（转）Objective-C 使用KVC

KVC ：Key Value Coding，直译是：键值编码。

Book类的代码，Book.h

1 #import <Foundation/Foundation.h>
2 
3 @interface Book : NSObject { 
4     NSString *name;
5 
6 }
7 
8 @end

Book.m

1 #import "Book.h"
2 
3 @implementation Book
4 
5 @end

这个类太简单，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：

 1 int main (int argc, const char * argv[]) { 
 2     NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
 3 
 4     Book *book = [[Book alloc] init];
 5     [book setValue:@"《Objective-C 入门》" forKey:@"name"];
 6     NSString *name = [book valueForKey:@"name"];
 7     NSLog(@"book name: %@", name];
 8     
 9     [pool drain]; 
10     return 0; 
11 }

这里会发现Objective-C的KVC很类似于Java中通过反射到类实例变量的方式。比如valueForKey方法先尝试在

Book实例上找到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，首先创建了一个新类Author，图书的作者，Author.h

1 #import <Cocoa/Cocoa.h>
2 
3 @interface Author : NSObject { 
4     NSString *name; 
5 }
6 
7 @end

也有个name属性，表示作者的姓名。Author.m

1 #import "Author.h"
2 
3 @implementation Author
4 
5 @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：

 1 #import <Foundation/Foundation.h>
 2 
 3 @class Author;
 4 
 5 @interface Book : NSObject { 
 6     NSString *name; 
 7     Author *author; 
 8 }
 9 
10 @end

实现文件还是什么都没有：

1 #import "Book.h"
2 
3 @implementation Book
4 
5 @end

在main方法中，通过KVC方式获取book的author的name属性：

 1 int main (int argc, const char * argv[]) { 
 2     NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
 3 
 4     Book *book=[[[Book alloc] init] autorelease]; 
 5     [book setValue:@"《Objective C入门》" forKey:@"name"]; 
 6     NSString *name=[book valueForKey:@"name"]; 
 7     NSLog(@"book name: %@",name); 
 8     
 9     Author *author=[[[Author alloc] init] autorelease]; 
10     [author setValue:@"Marshal Wu" forKey:@"name"]; 
11     [book setValue:author forKey:@"author"]; 
12     NSString *authorName=[book valueForKeyPath:@"author.name"]; 
13     NSLog(@"author name: %@",authorName); 
14     
15     [pool drain]; 
16     return 0; 
17 }

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：

 1 #import <Foundation/Foundation.h>
 2 
 3 @class Author;
 4 
 5 @interface Book : NSObject { 
 6     NSString *name; 
 7     Author *author; 
 8     float price; 
 9 }
10 
11 @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：

 1 int main (int argc, const char * argv[]) { 
 2     NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
 3 
 4     Book *book=[[[Book alloc] init] autorelease]; 
 5     [book setValue:@"《Objective C入门》" forKey:@"name"]; 
 6     NSString *name=[book valueForKey:@"name"]; 
 7     NSLog(@"book name: %@",name); 
 8     
 9     Author *author=[[[Author alloc] init] autorelease]; 
10     [author setValue:@"Marshal Wu" forKey:@"name"]; 
11     [book setValue:author forKey:@"author"]; 
12     NSString *authorName=[book valueForKeyPath:@"author.name"];
13     NSLog(@"author name: %@",authorName); 
14     
15     [book setValue:@"zhangsan" forKeyPath:@"author.name"]; 
16     
17     [book setValue:@"10.4" forKey:@"price"]; 
18     NSLog(@"book price is %@",[book valueForKey:@"price"]); 
19     
20     
21     [pool drain]; 
22     return 0; 
23 }

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：

 1 #import <Foundation/Foundation.h>
 2 
 3 @class Author;
 4 
 5 @interface Book : NSObject { 
 6     NSString *name; 
 7     Author *author; 
 8     float price; 
 9     NSArray *relativeBooks; 
10 }
11 
12 @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：

