What is dependency injection?

Jun 26, 2011  php, unit-testing

For quite some time, I have been working on getting my head around best practices for structuring my web applications. One of the strongest messages that has come through is to use dependency injection to facilitate unit testing.

As many others have noted, dependency injection is a very simple concept. If one thing (the “consumer”) depends upon something else (the “dependency”) to do its work, then the consumer should be given the dependency. The consumer should depend upon an agreed set of functionality that the dependency performs, but it should not care how the dependency actually gets the job done.

An easy example

A radio receives transmissions and plays sounds through its speakers. Nothing special there. But it needs a power source to function. The power source’s adapter has to fit into the hole on back of the radio. And there has to be some compatibility of voltages and currents and all that electrical magic. The radio is the consumer and the power source is the dependency. You can’t run the radio without the power source.

Note that, in principle, the power source could be a pack of AA batteries. Or it could be a solar cell. Or it could be a collection of gerbils running on flywheels. As long as it outputs the right voltage and current and the plug fits into the hole, then all is cool. The radio is not concerned by the power source implementation as long it lives up to what we expect a power source to do (i.e., its interface).

How about some code?

Using our radio and power source example, we could imagine PHP objects and interfaces as follows:

interface PowerSourceInterface 
{	
	public function getPower($numberOfPowerUnits);
}

class PowerSourceGerbils implements PowerSourceInterface
{
	protected $_gerbils;
	
	public function __construct($gerbils)
	{
		$this->_gerbils = $gerbils;
	}
	
	pubic function getPower($numberOfPowerUnits)
	{
		// Make the gerbils run like hell on the flywheel
		// to generate the requested amount of power.
	}
}

class Radio 
{
	protected $_powerSource;
	
	public function __construct(PowerSourceInterface $powerSource)
	{
		$this->_powerSource = $powerSource;
	}
	
	public function play()
	{
		// Call upon $this->_powerSource->getPower()
		// Feed that power into the various electrical 
		// components of the radio and push the sound
		// out through the speakers.
	}
}

The idea is that by clearly defining the responsibilities, we can create independent object that fulfill those responsibilities. Even better, by defining interfaces, we can set up a contract for what a particular consumer expects his dependencies to be able to do for him.

So, who cares? Unit testing cares.

Suppose you are a company that makes radios. You hire the best radio engineers and work out all the details for what makes a radio great. Maybe you have amazing noise-reduction algorithms that filter out static. Maybe your radios have balance and equalizer controls to allow the user to tweak the sound he gets out of your box. Maybe your radios are super efficient so that they pull less power from the power source, allowing them to play longer before recharge. Or maybe you simply put them in colorful plastic shells featuring photos of some random pop star.

The point is that your expertise is in the areas highlighted above. You make no representation to be experts in power generation; you are a power consumer. The quality of your work can only be fairly judged when the power source functions properly. Hey, man, if the power source tanks, then all bets are off, right?

So as part of your manufacturing process, you do a whole bunch of unit-testing on your radios using a “mock” power source, a power source that definitely functions correctly. This approach – explicitly defining and passing dependencies, combined with supplying mock dependencies during unit testing – clearly defines responsibilities and allows you to focus on fulfilling yours without worrying that the problem lies in someone else’s area of responsibility.

What’s the downside?

Well, creating objects is now a bit of a pain the neck. Consider what is required now just to play a radio.

$gerbils = new GerbilCollection();  // guess we need a Gerbil class and a GerbilCollection class, too
$powerSource = new PowerSourceGerbils($gerbils);
$radio = new Radio($powerSource);
$radio->play();

So, every time I want to play a radio, I need to construct a power source (which might have its own dependencies, like the gerbils in this case), feed that power source into the radio, and then hit the play button.

I gotta do all these steps just to play a freaking radio? Can’t I do the radio setup once – create and plug in the power source – and then when I want to listen to some music, all I have to do is hit play?

I will address this in the next post: Poor Man’s Dependency Injection using Zend Framework.

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