目标c“对于每个” （快速枚举） - 收集评估？ [英] Objective c "for each" (fast enumeration) -- evaluation of collection?

问题描述

从实验看来，集合表达式只被评估一次。考虑这个例子：

  static NSArray * a; 
 
  - （NSArray *）fcn 
 {
 if（a == nil）
a = [NSArray arrayWithObjects：@one，@two，@ 三，无]; 
 NSLog（@called）; 
返回一个; 
 
 $ b ... 
 
（[self fcn]中的NSString * s）
 NSLog（@％@，s）; 
  

输出结果为：

 2010-10-07 07：37：31.420 WidePhotoViewer Lite [23694：207]一个
 2010-10-07 07：37：31.425 WidePhotoViewer Lite [23694：207]两张
 2010-10-07 07：37：31.425 WidePhotoViewer Lite [23694：207]三张
  

指示[self fcn]只被调用一次。

任何人都可以确认这是指定（而不是仅仅观察）的行为？

我想要做的是这样的：

  for（UIView * v in [innerView subviews]）{
  

而不是这个：

$ $ $ $ $ $ $ $ NSArray * vs = [innerView subviews];
for（UIView * v in vs）{

想法？

解决方案

这种for循环称为快速枚举（查看NSFastEnumeration对象）。苹果公司的文档说，在为obj表达式，表达式产生一个符合NSFastEnumeration协议的对象，所以我想这是正确的行为：函数被调用一次，迭代器被创建一次，并在循环中使用。 p>

It seems from experimentation that the collection expression is evaluated only once. Consider this example:

static NSArray *a;

- (NSArray *)fcn
{
    if (a == nil)
        a = [NSArray arrayWithObjects:@"one", @"two", @"three", nil];
    NSLog(@"called");
    return a;
}

...

for (NSString *s in [self fcn])
    NSLog(@"%@", s);

The output is:

2010-10-07 07:37:31.419 WidePhotoViewer Lite[23694:207] called
2010-10-07 07:37:31.420 WidePhotoViewer Lite[23694:207] one
2010-10-07 07:37:31.425 WidePhotoViewer Lite[23694:207] two
2010-10-07 07:37:31.425 WidePhotoViewer Lite[23694:207] three

indicating that [self fcn] is called only once.

Can anyone confirm that this is the specified (as opposed to merely observed) behavior?

What I have in mind is doing something like this:

for (UIView *v in [innerView subviews]) {

instead of this:

NSArray *vs = [innerView subviews];
for (UIView *v in vs) {

Thoughts?

解决方案

This kind of for loop is called a "fast enumeration" (look at the NSFastEnumeration object). Apple's documentation says that in "for obj in expression", expression yields an object that conforms to the NSFastEnumeration protocol, so I guess that's the correct behaviour: the function is called once, an iterator is created once and used in the loop.

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