Exceptions - for exceptional situations?

From the Domain-Driven Design movement we’ve learned to go somewhat back to the roots of object-oriented design. Designing domain objects is all about offering meaningful behavior and insights through a carefully designed API. We know now that domain objects with setters for every attribute will allow for the objects to be in an inconsistent state. By removing setters and replacing them with methods which only modify the object’s state in valid ways, we can protect an object from violating domain invariants.

My previous post was about finding and loading configuration files. I now continue my Symfony2 Config Component quest with a post about the way you can “semantically expose your configuration” (using the TreeBuilder). I wrote this piece to later contribute it to the Symfony documentation so feedback is very welcome!

Validate configuration values

After loading configuration values from all kinds of resources, the values and their structure can be validated using the Definition part of the Symfony2 Config Component. Configuration values are usually expected to show some kind of hierarchy. Also, values should be of a certain type, be restricted in number or be one of a given set of values. For example, the following configuration (in Yaml) shows a clear hierarchy and some validation rules that should be applied to it (like: “the value for ‘auto_connect’ must be a boolean”):

One of the ugly things about Symfony 1 validators was that their dependencies were generally fetched from far away, mainly by calling sfContext::getInstance(). With Symfony2 and it’s service container, this isn’t necessary anymore. Validators can be services. The example below is quite simple, but given you can inject anything you want, you can make it quite complicated. I show you how to create a validator that checks with the router of the value-under-validation is a route.