你看看这个源码来意会一下：
/**
* Run a map over each of the items.
*
* @param callable $callback
* @return static
*/
public function map(callable $callback)
{
$keys = array_keys($this->items);
$items = array_map($callback, $this->items, $keys);
return new static(array_combine($keys, $items));
}
public function each(callable $callback)
{
foreach ($this->items as $key => $item) {
if ($callback($item, $key) === false) {
break;
}
}
return $this;
}
