Promise

Promise is a class available in the Opal stdlib for helping structure asynchronous code.

It can be required inside any Opal applicaton:

require 'promise'

Usage

This example shows how to use a HTTP request from opal-jquery from a callback style, into a promise style handler.

def get_json(url)
  promise = Promise.new

  HTTP.get(url) do |response|
    if response.ok?
      promise.resolve response.json
    else
      promise.reject response
    end
  end

  promise
end

get_json('/users/1.json').then do |json|
  puts "Got data: #{json}"
end.fail do |res|
  alert "It didn't work :( #{res}"
end

A promise can only be resolved or rejected once.

Chaining Promises

Promises become useful when chained together. The prevous example could be extended to get another object from the result of the first request.

get_json('/users/1.json').then do |json|
  get_json("/posts/#{json[:post_id]}.json")
end.then do |post|
  puts "got post: #{post}"
end

Composing Promises

Promise.when can be used to wait for more than 1 promise to resolve (or reject). Lets assume we wanted to get 2 different users:

first = get_json '/users/1.json'
second = get_json '/users/2.json'

Promise.when(first, second).then do |user1, user2|
  puts "got users: #{user1}, #{user2}"
end.fail do
  alert "Something bad happened"
end