owl/doc/store.md

🦉 Store 🦉

Content

• Overview
• Example
• Reference
  • Public API
  • Mutations
  • Actions
  • Getters
  • Connecting a Component
  • Semantics
  • Good Practices

Overview

Managing the state in an application is not an easy task. In some cases, the state of an application can be part of the component tree, in a natural way. However, there are situations where some part of the state need to be displayed in various parts of the user interface, and then, it is not obvious which component should own which part of the state.

Owl's solution to this issue is a centralized store. It is a class that owns some state, and let the developer update it in a structured way (through mutations and actions). Owl components can then connect to the store, and will be updated if necessary.

Note: Owl's store is inspired by React Redux and VueX.

Example

Here is what a simple store looks like:

const actions = {
  addTodo({ commit }, message) {
    commit("addTodo", message);
  }
};

const mutations = {
  addTodo({ state }, message) {
    const todo = {
      id: state.nextId++,
      message,
      isCompleted: false
    };
    state.todos.push(todo);
  }
};

const state = {
  todos: [],
  nextId: 1
};

const store = new owl.Store({ state, actions, mutations });
store.on("update", () => console.log(store.state));

// updating the state
store.dispatch("addTodo", "fix all bugs");

Reference

The store is a simple owl.EventBus that triggers update events whenever its state is changed. Note that these events are triggered only after a microtask tick, so only one event will be triggered for any number of state changes in a call stack.

Also, it is important to mention that the state is observed (with an owl.Observer), which is the reason why it is able to know if it was changed. This implies that state changes need to be done carefully in some cases (adding a new key to an object, or modifying an array with the arr[i] = newValue syntax). See the Observer's documentation for more details.

Public API

1. constructor
2. commit
3. dispatch

Mutations

Mutations are the only way to modify the state. Changing the state outside a mutation is not allowed (and should throw an error). Mutations are synchronous.

const mutations = {
  setLoginState({ state }, loginState) {
    state.loginState = loginState;
  }
};

Mutations are called with the commit method on the store, and can receive an arbitrary number of arguments.

store.commit("setLoginState", "error");

Actions

Actions are used to coordinate state changes. It is also useful whenever some asynchronous logic is necessary. For example, fetching data should be done in an action.

const actions = {
  async login({ commit }, info) {
    commit("setLoginState", "pending");
    try {
      const loginInfo = await doSomeRPC("/login/", info);
      commit("setLoginState", loginInfo);
    } catch {
      commit("setLoginState", "error");
    }
  }
};

Actions are called with the dispatch method on the store, and can receive an arbitrary number of arguments.

store.dispatch("login", someInfo);

Note that anything returned by an action will also be returned by the dispatch call.

Getters

Usually, data contained in the store will be stored in a normalized way. For example,

{
    posts: [{id: 11, authorId: 4, content: 'Greetings'}],
    authors: [{id: 4, name: 'John'}]
}

However, the user interface will probably need some denormalized data like

{id: 11, author: {id: 4, name: 'John'}, content: 'Greetings'}

This is what getters are for: they give a centralized way to process and transform the data contained in the store.

const getters = {
  getPost({ state }, id) {
    const post = state.posts.find(p => p.id === id);
    const author = state.authors.find(a => (a.id = post.id));
    return {
      id,
      author,
      content: post.content
    };
  }
};

// somewhere else
const post = store.getters.getPost(id);

Getters take at most one argument.

Note that getters are cached if they don't take any argument, or their argument is a string or a number.

Connecting a Component

At some point, we need a way to access the state in the store from a component. By default, an Owl Component is not connected to any store. To do that, we need to create a component inheriting from OwlComponent:

const actions = {
  increment({ commit }) {
    commit("increment", 1);
  }
};
const mutations = {
  increment({ state }, val) {
    state.counter += val;
  }
};
const state = {
  counter: 0
};
const store = new owl.Store({ state, actions, mutations });

class Counter extends owl.ConnectedComponent {
  static mapStoreToProps(state) {
    return {
      value: state.counter
    };
  }
  increment() {
    this.env.store.dispatch("increment");
  }
}

const counter = new Counter({ store, qweb });

<button t-name="Counter" t-on-click="increment">
  Click Me! [<t t-esc="props.value"/>]
</button>

The ConnectedComponent class can be configured with the following fields:

• mapStoreToProps: a function that extracts the props of the Component from the state of the Store and returns them as a dict.
• getStore: a function that takes the env in arguments and returns an instance of Store to connect to (if not given, connects to env.store)
• hashFunction: the function to use to detect changes in the state (if not given, generates a function that uses revision numbers, incremented at each state change)
• deep (boolean): [only useful if no hashFunction is given] if false, only watch for top level state changes (true by default)

Semantics

The Store and the ConnectedComponent try to be smart and to optimize as much as possible the rendering and update process. What is important to know is:

• components are always updated in the order of their creation (so, parent before children)
• they are updated only if they are in the DOM
• if a parent is asynchronous, the system will wait for it to complete its update before updating other components.
• in general, updates are not coordinated. This is not a problem for synchronous components, but if there are many asynchronous components, this could lead to a situation where some part of the UI is updated and other parts of the UI is not updated.

Good Practices

• avoid asynchronous components as much as possible. Asynchronous components lead to situations where parts of the UI is not updated immediately.
• do not be afraid to connect many components, parent or children if needed. For example, a MessageList component could get a list of ids in its mapStoreToProps and a Message component could get the data of its own message
• since the mapStoreToProps function is called for each connected component, for each state update, it is important to make sure that these functions are as fast as possible.