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Managing Dependencies

As far as I get it and though I don’t really know if i can convince you, i realize that models/entities/classes shouldn’t be dependant of each other. The less they are dependant, the better.

This, translated to common words, can be sum up VERY ROUGHLY :

Do not use 'new' keyword within any of your classes.

Another way to check if your class is tightly dependent : check the 'use' keyword : if your object is using TOO much different objects and moreover from different modules/libraries/concepts, than we can say there’s something wrong with the way you build it up.

Then, HOW to use other classes without creating new objects ?

We instanciate objects and then, INJECT them into our classes.

How to inject dependencies ?

There’s different strategies :

You can use the constructor, create a constructor with as many arguments as you have dependencies and set each argument to a property. From outside that object, you create all the objects you need and pass them through the constructor. With this strategy, you ensure that the dependencies are mandatory.

You can use setter methods, just like for any of your property. With this strategy, you cannot be sure the dependencies are mandatory. You have to manage de default status if any.

You can use interfaces, though i don’t get this right so i won’t talk about that part.

Within our class, the dependency will be a property just like another and, still within the class, we will manipulate and return such property and barely not caring whether the dependency is an instance of class A or class B… All our class has to know is just HOW to use its dependency.

There are exceptions to this rule :

Factory object HAS to use the 'new' keyword. It is because they are factories. They are meant to return a new object.

And for my part, I don’t inject ArrayObject either. I consider it as a type more than a dependency… But it is a personal point of view and i am still thinking whether it is right or wrong.

Instanciating objects

Oh… of course… I said earlier : We instanciate objects and then, INJECT them into our classes. I have covered roughly the injection part but now, WHERE to instanciate objects, since no classes can use the 'new' keyword…

We will use a higher level object. In that object, we will instanciate all our objects : the dependencies and all the other objects, with the injections whether by constructor or by setters.

Zend Framework 2 has that kind of useful component : the Service Manager. I’m quite sure that other frameworks do have a similar component but i haven’t tested them yet so i don’t know how the Service Manager is called with Symfomy, Laravel etc… Just find out how to use it.

The Service Manager is a kind of index where we can declare each of your classes. This is, roughly, the only component of the module where we will use all our 'new' instructions and instanciate each of our objects at least once. The only one.

Using the objects

All our model objects (entities, mappers, factories, forms) are instanciated (therefore created) within the Service Manager as well as their dependencies. So, technically, you won’t need to access the Service Manager from within the model objects. Because you won’t have to call for any external classes from within your class since all the dependencies are already injected from the Service Manager.

For the particular case of an array of object, the old fashioned way to do this is to have a foreach loop and instanciate a 'new' object for each iteration. Since the object is injected only once in our object, use the clone functionality instead. Do the foreach loop and clone the injected dependency rather than instanciating from nowhere.

It leaves the Views : objects used by the view should be pushed by the Controller. It leaves the plugins… I’ll cover that part once i’ll have to work with views plugins more…

Then the Controller : Controller classes in Zend Framework implements the ServiceLocatorAware interface which allow them to access to the Service Manager.

When i started to use ZF2, i was looking everywhere for tips on how to access the Service Manager. But you likely do not need that answer. You shouldn’t need that answer.

I find benefits while instanciating all my objects in the Service Manager : if i ever do a refactoring of my code, rearranging the namespace, splitting one module in two differents modules, it is easy peasy… just change the links in your Service Manager and the rest of the code remain unmoved. This save my time several time.