WordPress: cannot access callback_args key values inside callback function - wordpress

Question

I am developing a simple plugin based custom post type. The post type is registered fine. Now I want to create some meta boxes and pass a few property values via callback arguments. This is what I tried:
function wpcd_add_dealer_meta_boxes() {
add_meta_box(
'dealer_first_name',
'First Name',
array($this, 'wpcd_meta_box_first_name_markup'),
'dealers',
'normal',
'default',
array(
'id' => 'first_name',
'name' => 'first_name',
'type' => 'text',
'placeholder' => 'Enter first name',
'maxlength' => '30',
'spellcheck' => 'true',
'autocomplete' => 'off'
)
);
}
And here is my callback function with an argument:
function wpcd_meta_box_first_name_markup($args) {
$html = '<input ';
$html.= 'type="' . $args->type . '" ';
$html.= 'id="' .$args->id . '" ';
$html.= 'name="' .$args->name . '" ';
if( isset($args->required) && ($args->required == 'true' || $args->required == '1' ) ) {
$html.= 'required ';
}
if( isset($args->placeholder) && $args->placeholder != '' ) {
$html.= 'placeholder="' . esc_attr( $args->placeholder ) . '" ';
}
if( isset($args->maxlength) ) {
$html.= 'maxlength="' . $args->maxlength . '" ';
}
if( isset($args->spellcheck) && ($args->spellcheck == 'true' ) ) {
$html.= 'spellcheck="' . $args->spellcheck . '" ';
}
if( isset($args->autocomplete) && ($args->autocomplete == 'on' ) ) {
$html.= 'autocomplete="' . $args->autocomplete . '" ';
}
$html.= '/>';
echo $html;
}
But I cannot get the values like $args->id, $args->name etc inside the function. To be precise, all the values are empty, where I did not check if(isset(...)). And where I did those are simply ignored.
With my above code I am expecting the following markup as output:
<input type="text" id="first_name" name="last_name" required placeholder="Enter firstname" maxlength="30" autocomplete="off" spellcheck="true" />
while the actual output is
<input type="" id="" name="" />
Attributes type, id and name are not wrapped inside an if(isset()) block so they are being generated (with empty values) and whatever is wrapped inside if(isset()) block, are simply being ignored, as if they are not set at all!
What I am missing or doing wrong?
Any suggestion would be a life saver for me.

Answer 1

If you check the documentation for add_meta_box() carefully, you'll see:
($callback_args (array) (Optional) Data that should be set as the
$args property of the box array (which is the second parameter passed
to your callback).
The first parameter passed to your callback is a WP_Post object. The second is your parameter array. So, try:
function wpcd_meta_box_first_name_markup($post, $args) { ...
And then access your arguments as you'd expect:
$args['type']