Have You Ever Used A Symfony Compiler Pass?

It’s been JavaScript, JavaScript, JavaScript for the past few weeks, so this week I thought I’d vary things up a little and, heck, cover a little Symfony 🙂

Yes, two new – dare I say – useful videos were added to CodeReviewVideos this week.

Lets go ahead and jump right into it.

First up we venture into the wonderful – and scary sounding – world of Compiler Passes. Now, this is one of those Symfony things that is really useful, but also something you’re unlikely to try until you’ve seen it working before. Ahh, the old catch 22. Not so bad if you work in a huge team of devs who all share knowledge (or you are particularly good at reverse engineering), but if you’re a lone wolf, or part of a smaller, less experienced team, things like this may completely pass you by.

To begin with, we start with a code smell that I saw recently when working with a client. This problematic code is fairly common, and isn’t specific to Symfony projects. You may even have it in your own projects:

Controller bloat.

This particularly issue starts off with an if  / else , which turns into an if  / elseif  / else , and within a few further iterations you’ve got this nasty conditional dominating the method. Actually in the code I was looking at, this was a common theme throughout multiple controllers, and multiple actions in each controller. It’s things like this that make people believe Symfony is hard, when in reality, this problem would follow you if you switched to Laravel, or Zend, or Slim, or any other.

We’ll use an example of having to convert an array of data into a variety of formats – CSV, XML, YAML, and so on.

To begin with, these various conversion strategies live inside our controller action.

In the first video we cover a simple way to start refactoring this code:


At this stage all we are doing is extracting the problem code from the controller. This is really easy to do, and can immediately start to reduce duplication, and increase code re-use.

Unfortunately this doesn’t actually solve the tight coupling problem. But sometimes, you know what? You don’t need too. Over engineered code can be just as painful to work with as sloppy code. It’s just a different kind of pain.

Anyway, on larger systems simply extracting code out like this may not give you the flexibility you need. For that we can use a technique that third party bundles use:


In this video we cover how to use a Symfony Compiler Pass to decouple the Conversion service we’ve created from the various conversion strategies (e.g. convert to YAML, convert to JSON, and so on).

Let’s imagine that you wanted to create a Symfony Bundle that you could then share with the world in order to help others who might need to convert arrays in the future. Whilst you might provide a variety of implementations out of the box – ConvertToJSON, ConvertToXML, and so on – you don’t want to be solely responsible for adding every possible implementation in the future.

Instead, you could allow users of your library to implement the expected interface, and then use a Symfony service Tag that tells your Conversion service to also include this custom converter that your library user has created.

There’s loads of uses for this approach, and honestly it’s easier to do than you might think. Much like a lot of Symfony concepts, actually 🙂

All of this weeks videos are free to watch, though I will be making the Compiler Pass video a Members Only video at the end of this month.

Until next week, take care and happy coding



Published by

Code Review

Code Review

CodeReviewVideos is a video training site helping software developers learn Symfony faster and easier.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.