Queries

The simplest way to access data in ECHO from react is by using a useQuery hook on a space. This will return generic objects which can be mutated like regular JavaScript objects. useQuery<T> can also return strongly typed results as will be shown below.

Untyped queries

The first argument to useQuery from package @dxos/react-client is the space and the second is an optional filter which matches all objects which have all the keys and values specified in the filter. The return type is an iterable array of Document objects.

import React from 'react';
import { createRoot } from 'react-dom/client';
import { ClientProvider } from '@dxos/react-client';
import { useQuery, useSpaces } from '@dxos/react-client/echo';
import { useIdentity } from '@dxos/react-client/halo';

export const App = () => {
  useIdentity();
  const [space] = useSpaces();
  const tasks = useQuery(space, { type: 'task' });
  return <>
    {tasks.map((task) => (
      <div key={task.id}>{task.title}</div>
    ))}
  </>;
};

const root = createRoot(document.getElementById('root')!);
root.render(
  <ClientProvider>
    <App />
  </ClientProvider>
);

The API definition of useQuery is below. It returns a generic TypedObject type which supports the ability to set and read arbitrary keys and values. See below for how to add type safety.

useQuery([space], [filter], [options], [deps])

Create subscription.

Returns: TypedObject[]

Arguments:

space: Space

filter: Filter<T>

options: QueryOptions

deps: any[]

Typed Queries

It's possible to obtain strongly typed objects from useQuery.

Because useQuery returns tracked ECHO objects, their type must descend from TypedObject.

DXOS provides a tool to generate these types from a schema definition file.

Benefits of schema declarations

• ability to generate type-safe data access code, which makes development faster and safer. :::

Protobuf is well oriented towards schema migrations, while at the same time being compact and efficient on the wire and in-memory.

Consider this expression of schema declared in protobuf:

syntax = "proto3";

package example.tasks;

message Task {
  option (object) = true;

  string title = 1;
  bool completed = 2;
}

message TaskList {
  option (object) = true;

  string title = 1;
  repeated Task tasks = 2;
}

Note

Note the directives option (object) = true; which instruct the framework to generate TypeScript classes from the marked messages.

Using a tool called dxtype from @dxos/echo-typegen we can generate corresponding classes for use with DXOS Client.

Install the dxtype tool as a dev dependency:

npm install --save-dev @dxos/echo-typegen

Install base types for the generated code:

npm install @dxos/echo-schema

Now scripts have access to dxtype:

dxtype <input protobuf file> <output typescript file>

Tip

If you're using one of the DXOS application templates, this type generation step is pre-configured as a prebuild script for you.

To use the type declarations, simply import the relevant type like Task from the location where dxtype produces output and pass it to useQuery<T>.

For example, defining types in a folder named schema:

The schema protobuf file:

schema/schema.proto

syntax = "proto3";

package example.tasks;

message Task {
  option (object) = true;

  string title = 1;
  bool completed = 2;
}

message TaskList {
  option (object) = true;

  string title = 1;
  repeated Task tasks = 2;
}

The script in package.json:

package.json

{
  "scripts": {
    "prebuild": "dxtype schema/schema.proto schema/index.ts"
  }
}

After executing npm run prebuild, types are available in schema/index.ts:

import React from 'react';
import { createRoot } from 'react-dom/client';

import { ClientProvider } from '@dxos/react-client';
import { useQuery, useSpaces } from '@dxos/react-client/echo';
import { useIdentity } from '@dxos/react-client/halo';

import { Task, types } from './schema';

export const App = () => {
  useIdentity();
  const [space] = useSpaces();
  const tasks: Task[] = useQuery(space, Task.filter());
  return (
    <>
      {tasks.map((task) => (
        <div key={task.id}>
          {task.title} - {task.completed}
        </div>
      ))}
    </>
  );
};

const root = createRoot(document.getElementById('root')!);
root.render(
  <ClientProvider
    onInitialized={async (client) => {
      client.addSchema(types);
    }}
  >
    <App />
  </ClientProvider>,
);

You can pass Task.filter to useQuery to locate items that match specific criteria.

Note the client.addSchema(types) call which registers the generated types with the client.