src/Leevel/Pipeline/Pipeline.php
protected function stageGenerator(array $stage): Generator
    {
        array_unshift($stage, null);

        foreach ($stage as $item) {
            yield $this->stageCallback($item);
        }
    }
protected function stageGenerator(array $stage): Generator
    {
        array_unshift($stage, null);

        foreach ($stage as $item) {
            yield $this->stageCallback($item);
        }
    }
工序迭代器
添加一个空的迭代器，第一次迭代 next 自动移除.

@param array $stage

@return \Generator