Add a checkbox in the post meta box - wordpress

Question

I want to add a checkbox in product post type. So i wrote the code
add_action( 'add_meta_boxes', 'smashing_add_post_meta_boxes' );
/* Create one or more meta boxes to be displayed on the post editor screen. */
function smashing_add_post_meta_boxes() {
add_meta_box(
'smashing-post-class', // Unique ID
esc_html__( 'Post Class', 'example' ), // Title
'smashing_post_class_meta_box', // Callback function
'product', // Admin page (or post type)
'side', // Context
'default' // Priority
);
}
/* Display the post meta box. */
function smashing_post_class_meta_box( $object, $box ) { ?>
<?php wp_nonce_field( basename( __FILE__ ), 'smashing_post_class_nonce' ); ?>
<p>
<label for="smashing-post-class"><?php _e( "Add a custom CSS class, which will be applied to WordPress' post class.", 'example' ); ?></label>
<br />
<input class="widefat" type="checkbox" name="smashing-post-class" id="smashing-post-class" value="<?php echo esc_attr( get_post_meta( $object->ID, 'smashing_post_class', true ) ); ?>" size="30" />What's New
</p>
<?php }
/* Save post meta on the 'save_post' hook. */
add_action( 'save_post', 'smashing_save_post_class_meta', 10, 2 );
/* Save the meta box's post metadata. */
function smashing_save_post_class_meta( $post_id, $post ) {
/* Verify the nonce before proceeding. */
if ( !isset( $_POST['smashing_post_class_nonce'] ) || !wp_verify_nonce( $_POST['smashing_post_class_nonce'], basename( __FILE__ ) ) )
return $post_id;
/* Get the post type object. */
$post_type = get_post_type_object( $post->post_type );
/* Check if the current user has permission to edit the post. */
if ( !current_user_can( $post_type->cap->edit_post, $post_id ) )
return $post_id;
/* Get the posted data and sanitize it for use as an HTML class. */
$new_meta_value = ( isset( $_POST['smashing-post-class'] ) ? sanitize_html_class( $_POST['smashing-post-class'] ) : '' );
/* Get the meta key. */
$meta_key = 'smashing_post_class';
/* Get the meta value of the custom field key. */
$meta_value = get_post_meta( $post_id, $meta_key, true );
/* If a new meta value was added and there was no previous value, add it. */
if ( $new_meta_value && '' == $meta_value )
add_post_meta( $post_id, $meta_key, $new_meta_value, true );
/* If the new meta value does not match the old value, update it. */
elseif ( $new_meta_value && $new_meta_value != $meta_value )
update_post_meta( $post_id, $meta_key, $new_meta_value );
/* If there is no new meta value but an old value exists, delete it. */
elseif ( '' == $new_meta_value && $meta_value )
delete_post_meta( $post_id, $meta_key, $meta_value );
}
How can i change the code to get the checkbox value and checked if someone check the checkbox before.
Here, i changed only the input box "text" to "checkbox". But i do not know what to do to create a single checkbox. Please help me. I am new in wordpress.

Answer 1

you prob need to assign a value for the checkbox. You might want to think what this value is if not already set.
value="<?php echo esc_attr( get_post_meta( $object->ID, 'smashing_post_class', true ) ); ?>"
on a new post this is not set. So you could do something like
value="<?php if ($x=get_post_meta( $object->ID, 'smashing_post_class', true ) ) {echo $x;}else{echo "whatever";} ?>"
You are already saving the value to post_meta(see $new_meta_value) so the code to set the value already pulls it from the database if it exists.
you might want to rethink the way you are doing the above. The checkbox value is always going to be the same from now on. You might as well set the value to whatever you want it to be right now. E.g.
<input type="checkbox" name="whatever" value="true"> select me for fun
you can access the posted value in your save meta box function
$value= $_POST['whatever'];
and save to post meta (a seperate database table for custom values and a few other values that dont fit wp_posts)
update_post_meta($post_id, '_keyname', $value);