iOS中category动态添加属性的方法(关联和KVC)

本文转载自：http://www.cnblogs.com/wengzilin/p/4331685.html?utm_source=tuicool
   想到要如何为所有的对象增加实例变量吗？我们知道，使用Category可以很方便地为现有的类增加方法，但却无法直接增加实例变量。不过从Mac OS X v10.6开始，系统提供了Associative References，这个问题就很容易解决了。这种方法也就是所谓的关联(association)，我们可以在runtime期间动态地添加任意多的属性，并且随时读取。所用到的两个重要runtime API是：

OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)

OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key) 
__OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

现在我们结合一个实际的例子来说明他们的用法。假设我们现在打算利用category对UILabel进行属性补充，可以这么做：

#import <UIKit/UIKit.h> #import <objc/runtime.h> @interface UILabel (Associate)

- (void) setFlashColor:(UIColor *) flashColor;

- (UIColor *) getFlashColor;

@end

#import "UILabel+Associate.h" 

@implementation UILabel (Associate) 

static char flashColorKey; 

- (void) setFlashColor:(UIColor *) flashColor {
    objc_setAssociatedObject(self, &flashColorKey, flashColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (UIColor *) getFlashColor { 

    return objc_getAssociatedObject(self, &flashColorKey);
} 

@end

上面的例子有几个需要注意的地方：

1、key：我们注意到在函数签名中key的类型const void *，这表示key仅仅是一个地址，而不是字符串的内容，这也是为说明flashColorKey没有初始化的原因，因为具体指向什么内容我们无所谓，我们要的仅仅是地址！如果在setAssocaitedObject中你传入的是flashColorKey，那get方法得到的值将会是nil。正确的应该是传入地址&flashColorKey。

2、policy：这里的policy跟属性声明中的retain、assign、copy是一样的，不再赘述

下面我们再来看另一个例子，来源于APPLE GUIDE：

#import  

#import  

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

 /*Seciton 0. 关联数据的Key和Value*/ 

 static char overviewKey;  

 static const char *myOwnKey = "VideoProperty\0";  

 static const char intValueKey = 'i';
    NSArray *array = [[NSArray alloc]   initWithObjects:@ "One", @"Two", @"Three", nil];

  // For the purposes of illustration, use initWithFormat: to ensure // we get a deallocatable string 

  NSString *overview = [[NSString alloc]   initWithFormat:@"%@", @"First three numbers"];  

  NSString *videoKeyValue = @"This is a video";

  NSNumber *intValue = [[NSNumber alloc] initWithInt:5];

  /*Section 1. 关联数据设置部分*/ 

  objc_setAssociatedObject (   array,   &overviewKey,   overview,   OBJC_ASSOCIATION_RETAIN  );

  [overview release];

  objc_setAssociatedObject (   array,   myOwnKey,   videoKeyValue,   OBJC_ASSOCIATION_RETAIN  );

  objc_setAssociatedObject (   array,   &intValueKey,   intValue,   OBJC_ASSOCIATION_RETAIN  );

  /*Section 3. 关联数据查询部分*/

  NSString *associatedObject =  (NSString *) objc_getAssociatedObject (array, &overviewKey);
    NSLog(@"associatedObject: %@", associatedObject);

  NSString *associatedObject2 = (NSString *) objc_getAssociatedObject(array, myOwnKey);

  NSLog(@"Video Key value is %@", associatedObject2);

  NSString *assObject3 = (NSString *) objc_getAssociatedObject(array, &myOwnKey);

  if( assObject3 )  {

   NSLog(@"不会进入这里! assObject3 应当为nil!");  }

  else  {   NSLog(@"OK. 通过myOwnKey的地址是得不到数据的!");  }
        NSNumber *assKeyValue = (NSNumber *) objc_getAssociatedObject(array, &intValueKey);

      NSLog(@"Int value is %d",[assKeyValue intValue]);

  /*Section 3. 关联数据清理部分*/ 

  objc_setAssociatedObject (   array,   &overviewKey,   nil,   OBJC_ASSOCIATION_ASSIGN  );

  objc_setAssociatedObject (   array,   myOwnKey,    nil,   OBJC_ASSOCIATION_ASSIGN  );

  objc_setAssociatedObject (   array,   &intValueKey,   nil,   OBJC_ASSOCIATION_ASSIGN  );  [array release];
    } return 0;
}

2、kvc的方式
// 对NSObject扩展Simon特有的方法及属性
#import

@interface NSObject (Simon)

// book id
- (NSInteger)bookId;
- (void)setBookId:(NSInteger)_bookId;

// book name
- (NSString *)bookName;
- (void)setBookName:(NSString *)_bookName;

// otherInfo
- (NSDictionary *)otherInfo;
- (void)setOtherInfo:(NSDictionary *)_otherInfo;

@end

#import "NSObject+Simon.h"
@implementation NSObject (Simon)

static NSMutableDictionary *_td = nil;

+ (NSMutableDictionary *)_td{
    if (_td == nil) {
        _td = [NSMutableDictionary dictionary];
    }
    return _td;
}


// book id
- (NSInteger)bookId {
    return [[[[self class] _td] objectForKey:@"bookId"] intValue];
}

- (void)setBookId:(NSInteger)_bookId {
    NSNumber *bookId = [NSNumber numberWithInt:_bookId];
    [[[self class] _td] setObject:bookId forKey:@"bookId"];
}

// book name
- (NSString *)bookName{
    return [[[self class] _td] objectForKey:@"bookName"];
}
- (void)setBookName:(NSString *)_bookName{
    NSString *bookName = [_bookName copy];
    [[[self class] _td] setObject:bookName forKey:@"bookName"];
    [bookName release];
}

// otherInfo
- (NSDictionary *)otherInfo{
    return [[[self class] _td] objectForKey:@"otherInfo"];
}

- (void)setOtherInfo:(NSDictionary *)_otherInfo{
    NSDictionary *otherInfo = [_otherInfo copy];
    [[[self class] _td] setObject:otherInfo forKey:@"otherInfo"];
    [otherInfo release];
}


@end


#import

@interface SFBook : NSObject

@end

#import "SFBook.h"

@implementation SFBook

@end

// 使用这个属性扩展

SFBook *book = [[SFBook alloc] init];
book.bookId = 1;
book.bookName = @"IOS设计模式";
   
NSLog(@">>>>>%d>>>>>>%@",book.bookId,book.bookName);
[book release];