NSEnumerator的价值是什么？

Question

什么时候...在...可以吗？

具体来说,当我们可以写:

NSArray *array;
// array allocated and initialized here.
for (id obj in array) {
  // do something to the object here
}

我们为什么要使用NSEnumerator？

Answer 1

NSEnumerator是在快速枚举(for/in循环)可用之前创建的.如果您愿意,可以将其视为向后兼容性.

但是使用NSEnumerator,您可以按自定义顺序枚举集合,例如向后:

NSEnumerator* enu = [array reverseObjectEnumerator];
id object;
while ((object = [enu nextObject])) {
 ...
}

(当然,既然NSEnumerator也支持/ in循环,你可以使用更好的方法:

for (id object in [array reverseObjectEnumerator]) {
  ...
}

)

或者通过继承NSEnumerator来定义自己的迭代器类,例如

#import <Foundation/Foundation.h>

@interface RangeEnumerator : NSEnumerator {
    int cur, len;
}
+(RangeEnumerator*)enumeratorWithLength:(int)length;
-(id)initWithLength:(int)length;
-(id)nextObject;
@end
@implementation RangeEnumerator
-(id)initWithLength:(int)length {
    if ((self = [super init]))
        len = length;
    return self;
}
+(RangeEnumerator*)enumeratorWithLength:(int)length {
    return [[(RangeEnumerator*)[self alloc] initWithLength:length] autorelease];
}
-(id)nextObject {
    if (cur < len)
        return [NSNumber numberWithInt:cur++];
    else
        return nil;
}
@end

int main () {
    NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];

    for (NSNumber* num in [RangeEnumerator enumeratorWithLength:12])
        printf("%d\n", [num intValue]);

    [pool drain];

    return 0;
}