PHP - Iterating twice a generic iterable

Question

In PHP 7.1 there is a new iterable psudo-type which abstracts arrays and Traversable objects.

Suppose that in my code I have a class like the following:

class Foo
{
    private $iterable;

    public function __construct(iterable $iterable)
    {
        $this->iterable = $iterable;
    }

    public function firstMethod()
    {
        foreach ($this->iterable as $item) {...}
    }

    public function secondMethod()
    {
        foreach ($this->iterable as $item) {...}
    }
}

This works fine is $iterable is an array or an Iterator, except when $iterable is a Generator. In that case in fact, calling firstMethod() and then secondMethod() would produce the following Exception: Cannot traverse an already closed generator.

Is there a way to avoid this issue?

Choli

Answer 1

Generators can't be rewound. If you want to avoid this issue, you have to make a new generator. This can be done automatically if you create an object that implements IteratorAggregate:

class Iter implements IteratorAggregate
{
    public function getIterator()
    {
        foreach ([1, 2, 3, 4, 5] as $i) {
            yield $i;
        }
    }
}

Then just pass an instance of this object as your iterator:

$iter = new Iter();
$foo = new Foo($iter);
$foo->firstMethod();
$foo->secondMethod();

Output:

1
2
3
4
5
1
2
3
4
5

Recept

Answer 2

After some research, I've found:

• non-working native CachingIterator, which does just keep values inside, but doesn't return them on consecutive iteration.
• splendid CachingIteratorAggregate from loophp/iterators library. It does the exact same thing we expect it to - keeps iteration results and returns them on second iteration.

Hence, consider checking out https://github.com/loophp/iterators

Ottie