Adding custom column displaying in Wordpress Manage Category/Custom Taxonomy editing page

add_custom_column_to_edit_tag.php

/*
    Purpose: add custom column header and custom column content to respective custom header in Manage Category Editing Page
    Version Tested: 3.8
    
    Story:
    Because I found no explanation nor documents in Wordpress.org, I tried to trace the code in wp-admin folder
    after understanding the operation of apply_filter(), add_action() and add_filter()
    
    Logic:
    The table list in edit_tag.php is based on 
    wp-admin/includes/class-wp-list-table.php and
    wp-admin/includes/class-wp-terms-list-table.php
    Note: actually class-wp-terms-list-table.php is creating a class extend the basic table list class in class-wp-list-table.php
    
    Reference:
    http://wordpress.stackexchange.com/questions/6883/how-can-i-add-a-custom-column-to-the-manage-categories-table/129545#129545
*/
// Example

// these filters will only affect custom column, the default column will not be affected
// filter: manage_edit-{$taxonomy}_columns
function custom_column_header( $columns ){
	$columns['order_unit'] = 'Order Unit';
	return $columns;
}
add_filter( "manage_edit-shop-subcategory_columns", 'custom_column_header', 10);


// parm order: value_to_display, $column_name, $tag->term_id
// filter: manage_{$taxonomy}_custom_column
function custom_column_content( $value, $column_name, $tax_id ){
    // var_dump( $column_name );
    // var_dump( $value );
    // var_dump( $tax_id );

    // for multiple custom column, you may consider using the column name to distinguish

    // although If clause is working, Switch is a more generic and well structured approach for multiple columns
    // if ($column_name === 'header_name') {
        // echo '1234';
    // }
    switch( $column_name ) {
          case 'header_name1':
               // your code here
               $value = 'header name 1';
          break;

          case 'header_name2':
               // your code here
               $value = 'header name 2';
          break;

          // ... similarly for more columns
          default:
          break;
    } 

    return $value; // this is the display value
}
add_action( "manage_shop-subcategory_custom_column", 'custom_column_content', 10, 3);