ObjC: 使用KVC(转)

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：

#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; 
}

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]);

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。