KVO与KVC的使用（转）

转自http://www.cnblogs.com/mybkn/archive/2013/05/29/3104166.html

KVO:

在看KVO之前，有必要先了解下KVC，即，Key-Value Coding 键值对编程。通过key-value可以方便的存取数据。

具体的操作简单说就是：setValue:forKey:   valueForKey：

//book Object
//.h
#import <Foundation/Foundation.h>
@class Author;
@interface Book : NSObject{
    NSString *name;
    Author *author;
    float price;
    NSArray *relativeBooks;
}
@end

#import "Book.h"
@implementation Book
@end
Book *book = [[Book alloc] init];
    [book setValue:@"iOS book" forKey:@"name"];//这里要注意实例变量的名字不要写错，写错了不会有提示
    NSString *name = [book valueForKey:@"name"];
    NSLog(@">> %@",name);

    Author *author = [[Author alloc] init];
    [author setValue:@"Zhan" forKey:@"name"];
    [book setValue:author forKey:@"author"];

    NSString *authorName = [book valueForKeyPath:@"author.name"];
    NSLog(@">> %@",authorName);

    [book setValue:@"" forKey:@"price"];
    NSLog(@">> %@",[book valueForKey:@"price"]);

    Book *book1 = [[Book alloc] init];
    [book1 setValue:@"" forKey:@"price"];
    Book *book2 = [[Book alloc] init];
    [book2 setValue:@"" forKey:@"price"];

    NSArray *books = [NSArray arrayWithObjects:book1,book2,nil];
    [book setValue:books forKey:@"relativeBooks"];
    NSLog(@">>%@",[book valueForKeyPath:@"relativeBooks.price"]);

KVO是基于kvc实现的，采取的是观察者的模式：

    book4=[[Book alloc] init];
    //增加观察者，为本类，keypath为book中的price对象，所以为price
    [book4 addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:nil];
    //修改值
    [book4 setValue:@"" forKey:@"price"];

//回掉方法，只要book4的值一改变就会回调这个方法

-(void) observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context{
NSLog(@"cel back");
if([keyPath isEqual:@"price"]){
NSLog(@">>>>>>>price is changed");
NSLog(@"old price is %@",[change objectForKey:@"old"]);
NSLog(@"new price is %@",[change objectForKey:@"new"]);
}
}