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Introduction

The IoC principle suggests to invert the control, meaning that instead of driving the car yourself, you hire a cab, where another person will drive the car. Thus, this is called inversion of the control - from you to the cab driver.

The Laravel inversion of control container is a powerful tool for managing class dependencies. Dependency injection is a method of removing hard-coded class dependencies. Instead, the dependencies are injected at run-time, allowing for greater flexibility as dependency implementations may be swapped easily.

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Understanding the Laravel IoC container is essential to building a powerful, large application, as well as for contributing to the Laravel core itself.

Basic Usage

Binding A Type Into The Container

There are two ways the IoC container can resolve dependencies: via Closure callbacks or automatic resolution. First, we'll explore Closure callbacks. First, a 'type' may be bound into the container:

Resolving A Type From The Container

When the App::make method is called, the Closure callback is executed and the result is returned.

Binding A 'Shared' Type Into The Container

Sometimes, you may wish to bind something into the container that should only be resolved once, and the same instance should be returned on subsequent calls into the container:

Binding An Existing Instance Into The Container

You may also bind an existing object instance into the container using the instance method:

Where To Register Bindings

IoC bindings, like event handlers or route filters, generally fall under the title of 'bootstrap code'. In other words, they prepare your application to actually handle requests, and usually need to be executed before a route or controller is actually called. Like most other bootstrap code, the start files are always an option for registering IoC bindings. Alternatively, you could create an app/ioc.php (filename does not matter) file and require that file from your start file.

If your application has a very large number of IoC bindings, or you simply wish to organize your IoC bindings in separate files by category, you may register your bindings in a service provider.

Automatic Resolution

Resolving A Class

The IoC container is powerful enough to resolve classes without any configuration at all in many scenarios. For example:

Note that even though we did not register the FooBar class in the container, the container will still be able to resolve the class, even injecting the Baz dependency automatically!

When a type is not bound in the container, it will use PHP's Reflection facilities to inspect the class and read the constructor's type-hints. Using this information, the container can automatically build an instance of the class.

Binding An Interface To An Implementation

However, in some cases, a class may depend on an interface implementation, not a 'concrete type'. When this is the case, the App::bind method must be used to inform the container which interface implementation to inject:

Now consider the following controller:

Since we have bound the UserRepositoryInterface to a concrete type, the DbUserRepository will automatically be injected into this controller when it is created.

Practical Usage

Laravel provides several opportunities to use the IoC container to increase the flexibility and testability of your application. One primary example is when resolving controllers. All controllers are resolved through the IoC container, meaning you can type-hint dependencies in a controller constructor, and they will automatically be injected.

Type-Hinting Controller Dependencies

In this example, the OrderRepository class will automatically be injected into the controller. This means that when unit testing a 'mock' OrderRepository may be bound into the container and injected into the controller, allowing for painless stubbing of database layer interaction.

Other Examples Of IoC Usage

Filters, composers, and event handlers may also be resolved out of the IoC container. When registering them, simply give the name of the class that should be used:

Service Providers

Service providers are a great way to group related IoC registrations in a single location. Think of them as a way to bootstrap components in your application. Within a service provider, you might register a custom authentication driver, register your application's repository classes with the IoC container, or even setup a custom Artisan command.

In fact, most of the core Laravel components include service providers. All of the registered service providers for your application are listed in the providers array of the app/config/app.php configuration file.

Defining A Service Provider

To create a service provider, simply extend the IlluminateSupportServiceProvider class and define a register method:

Note that in the register method, the application IoC container is available to you via the $this->app property. Once you have created a provider and are ready to register it with your application, simply add it to the providers array in your app configuration file.

Registering A Service Provider At Run-Time

You may also register a service provider at run-time using the App::register method:

Container Events

Registering A Resolving Listener

The container fires an event each time it resolves an object. You may listen to this event using the resolving method:

Note that the object that was resolved will be passed to the callback.

In this chapter, we will learn about IoC and how to implement it. This is the first step towards achieving loose coupled design, as illustrated by the following figure:

Inversion of Control (IoC) is a design principle (although, some people refer to it as a pattern). As the name suggests, it is used to invert different kinds of controls in object-oriented design to achieve loose coupling. Here, controls refer to any additional responsibilities a class has, other than its main responsibility. This include control over the flow of an application, and control over the flow of an object creation or dependent object creation and binding.

IoC is all about inverting the control. To explain this in layman's terms, suppose you drive a car to your work place. This means you control the car. The IoC principle suggests to invert the control, meaning that instead of driving the car yourself, you hire a cab, where another person will drive the car. Thus, this is called inversion of the control - from you to the cab driver. You don't have to drive a car yourself and you can let the driver do the driving so that you can focus on your main work.

The IoC principle helps in designing loosely coupled classes which make them testable, maintainable and extensible.

