gem 'fcmpush'

Fcmpush.configure do |config|
  ## for message push
  # firebase web console => project settings => service account => firebase admin sdk => generate new private key

  # pass string of path to credential file to config.json_key_io
  config.json_key_io = "#{Rails.root}/path/to/service_account_credentials.json"
  # Or content of json key file wrapped with StringIO
  # config.json_key_io = StringIO.new('{ ... }')

  # Or set environment variables
  # ENV['GOOGLE_ACCOUNT_TYPE'] = 'service_account'
  # ENV['GOOGLE_CLIENT_ID'] = '000000000000000000000'
  # ENV['GOOGLE_CLIENT_EMAIL'] = '[email protected]'
  # ENV['GOOGLE_PRIVATE_KEY'] = '-----BEGIN PRIVATE KEY-----\n...\n-----END PRIVATE KEY-----\n\'

  ## for topic subscribe/unsubscribe because they use regacy auth
  # firebase web console => project settings => cloud messaging => Project credentials => Server key
  config.server_key = 'your firebase server key'
  # Or set environment variables
  # ENV['FCM_SERVER_KEY'] = 'your firebase server key'
end

require 'fcmpush'

project_id   = "..." # Your project_id
device_token = "..." # The device token of the device you'd like to push a message to

client  = Fcmpush.new(project_id)
payload = { # ref. https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages
  message: {
    token: device_token,
    notification: {
      title: "this is title",
      body: "this is message body"
    }
  }
}

response = client.push(payload)

json = response.json
json[:name] # => "projects/[your_project_id]/messages/0:1571037134532751%31bd1c9631bd1c96"

require 'fcmpush'

project_id   = "..." # Your project_id
device_tokens = ["...A", "...B", "...C"] # The device token of the device you'd like to push a message to

client  = Fcmpush.new(project_id)

payloads = device_tokens.map do |token|
  { # ref. https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages
    message: {
      token: token,
      notification: {
        title: "this is title",
        body: "this is message body"
      }
    }
  }
end

response = client.batch_push(payloads)

response_array = response.json
response_array.first[:name] # => "projects/[your_project_id]/messages/0:1571037134532751%31bd1c9631bd1c96"

require 'fcmpush'

project_id   = "..." # Your project_id
topic = "your_topic_name"
device_tokens = ["device_tokenA", "device_tokenB", ...] # The device tokens of the device you'd like to subscribe

client  = Fcmpush.new(project_id)

response = client.subscribe(topic, device_tokens)
# response = client.unsubscribe(topic, device_tokens)

json = response.json
json[:results] # => [{}, {"error":"NOT_FOUND"}, ...]  ref. https://developers.google.com/instance-id/reference/server#example_result_3