NSFastEnumeration

实现了NSFastEnumeration协议的集合的都可以使用快速枚举的特性，如NSArray, NSDictionary, NSSet, NSEnumerator等。注意这里NSEnumerator也实现了NSFastEnumeration协议，这样可以执行一些特殊的任务，如使用reverseObjectEnumerator来对数组进行反同查询。

快速枚举的格式如下：

for(type loopVariable in expression)
{
  语句
}

下面的例子关注一下不同退出循环的方式，loopVariable值的变化：

#import <Foundation/Foundation.h>
#import "Photo.h"

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

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSArray *array = [NSArray arrayWithObjects:[[Photo alloc] init],[[Photo alloc] init],[[Photo alloc] init], nil];
    
    Photo *photo;
    for(photo in array)
    {
        [photo draw];
    }
    NSLog(@"经过完整遍历后，循环变量photo的值为 = %@",photo);
    
    int n=0;
    for(photo in array)
    {
        if(n == 2)break;
        [photo draw];
        n++;
    }
    NSLog(@"通过break退出循环，循环变量photo的值为 = %@，是最后一个遍历的值。",photo);
    

    [pool drain];
    return 0;
}

运行结果：

[Switching to process 4357 thread 0x0]
2011-05-11 10:27:04.235 demo06[4357:903] 55
2011-05-11 10:27:04.249 demo06[4357:903] 18
2011-05-11 10:27:04.252 demo06[4357:903] 51
2011-05-11 10:27:04.257 demo06[4357:903] 经过完整遍历后，循环变量photo的值为 = (null)
2011-05-11 10:27:04.262 demo06[4357:903] 60
2011-05-11 10:27:04.263 demo06[4357:903] 33
2011-05-11 10:27:04.264 demo06[4357:903] 通过break退出循环，循环变量photo的值为 = <Photo: 0x10010ca70>，是最后一个遍历的值。

NSDictionary

#import <Foundation/Foundation.h>
#import "Photo.h"

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

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:
                          [[Photo alloc] init], @"p1",
                          [[Photo alloc] init], @"p2",
                          [[Photo alloc] init], @"p3", nil];
    
    //for...in 只能遍历NSDictionary的Key
    NSString *key;
    for(key in dict)
    {
        [[dict objectForKey:key] draw];
    }
    
    //如果要遍历对象，需要通过objectEnumerator进行转换
    for(id obj in [dict objectEnumerator])
    {
        [obj draw];
    }
    

    [pool drain];
    return 0;
}