class InputStream

class InputStream implements \IteratorAggregate {
    private ?\Closure $onEmpty = null;
    private array $input = [];
    private bool $open = true;
    
    /**
     * Sets a callback that is called when the write buffer becomes empty.
     */
    public function onEmpty(?callable $onEmpty = null) : void {
        $this->onEmpty = null !== $onEmpty ? $onEmpty(...) : null;
    }
    
    /**
     * Appends an input to the write buffer.
     *
     * @param resource|string|int|float|bool|\Traversable|null $input The input to append as scalar,
     *                                                                stream resource or \Traversable
     */
    public function write(mixed $input) : void {
        if (null === $input) {
            return;
        }
        if ($this->isClosed()) {
            throw new RuntimeException(\sprintf('"%s" is closed.', static::class));
        }
        $this->input[] = ProcessUtils::validateInput(__METHOD__, $input);
    }
    
    /**
     * Closes the write buffer.
     */
    public function close() : void {
        $this->open = false;
    }
    
    /**
     * Tells whether the write buffer is closed or not.
     */
    public function isClosed() : bool {
        return !$this->open;
    }
    public function getIterator() : \Traversable {
        $this->open = true;
        while ($this->open || $this->input) {
            if (!$this->input) {
                (yield '');
                continue;
            }
            $current = array_shift($this->input);
            if ($current instanceof \Iterator) {
                yield from $current;
            }
            else {
                (yield $current);
            }
            if (!$this->input && $this->open && null !== ($onEmpty = $this->onEmpty)) {
                $this->write($onEmpty($this));
            }
        }
    }

}