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;

}