Let's understand how IoC inverts the different kinds of control.

Control Over the Flow of a Program

In a typical console application in C#, execution starts from the Main() function. The Main() function controls the flow of a program or, in other words, the sequence of user interaction. Consider the following simple console program.

In the above example, the Main() function of the program class controls the flow of a program. It takes the user's input for the first name and last name. It saves the data, and continues or exits the console, depending upon the user's input. So here, the flow is controlled through the Main() function.

IoC can be applied to the above program by creating a GUI-based application such as the following windows-based application, wherein the framework will handle the flow of a program by using events.

This is a simple example of implementing IoC in the flow of a program.

Control Over the Dependent Object Creation

IoC can also be applied when we create objects of a dependent class. First of all, let's understand what we mean by dependency here.

Consider the following example.

In the above example, class A calls b.SomeMethod() to complete its task1. Class A cannot complete its task without class B and so you can say that 'Class A is dependent on class B' or 'class B is a dependency of class A'.

In the object-oriented design approach, classes need to interact with each other in order to complete one or more functionalities of an application, such as in the above example - classes A and B. Class A creates and manages the life time of an object of class B. Essentially, it controls the creation and life time of objects of the dependency class.

The IoC principle suggests to invert the control. This means to delegate the control to another class. In other words, invert the dependency creation control from class A to another class, as shown below.

As you can see above, class A uses Factory class to get an object of class B. Thus, we have inverted the dependent object creation from class A to Factory. Class A no longer creates an object of class B, instead it uses the factory class to get the object of class B.

Let's understand this using a more practical example.

In an object-oriented design, classes should be designed in a loosely coupled way. Loosely coupled means changes in one class should not force other classes to change, so the whole application can become maintainable and extensible. Let's understand this by using typical n-tier architecture as depicted by the following figure:

In the typical n-tier architecture, the User Interface (UI) uses Service layer to retrieve or save data. The Service layer uses the BusinessLogic class to apply business rules on the data. The BusinessLogic class depends on the DataAccess class which retrieves or saves the data to the underlying database. This is simple n-tier architecture design. Let's focus on the BusinessLogic and DataAccess classes to understand IoC.

The following is an example of BusinessLogic and DataAccess classes for a customer.

As you can see in the above example, the CustomerBusinessLogic class depends on the DataAccess class. It creates an object of the DataAccess class to get the customer data.

Now, let's understand what's wrong with the above classes.

In the above example, CustomerBusinessLogic and DataAccess are tightly coupled classes because the CustomerBusinessLogic class includes the reference of the concrete DataAccess class. It also creates an object of DataAccess class and manages the lifetime of the object.

Problems in the above example classes:

1. CustomerBusinessLogic and DataAccess classes are tightly coupled classes. So, changes in the DataAccess class will lead to changes in the CustomerBusinessLogic class. For example, if we add, remove or rename any method in the DataAccess class then we need to change the CustomerBusinessLogic class accordingly.
2. Suppose the customer data comes from different databases or web services and, in the future, we may need to create different classes, so this will lead to changes in the CustomerBusinessLogic class.
3. The CustomerBusinessLogic class creates an object of the DataAccess class using the new keyword. There may be multiple classes which use the DataAccess class and create its objects. So, if you change the name of the class, then you need to find all the places in your source code where you created objects of DataAccess and make the changes throughout the code. This is repetitive code for creating objects of the same class and maintaining their dependencies.
4. Because the CustomerBusinessLogic class creates an object of the concrete DataAccess class, it cannot be tested independently (TDD). The DataAccess class cannot be replaced with a mock class.

To solve all of the above problems and get a loosely coupled design, we can use the IoC and DIP principles together. Remember, IoC is a principle, not a pattern. It just gives high-level design guidelines but does not give implementation details. You are free to implement the IoC principle the way you want.

The following pattern (but not limited) implements the IoC principle.

Let's use the Factory pattern to implement IoC in the above example, as the first step towards attaining loosely coupled classes.

First, create a simple Factory class which returns an object of the DataAccess class as shown below.

Now, use this DataAccessFactory class in the CustomerBusinessLogic class to get an object of DataAccess class.

Windows Drivers Ioctl

As you can see, the CustomerBusinessLogic class uses the DataAccessFactory.GetCustomerDataAccessObj() method to get an object of the DataAccess class instead of creating it using the new keyword. Thus, we have inverted the control of creating an object of a dependent class from the CustomerBusinessLogic class to the DataAccessFactory class.

This is a simple implementation of IoC and the first step towards achieving fully loose coupled design. As mentioned in the previous chapter, we will not achieve complete loosely coupled classes by only using IoC. Along with IoC, we also need to use DIP, Strategy pattern, and DI (Dependency Injection).

Drivers Ioc Jobs

Let's move to the second step to understand DIP and how it helps in achieving loose coupled design in the next chapter.