LitState

Simple shared app state management for LitElement.

LitState automatically re-renders your LitElement components, when a shared app state variable they use changes. It's like LitElement's properties, but then shared over multiple components.

It's tiny, simple but still powerful, just like LitElement and lit-html.

Installation

npm install lit-element-state

The name lit-state is unfortunately already taken on npm, so therefore the slightly awkward package name.

Current version: v1.7.0

For more information and update instructions, See the changelog.

Basic idea

You keep your shared state in a LitState derived class. This class contains stateVar variables that contain the state. This class can also contain helper functions that modify the state. Decorate your LitElement classes with the observeState() mixin. This makes your components automatically re-render whenever a stateVar they use changes.

Usage

1. Create a `LitState` object:

import { LitState, stateVar } from 'lit-element-state';

class MyState extends LitState {
    @stateVar() counter = 0;
}

export const myState = new MyState();

Or without javascript decorators:

import { LitState, stateVar } from 'lit-element-state';

class MyState extends LitState {
    static get stateVars() {
        return {
            counter: 0
        };
    }
}

export const myState = new MyState();

2. Make your component aware of your state:

By using the observeState() mixin on your LitElement class and then just using the stateVar variables in your render method:

import { LitElement, html } from 'lit-element';
import { observeState } from 'lit-element-state';
import { myState } from './my-state.js';

class MyComponent extends observeState(LitElement) {

    render() {
        return html`
            <h1>Counter: ${myState.counter}</h1>
            <button @click=${() => myState.counter++}></button>
        `;
    }

}

The components that read myState.counter will automatically re-render when any (other) component updates it.

In more technical words:

A component using the observeState() mixin will re-render when any stateVar - which it read in the last render cycle - changes.

Docs

For more information about how to use LitState, check the docs.

How does this work?

Basics

When you define a stateVar variable, LitState will observe those variables whenever they're get or set. When using the observeState() mixin on a component, during the render of that component, there is a recorder active that records any stateVar that is accessed during the render of that component. At the end of the render, the recorded stateVar variables are collected and whenever one of them changes, the component will be re-rendered. If the re-render uses different stateVar variables, they are again recorded and observed for possible rerenders.

Implementation details

To re-render the component, the observeState() mixin calls LitElement's this.requestUpdate() (with no arguments). This will enqueue an update request for the component. The component will re-render at the end of the execution queue. this.requestUpdate() can be called multiple times during a particular JavaScript event (like a click), and it will only update the component once, at the end of the execution queue. So it doesn't matter when it is called multiple times when multiple stateVar variables are changed during a JavaScript event. This is an optimization feature built-in in LitElement. LitElement uses this optimization for it's own properties. This optimization works in the same way for LitState's stateVar variables.

Also, LitElement uses lit-html, which sees which parts of the template are changed or not. And it will only re-render the HTML elements that have changes.

Notes

You can create and use multiple `LitState` classes at the same time.

It is even encouraged to keep things separate. You can of course have one big LitState derived class which contains all global app state variables. But it is probably cleaner if you categorize it into multiple smaller LitState derived classes. For example, you can put each state class in a separate file, collected in a state/ folder, and import them at the places you need.

You can nest states

If your state requires more hierarchy, you can also nest states. It doesn't matter to your components how your state is structured, as long as it uses the correct references to your stateVar variables. Refer to the docs for more information about nesting states.

Only new assigns trigger a re-render. Updating an object/array won't trigger a re-render.

Just like LitElement's properties, only a new assign of the stateVar triggers a re-render. For example if you have a state like this:

MyState extends LitState {
    @stateVar() myObj = {myKey: 'myValue'};
    @stateVar() myArray = ['one', 'two', 'three'];
}

Then this won't trigger a re-render:

myState = new MyState();
myState.myObj.mykey = 'newValue';
myState.myArray.push('four');

You'll instead need to assign a new object to the stateVar:

myState.myObj = {...myState.myObj, myKey: 'newValue'};
myState.myArray = [...myState.myArray, 'four'];

Watching for changes inside objects is very complex matter and would make LitState way more complicated than desirable. If you are interested in this kind of thing, check out observable-slim.

Extra features

Custom `stateVar` variables

You can easily extend LitState with a custom stateVar handler. An example of this is the asyncStateVar, which is a stateVar variation that makes handling with asynchronous data easy. To make a custom stateVar yourself, create a class that extends from StateVar, exported by LitState. Check out the documentation on this.

FAQ

Why should I use shared state for my components? Doesn't that oppose the concept of web components?

The big feature of web components is that they are encapsulated through the Shadow DOM. That means that their internal state isn't affected by state from the outside. And also that the component's internal state doesn't affect other elements on the page. This makes web components great for creating reusable elements. Reusable elements should have no side-effects, meaning that they shouldn't change state outside of themselves.

Reusable elements are great and we should use them a lot. When you're building a full application however, it is also desirable to have application-specific components that have application-specific side-effects. For example, changing the global app state. And it is of course desirable, that when this global app state changes, the components that use this global app state are synchronized with it.

And you can also have a reusable component that has several internal sub-components. They all might need to share some common internal state.

LitState is created for these use cases, and is meant to make it as simple as possible for the developer.

Why not use Redux or MobX? Any benefits by using this?

To use Redux (or something similar, like unistore) you need a lot of of boiler plate code: Actions, Action Creators and Reducers, you need to dispatch created actions and you need to map the state to your props inside your components. You have helper libraries that make it a bit easier, but I still think it is unnecessarily complicated.

I think MobX is much easier to use, because you don't need to write any boilerplate. However, MobX is a quite large library with a lot of whistles and bells. And for more advanced use-cases it can become relatively complicated to use.

I think a lot of features from MobX are not really necessary when you use LitElement. MobX is mainly created for React. Therefore MobX has optimizations aimed at how React works. LitState is specifically aimed at LitElement. And most of the optimizations MobX created for React are not required for LitElement.

See the section How does this work? to see how LitState works together with LitElement.

Also LitState doesn't try to track changes inside objects, like MobX does. That is also a reason why MobX is complicated. It's nice that you can modify objects and MobX detects that, but it's not very hard to just set a new object. That makes the source code of LitState a lot smaller and simpler, and therefore also easier to understand what is happening. Look here for more details on this.

Basically it comes down to the fact that LitState is written for, and with the same philosophy as, LitElement and lit-html. Which makes it more suitable for developers that like this philosophy.

If you however do want to use MobX with LitElement for some reason, check out lit-mobx.

Development

LitState is brand-new. I created it because I wanted an easy way to deal with shared app state in LitElement, for my own projects. I hope it can make the lives of other developers easier too.

I want to keep LitState small and simple, just like LitElement. So I don't expect to add a lot of features. Only things that are a very common patterns for shared app state management would be suitable to include.

In any case, I will keep expanding the documentation to make the library more accessible. Also I would like to add unit tests, to automatically test the library. I don't have much experience with unit testing in JavaScript, so I need to dive into that.

If you have comments, suggestions, questions, any kind of feedback, or you want to contribute, I would be pleased to hear from you. Feel free to open an issue.

Also see

asyncStateVar - asyncStateVar for LitState, easy handing of async data
LitStyle - Shared component styles for LitElement
LitDocumentEvent - Easily add listeners to the document object
LitDocs - Utilities to create documentation for LitElement related projects

krruzic/lit-state