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