依赖注入如何知道要传递哪些变量？

Question

I am struggling with the idea of a dependency injection container.

Not sure how to explain but I will try with a few examples:

class foo
{
    public function __construct(\Somenamespace\Bar $bar, $x)
    {

    }
}

How would the DIC know what 'x' would be?

And if we go further down the road and look at the bar class. The DIC can with no problem with reflection create Bar.

namespace \Somenamespace;
class bar
{
    public function __construct(\SomethingElse\Tap $tap, $y)
    {

    }   
}

The bar wants taps and the tap could also be found by the DIC but how does it know about 'y'?

And if we create the tap.

namespace\SomethingElse;
class tap
{
    public function __construct($amountOfTaps)
    {

    }
}

How would the DIC know about the amount of taps or whatever variable?

EDIT:

Same question does for if the hinted type is an interface or an abstract class. How does a DIC figure out what he needs to create?

Answer 1

That's a good and a valid question.

Here is one of the possible answers: the container can't know

public function __construct(\Somenamespace\Bar $bar, $x)

• $bar is an instance of \Somenamespace\Bar, easy
• $x could be anything... The container can't know for sure what you want.

So most containers stop here and ask you to explicitly define (e.g. in a configuration file) what dependency should be injected for $x.

---

However some containers can try to guess. For example:

public function __construct($exportService)

The container can assume that the variable name can be the service name, so it will try to inject the dependency named exportService.

I don't know any PHP container doing that, I'm planning to implement that in the future in PHP-DI.

---

The same goes for "interface injection":

public function __construct(Some\GoodInterface $x)

In Java, the Container can know all classes that are in a project (they are all grouped in a WAR/JAR archive). So the Java container can look for all implementation of the interface.

If there is only one, it automatically uses it.

In PHP, with autoloading, the container can't know all the classes of your project. So all in all, it can't guess what implementation to inject.

So like above, you will have to define which implementation to use manually (configuration file, …)