Category: coding

Dependency Injection is a software design pattern that allows avoiding hard-coding dependencies and makes possible to change the dependencies both at runtime and compile time.

By using Dependency Injection we can write more maintainable, testable, and modular code. All projects have dependencies. The larger the project the more dependencies is it bound to have; now having a great number of dependencies is nothing bad by itself however how those dependencies are managed and maintained is.

Dependency Injection is not a new pattern and it has been commonly used on many languages like Java, but this pattern is somewhat new in the PHP world and it’s gaining traction quickly thanks for frameworks like laravel

Let’s exemplify these concepts by creating a pair of classes first without dependency injection and then rewriting the code to use the dependency injection pattern; since I’m primarily a Magento developer I’ll be really original (wink, wink) and create a Product and a StockItem class.

At first glance the code looks pretty normal and it’s what many PHP developers would call good code, however if we take a closer look at it using the S.O.L.I.D principle we can see that the code actually has many problems:

• The StockItem is tightly coupled with the Product class, and while this might not look bad on this particular example. Let’s imagine that we made a change to the StockItem class to include a new parameter, we would then have to modify every single class where the StockItem object was created.

• The Product class knows too much, in this case the stock status and quantity, let’s say that our application can handle inventories from multiple sources and stores but for the same product. With that in mind it would be in our best interest to make the Product class know less about its inventory.

• We just made our life harder when it comes to unit testing the code. Since we are instantiating the stockItem inside the constructor it would be impossible to unit test the Product class without also testing the StockItem class.

Let’s Inject something!

In the other hand by using dependency injection we can correct most of these problems, let’s take at the same code but using dependency injection:

Constructor Injection

By using dependency injection we have more maintainable code, in the previous example we are using a type of dependency injection normally referred as Constructor Injection. By doing a simple change we can reduce the level of complexity of our code and improve the overall quality; not to mention that now we can easily run unit tests.

Constructor injection is by far the most common method used and there are several advantages by using this particular type of injection:

• If the dependency is required by the class and cannot work without it, by using constructor injection we guarantee that the required dependencies are present.

• Since the constructor is only ever called when instantiating our object we can be sure that the dependency can’t be changed or altered during the object lifetime.

These two points make Constructor Injection extremely useful, however there is also a few drawbacks that make it unsuitable for all scenarios:

• Since all dependencies are required, it’s not suitable when optional dependencies are needed.

• While using class inheritance trying to extend and override the constructor becomes difficult.

Setter Injection

Another common type of dependency injection is called setter injection and the same code as above would look something like this:

As we can see, with Setter Injection the dependencies are provided to our class after it has been instantiated using setter methods. Setter Injection has a few advantages:

• Allows for optional dependencies and the class can be created with default values.

• Adding new dependencies is as easy as adding a new setter method and it won’t break any existing code.

Setter Injection might be more suitable for situations where more flexibility is required.

One of the problems of PHP as a programming language is the fact that you can only have single inheritance. This means a class can only inherit from one other class.

However, a lot of the time it would be beneficial to inherit from multiple classes. For example, it might be desirable to inherit methods from a couple of different classes in order to prevent code duplication.

This problem can lead to class that has a long family history of inheritance which often does not make sense.

In PHP 5.4 a new feature of the language was added known as Traits. A Trait is kind of like a Mixin in that it allows you to mix Trait classes into an existing class. This means you can reduce code duplication and get the benefits whilst avoiding the problems of multiple inheritance.

PHP Traits?

A Trait is simply a group of methods that you want include within another class. A Trait, like an abstract class, cannot be instantiated on it’s own.

An example of a Trait could be:

You could then include this Trait within other classes like this:

How do Traits work?

As you can see from the example above, both the Post and the Comment objects have the share() method available despite not having that method defined.

A Trait is basically just a way to “copy and paste” code during run time.

This means the Trait is copied in to the Post and Comment classes so when you instantiate a new instance, the share() method code will be available.

How are Traits different to Abstract classes?

A Trait is different to an Abstract class (What are Abstract classes?) because they do not rely on inheritance.

Imagine if the Post and the Comment class had to inherit from a Social class. We are most likely going to want to do more than just share posts and comments on social media websites, so we’ll probably end up with a complicated inheritance tree like this:

How are Traits different to Interfaces?

Traits kind of look a lot like Interfaces. Both Traits and interfaces are usually simple, concise and not much use without an actual implemented class. However the difference between the two is important.

