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
	// if ($column_name === 'order_unit') {
		// echo '1234';
	// }
	// return $columns;
}
add_action( "manage_shop-subcategory_custom_column", 'custom_column_content', 10, 3);