Module dependencies

Aim

The aim of library is to provide a dependency injection solution that is feature rich, easy to use, easy to learn, and easy to extend up to your needs.

This tutorial will show how to register several components, and reference them as dependencies.

Just registering a component with predefined data, is not enough even in smallest applications, since an application is an amalgamation of interconnected components. Therefore the framework does allow client code to register components that have dependencies, and reference them in configuration of those components.

For example, in application that simulates cars, a car clearly is not a single god object. It is a set of interconnected components, represented by objects in application. Though as seen in listing, the car has it’s color and size hard-coded to specific values, and modifications to them aren’t possible to do. Having a car simulator such code is not desired, since a simulator should be able to simulate multiple types of cars with different sizes and colors.

class Car {
    private Color color_;
    private Size size;

    public this() {
        color_ = Color(255, 255, 100);
        sizes = Size(200, 150, 500);
    }
}

In improvement to the defined car in application (above), could be done by exposing color and size of car to configuration from exterior environment

class Car {
    private Color color_;
    private Size size;

    public this(Size size) {
        this.size = size;
    }

    public @property void color(Color color) {
        this.color_ = color;
    }
}

void main() {
    Car myCar = new Car(Size(200, 150, 500));
    myCar.color = Color(0, 255, 0);
}

Having the ability to change color and size of a car it is possible to have now multiple instances of a car, with different sizes and colors, though with increase of different types cars, the amount of code required to instantiate and configure them with right sizes increases tremendously, and in the end can become complex, and cumbersome to understand. Not to mention, the importance of micromanaging the order of how cars component instantiate. A car cannot be instantiated before, her components. Clearly the solution is more flexible, yet at a cost of increased amount of code.

A DI framework aims to lessen the burden of writing wiring code, and will eliminate the need of micromanaging the order component creation. It will do instead of developers, just like in following example

SingletonContainer container = singleton(); // Creating container that will manage a color

with (container.configure) {

    register!Color // Register color into container.
        .set!"r"(cast(ubyte) 0) // Set red color to 0
        .set!"g"(cast(ubyte) 255) // Set green color to 255
        .set!"b"(cast(ubyte) 0); // Set blue color to 0

    register!Size // Register a size into container
        .set!"width"(200UL) // As for color set width to 150.
        .set!"height"(150UL)
        .set!"length"(500UL);

    register!Car // Register a car in container
        .construct(lref!Size) // Construct car using size present in container
        .set!"color"(lref!Color); // Set car color to color present in container.
}

container.instantiate(); // Boot container (or prepare managed code/data).

container.locate!Car.print;

Referencing a dependency in a Car can be done using lref!Type notation, that allows framework to detect that a dependency is actually required to pass, and not a simple value. Therefore the framework will search for a value of a type Type and pass it as a dependency to Car. The result of a container can be seen by fetching a car from container and printing it to the stdout

At end we should see the car:

You bought a new car with following specs:
Size:	Size(200, 150, 500)
Color:	Color(0, 255, 0)

Try example in this module. Modify it, play with it to understand how to configure dependencies between components.