Dependencies

If we were to do everything in just one class that won’t be very SOLID of us.
And it’s not just about following patterns. A class in metz represents one component. If we do everything in one class, we would literally be designing a system which has just one component. Not a very useful design!

There are two ways you can talk to other classes. The first one is as old as OOP, simply create an instance and call its methods.

The second, can sometimes be more convenient. Let’s cover this first and then we can look at instantiating classes.

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Dependency Injection

Rather than you manually managing creation of classes, metz can do it for you. We maintain a DI container to store a singleton instance of a classe. These instances can then be retreived using std.resolve.

Simply decorate the class with @Injectable and it will start participating in dependency injection. Moreover, metz will take care of creating the entire dependency graph for you.

So to increase complexity for the sake of it, let’s divide our hello world Main class example into two classes.

So it should look like this:

You can use DI to make this representation easy:

@Injectable
class Hello {
	/*
	* We are using std.resolve to get the singleton instance of 'World'
	* and make it our member.
	*/
	private worldInstance = std.resolve(World);

    hello() {
        const result = this.worldInstance.world('Hello');
        std.log(result);
    }
}

@Injectable
class World {

    world(arg: string) {
        return `${arg} World!`
    }
}

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Manual Construction

You can always use new to create instances manually, regardless if the class is part of DI container or not.

This is best suited for scenarios where you need multiple instances of the same thing. For example the Poller in Introduction needs to have 2 instances.
So we instantiated them using new, you can see this in the code on playground as well.

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Best of both the worlds

You are free to mix and match. A class not participating in DI, can still access the container. And an @Injectable class can create instances of classes.

You can see how the system behaves when the two approaches are used simulatenously.

We got multiple classes, but we are still stuck in a single file. Let’s check out the next section to fix that.