Dependency injection: what belongs in the constructor?
- by Adam Backstrom
I'm evaluating my current PHP practices in an effort to write more testable code. Generally speaking, I'm fishing for opinions on what types of actions belong in the constructor. Should I limit things to dependency injection? If I do have some data to populate, should that happen via a factory rather than as constructor arguments? (Here, I'm thinking about my User class that takes a user ID and populates user data from the database during construction, which obviously needs to change in some way.)
I've heard it said that "initialization" methods are bad, but I'm sure that depends on what exactly is being done during initialization.
At the risk of getting too specific, I'll also piggyback a more detailed example onto my question.
For a previous project, I built a FormField class (which handled field value setting, validation, and output as HTML) and a Model class to contain these fields and do a bit of magic to ease working with fields. FormField had some prebuilt subclasses, e.g. FormText (<input type="text">) and FormSelect (<select>). Model would be subclassed so that a specific implementation (say, a Widget) had its own fields, such as a name and date of manufacture:
class Widget extends Model {
public function __construct( $data = null ) {
$this->name = new FormField('length=20&label=Name:');
$this->manufactured = new FormDate;
parent::__construct( $data ); // set above fields using incoming array
}
}
Now, this does violate some rules that I have read, such as "avoid new in the constructor," but to my eyes this does not seem untestable. These are properties of the object, not some black box data generator reading from an external source. Unit tests would progressively build up to any test of Widget-specific functionality, so I could be confident that the underlying FormFields were working correctly during the Widget test.
In theory I could provide the Model with a FieldFactory() which could supply custom field objects, but I don't believe I would gain anything from this approach. Is this a poor assumption?
