19:45:27.788 ReleaseMemoDemo[10289:a0f] relative books price sum: 9 

2011-05-26 19:45:27.788 ReleaseMemoDemo[10289:a0f] relative books price avg: 4.5 

2011-05-26 19:45:27.789 ReleaseMemoDemo[10289:a0f] relative books price avg: 5 

2011-05-26 19:45:27.789 ReleaseMemoDemo[10289:a0f] relative books price avg: 4

另外,如果想获得没有重复的价格集合,可以这样:

Book *book1=[[[Book alloc] init] autorelease]; 

[book1 setValue:@"5.0" forKey:@"price"];



Book *book2=[[[Book alloc] init] autorelease]; 

[book2 setValue:@"4.0" forKey:@"price"];



Book *book3=[[[Book alloc] init] autorelease]; 

[book3 setValue:@"4.0" forKey:@"price"];



NSArray *books=[NSArray arrayWithObjects:book1,book2,book3,nil]; 

[book setValue:books forKey:@"relativeBooks"];



NSLog(@"relative books price: %@",[book valueForKeyPath:@"relativeBooks.price"]); 

NSLog(@"relative books distinct price: %@",[book valueForKeyPath:@"[email protected]"]);



NSLog(@"relative books count: %@",[book valueForKeyPath:@"relativeBooks.@count"]); 

NSLog(@"relative books price sum: %@",[book valueForKeyPath:@"[email protected]"]); 

NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"[email protected]"]); 

NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"[email protected]"]); 

NSLog(@"relative books price avg: %@",[book valueForKeyPath:@[email protected]]);

这里增加了book3实例,它的价格和book2相同。在使用@distinctUnionOfObjects后,发现效果是消除重复的价格:

011-05-26 19:55:41.123 ReleaseMemoDemo[10378:a0f] book price is 10.4 

2011-05-26 19:55:41.124 ReleaseMemoDemo[10378:a0f] relative books price: ( 

    5, 

    4, 

    4 

) 

2011-05-26 19:55:41.124 ReleaseMemoDemo[10378:a0f] relative books distinct price: ( 

    4, 

    5 

)

KVC还可以在一个语句中为实例的多个属性赋值:

Book *book4=[[Book alloc] init]; 

NSArray *bookProperties=[NSArray arrayWithObjects:@"name",@"price",nil]; 

NSDictionary *bookPropertiesDictionary=[book4 dictionaryWithValuesForKeys:bookProperties]; 

NSLog(@"book values: %@",bookPropertiesDictionary);



NSDictionary *newBookPropertiesDictionary=[NSDictionary dictionaryWithObjectsAndKeys:@"《Objective C入门》",@"name", 

                                           @"20.5",@"price",nil]; 

[book4 setValuesForKeysWithDictionary:newBookPropertiesDictionary]; 

NSLog(@"book with new values: %@",[book4 dictionaryWithValuesForKeys:bookProperties]);

另外,还有两个比较高级的内容:

  • nil和覆盖setNilValueForKey方法
  • 覆盖valueForUndefinedKey方法

可自行看reference了解。

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