WebComponents-DI

This package aims to provide simple dependency injection for web components. It is based on an idea by Justin Fagnani. You can find more details about it here: https://www.youtube.com/watch?v=6o5zaKHedTE

The basic idea is that dependencies are requested and provided using events. This project aims to do the following:

• make something similar to the initial implementation available to everyone using web components
• add some icing on top to make it super easy to use

There is a simple demo that illustrates how to use this project. It uses LitElement, but WebComponents-DI should work with any web component library or framework.

The general idea goes like this: There is a child component which wants to request a dependency from a parent component. The example uses LitElement, but it should be easy to understand even if you do not know it:

import {customElement, html, LitElement, TemplateResult} from 'lit-element';
import {requestDI} from 'webcomponents-di';

@customElement('child-component')
export class ChildComponent extends requestDI(LitElement) {
    world: unknown | null = null;

    connectedCallback(): void {
        super.connectedCallback();
        this.world = this.requestInstance('world');
    }

    protected render(): TemplateResult | void {
        return html`
            <div style="background-color: red"><h2>I'm the Child</h2>
                <p>Hello, ${this.world}</p>
            </div>`;
    }
}

The only webcomponents-di parts are requestDI mixin and the method requestInstance. The mixin requestDI provides all basic functionality to request dependencies. You can find a more advanced example in the example project. requestInstance requests a dependency called world. Now to provide this dependency in a parent web component all you do is this:

import {customElement, html, LitElement, TemplateResult} from 'lit-element';
import {provideDI} from 'webcomponents-di';

@customElement('parent-component')
export class ParentComponent extends provideDI(LitElement) {

    resolveDependency(key: string, options?: unknown): unknown | null {
        if (key === 'world') {
            return 'DI (from parent)';
        }
        return super.resolveDependency(key, options);
    }

    protected render(): TemplateResult | void {
        return html`
        <div style="background-color: blue">
            <h1>I'm the parent</h1>
            <child-component></child-component>
            <advanced-child-component>
        </div>`
    }
}

Here the mixin provideDI provides a method you can implement called resolveDependency. It is called whenever a request for a dependency is received from a child component. Dependencies are resolved using their key. So if you want to provide a dependency from your component simply return it. resolveDependency takes an optional options parameter which can be used to transfer additional data about the requested dependency.

So far for the basics. There is also a more sophisticated example in packages/simple-example. It uses Typescript decorators to make the entire process a bit simpler still.

To test the example, check out this repository, go into packages/simple-example and run npm start.

Note that this project has only been tested with Typescript yet.

Contributing

If you want to contribute you can either open issues to start a discussion on bugs or ideas or you can submit a Pull Request directly. To get the project running you need to have lerna installed globally. After checking out the project just run lerna bootstrap in its root directory.

TODO

• Right now dependencies can only be provided if the requesting component is a child component. Need to figure out if it should be that way
• Some general cleanup/improvements
• Some more docs
• Tests