How to validate an API REST?

Hello, 

I'm extending Resource Controller

In situations that are passed as GET I can validate this way:

PHP Code:

public function index()
    {
        // this
        if (!$this->validate([
            'page'    => 'required|is_natural_no_zero|max_length[11]',
            'city_id' => 'required|is_natural_no_zero|max_length[11]',
            'search'  => 'permit_empty|max_length[50]'

        ])) return $this->fail($this->validator->getErrors());

      // continue
    } 


However, when it is necessary to pass the parameter through the function, how to solve this?

PHP Code:

    public function show($id = null)
    {
        // error
        if (!$this->validate([
            $id => 'required|is_natural_no_zero|max_length[11]',
            
        ])) return $this->fail($this->validator->getErrors());

       // continue
    } 


Can anyone help?