An interface is a contract that says “this object is able to do this thing”, whereas a Trait is giving the object the ability to do the thing.

For example:

The Sharable Trait implements the share() method and the like() method is implemented in the Post class.

So as you can see we could type hint the Post object to see if it is sociable (it implements the Sociable interface), whilst the Trait defines a reusable method that we can then mix in to other similar classes:

Benefits of Traits?

The benefit of using Traits is that you reduce code duplication whilst preventing complicated class inheritance that might not make sense within the context of your application.

This allows you to define simple Traits that are clear and concise and then mix in that functionality where appropriate.

Drawbacks of Traits?

However with that being said, there are possible drawbacks when using Traits too.

Traits make it very easy to write bloated classes that have too much responsibility. A Trait is essentially a way to “copy and paste” code between classes. By having a way to very simply add another group of methods to a class, it’s very easy to diverge from the single responsibility principle.

Other drawbacks to using Traits are not being able to see all the methods of a class when looking at the source code as well as method conflicts or duplication of logic.

I think Traits, when used correctly, are a fantastic tool to have at our disposal. However, Traits can also be crutch for lazy programming. It’s very easy to just add a Trait to solve your immediate problem. Often composition is the better approach over inheritance or using a Trait.

What are typical situations for using Traits?

So what would be a typical situation when using a Trait would be a good idea?

Well, I think Traits are an excellent way to reuse a chunk of code between a set of similar classes that should not inherit from the same abstract class.

Using the social application from earlier, imagine we had objects for Post, Photo, Note, Message and Link. For the most part, these objects are fairly interchangeable within our system as they are typically created and interacted with between users.

However, Post, Photo, Note and Link are all objects that are publicly shareable between users, whereas Message objects are private messages that are not made public.

Inheritance is a concept in object oriented programming. With the help of inheritance we can get all property and method of one class in another class. Inheritance in php is introduced from php5 version.

Here I will explore about basics concept of inheritance  After basic we will discuss implementation of inheritance in php. 

Inheritance?

Inheritance is nothing but a design principle in oop. By implementing inheritance you can inherit(or get) all properties and methods of one class to another class. The class who inherit feature of another class known as child class. The class which is being inherited is know as parent class. Concept of the inheritance in oop is same as inheritance in real world. For example, child inherits characteristics  of their parent. Same is here in oop. One class is inheriting characteristics of another class.

With the help of inheritance you can increase re-usability of code. Let us take an example in terms of generic programming practices. Suppose you are going to create classes to render different html tag(div, span, form, table etc).  Now you will create class with name html_div, html_span ,  html_form. You are creating different class because every element is different in nature. For example form has action and method and you will have different input element in form. But table will have tbody, tr, th and td.

Now just think for some moment. There are some element and their rendering is same in all element. For example all html mention above is having name, id, class attribute which is same. Also rendering of those element is also same. So in above case you can create parent class with name tagsDetails and you can inherit that class across all of your html tags like div, span, form. Following is the generic code structure of inheritance in oop taking your HTML attribute in consideration.

Above code is and example of basic inheritance in php. All method(protected and public) from tagDetails class is directly accessible in your class page class. In child class you no need to write rendering of id and name logic again and again. This really saves time and give some good modulations in the code.

Hope your basic understanding about inheritance is clear. Now let us move to implementation of inheritance in php.

Multilevel and Multiple inheritance in PHP

In php multilevel inheritance is possible but multiple inheritance is not possible. In simplified terms in php child class can not inherit more than one parent class. But hierarchical inheritance is possible in php. Hierarchical means Parent inherit property of grand parent class. Grand child inherit property of parent class. So in multilevel inheritance child can get some property of from grand parent class also.

Example of Multiple inheritance in PHP

Above code will not work in php. Because php is single inheritance language.

Example of Multilevel inheritance in PHP

This is very basic example of multilevel inheritance. In php it is possible to implement multilevel inheritance. In above example parent class is inheriting grand parent property. And and child is inheriting parent property. So child have some parent and grand parent property.

Static Methods and Property in Inheritance in PHP

As in our example of page class we have explored that we can use $this-> keyword to get all property and method of parent(tagsDetails) class. But if your parent or child method is static, then you can access static methods or properties using self and parent keyword. Also this is not necessary to make method static if you want to use self or parent keyword. This is very useful if your parent and child both method is having property or method with same name. If both classes having same property and you want to call specific property or method then you can use this keyword.

Self and parent in case of static methods:

Self and Parent without static