How to Add Plug-in Meta Boxes to WordPress Custom Post Type Editor Pages

This article came about sort of by accident. In a previous article, I explained how to create a custom post type to display content on a page other than my home page. I’m still going to post to the home page, but I wanted a dedicated page for my Lead Like Jesus devotionals. As I started adding content to the newly created custom post type, I wanted to make sure the Sharedaddy feature was turned on for each post. Many of my readers enjoy sharing the thrice weekly devotional messages. They are uplifting, thought provoking and inspirational. But lo and behold, as I went to click the Sharedaddy checkbox to turn sharing on, the Sharedaddy meta-box was missing from my custom post type editor page! Obviously I missed something, but I didn’t know what it is.

After a couple of hours of searching every corner of the internet for an article, any article, that would point me in the right direction, I finally found a snippet of information that briefly mentions adding code to a theme’s functions.php source code to show plug-in meta-boxes on custom post type editor pages. The article was neither clear nor complete enough for me to fully grasp what needs to be done. Yet, there was just enough information to get me started down the right track even though I had to figure almost everything out for myself. Here’s what I did. It works and it works well.

Let’s continue to use Sharedaddy as our example. First, open the plugin editor and retrieve sharedaddy.php. Please view the video below for details on opening the plugin’s source file and copying the pertinent code to an external text editor. We’ll be editing the code copied from the source file and creating our own custom function.

Now that the code is copied into the external text editor, open your theme’s functions.php file and copy it’s entire source into your text editor as well. I recommend doing it this way, because at least on my computer, the TinyMCE editor used by WordPress is a bit quirky. Every time I attempt to edit a file, I’ll start writing and the code inside the text editor jumps either up or down and I lose sight of what I’m doing. I could still type and the changes will take even though I can’t see what I am doing. I don’t know if this is a problem with the very problematic IE8 or whether the problem is with the editor itself. I’ll test the WordPress editor with Firefox later. We’re going to modify the code we copied from Sharedaddy.php and hook it into the function we wrote for creating custom post types. If you recall, my custom function looks like this:

I've highlighted line 40 in the source code example. Make some room above line 40 to insert the following function between register_post_type() and flush_rewrite_rules():

In the block of code we've just inserted, add_action is calling a custom function called add_plugin_meta_boxes(). This new function simply calls the add_meta_box() we copied from the sharedaddy.php source file. Still with me? I know you are!

Now this is something I can't fully explain, but I think it's related to a timing issue. In my first draft of this code, I did not have the if (is_admin()){} wrapper around the two add_actions. For a reason I haven't discovered, the first ad_action() line kept causing php errors with the message that add_meta_box was undefined or not found or something along those lines. Once I inserted if (is_admin()){}, everything worked beautifully. Go figure!

Going back to our inserted code above, the second add_action is calling a function called save_sharing_box(). This is simply a renamed, but otherwise unedited version of the sharing_meta_box_save() function lifted out of sharedaddy.php. Our final code with all the new functionality is:

By the way, I finished editing this post on Firefox. TinyMCE is well behaved on this browser.