ObjC: 使用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：
#import <Foundation/Foundation.h>
@interface Book : NSObject {
NSString *name;}
@end
实现文件：
#import "Book.h"
@implementation Book
@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;
}
增加的代码见黑体斜体部分。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：
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:@"relativeBooks.@sum.price"]);
NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"relativeBooks.@avg.price"]);
NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"relativeBooks.@max.price"]);
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：
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:@"relativeBooks.@distinctUnionOfObjects.price"]);NSLog(@"relative books count: %@",[book valueForKeyPath:@"relativeBooks.@count"]);
NSLog(@"relative books price sum: %@",[book valueForKeyPath:@"relativeBooks.@sum.price"]);
NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"relativeBooks.@avg.price"]);
NSLog(@"relative books price avg: %@",[book valueForKeyPath:@"relativeBooks.@max.price"]);
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：
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。