 1 int main (int argc, const char * argv[]) { 
 2     NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
 3 
 4     Book *book=[[[Book alloc] init] autorelease]; 
 5     [book setValue:@"《Objective C入门》" forKey:@"name"]; 
 6     NSString *name=[book valueForKey:@"name"]; 
 7     NSLog(@"book name: %@",name); 
 8     
 9     Author *author=[[[Author alloc] init] autorelease]; 
10     [author setValue:@"Marshal Wu" forKey:@"name"]; 
11     [book setValue:author forKey:@"author"]; 
12     NSString *authorName=[book valueForKeyPath:@"author.name"];
13     NSLog(@"author name: %@",authorName); 
14     
15     [book setValue:@"zhangsan" forKeyPath:@"author.name"]; 
16     
17     [book setValue:@"10.4" forKey:@"price"]; 
18     NSLog(@"book price is %@",[book valueForKey:@"price"]); 
19     
20     Book *book1=[[[Book alloc] init] autorelease]; 
21 [book1 setValue:@"5.0" forKey:@"price"]; 
22 
23 Book *book2=[[[Book alloc] init] autorelease]; 
24 [book2 setValue:@"4.0" forKey:@"price"]; 
25 
26 NSArray *books=[NSArray arrayWithObjects:book1,book2,nil]; 
27 [book setValue:books forKey:@"relativeBooks"]; 
28 NSLog(@"relative books price: %@",[book valueForKeyPath:@"relativeBooks.price"]); 
29     
30     [pool drain]; 
31     return 0; 
32 }

增加的代码见黑体部分。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：

1 NSArray *books=[NSArray arrayWithObjects:book1,book2,nil]; 
2 [book setValue:books forKey:@"relativeBooks"]; 
3 NSLog(@"relative books price: %@",[book valueForKeyPath:@"relativeBooks.price"]); 
4 NSLog(@"relative books count: %@",[book valueForKeyPath:@"relativeBooks.@count"]); 
5 NSLog(@"relative books price sum: %@",[book valueForKeyPath:@"relativeBooks.@sum.price"]); 
6 NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"relativeBooks.@avg.price"]); 
7 NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"relativeBooks.@max.price"]); 
8 NSLog(@"relative books price avg: %@",[book valueForKeyPath:@relativeBooks.@min.price]);

相关日志：

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：

 1 Book *book1=[[[Book alloc] init] autorelease]; 
 2 [book1 setValue:@"5.0" forKey:@"price"];
 3 
 4 Book *book2=[[[Book alloc] init] autorelease]; 
 5 [book2 setValue:@"4.0" forKey:@"price"];
 6 
 7 Book *book3=[[[Book alloc] init] autorelease]; 
 8 [book3 setValue:@"4.0" forKey:@"price"];
 9 
10 NSArray *books=[NSArray arrayWithObjects:book1,book2,book3,nil]; 
11 [book setValue:books forKey:@"relativeBooks"];
12 
13 NSLog(@"relative books price: %@",[book valueForKeyPath:@"relativeBooks.price"]); 
14 NSLog(@"relative books distinct price: %@",[book valueForKeyPath:@"relativeBooks.@distinctUnionOfObjects.price"]);
15 
16 NSLog(@"relative books count: %@",[book valueForKeyPath:@"relativeBooks.@count"]); 
17 NSLog(@"relative books price sum: %@",[book valueForKeyPath:@"relativeBooks.@sum.price"]); 
18 NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"relativeBooks.@avg.price"]); 
19 NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"relativeBooks.@max.price"]); 
20 NSLog(@"relative books price avg: %@",[book valueForKeyPath:@relativeBooks.@min.price]);

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：

1 Book *book4=[[Book alloc] init]; 
2 NSArray *bookProperties=[NSArray arrayWithObjects:@"name",@"price",nil]; 
3 NSDictionary *bookPropertiesDictionary=[book4 dictionaryWithValuesForKeys:bookProperties]; 
4 NSLog(@"book values: %@",bookPropertiesDictionary);
5 
6 NSDictionary *newBookPropertiesDictionary=[NSDictionary dictionaryWithObjectsAndKeys:@"《Objective C入门》",@"name", 
7                                            @"20.5",@"price",nil]; 
8 [book4 setValuesForKeysWithDictionary:newBookPropertiesDictionary]; 
9 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。　

转自：http://marshal.easymorse.com/tech/objc-%E4%BD%BF%E7%94%A8kvc

posted @ 2012-03-16 14:23 流浪的剑客阅读(522) 评论(0) 收藏举报

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（转）Objective-C 使用KVC

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