discourse/app/controllers/metadata_controller.rb

class MetadataController < ApplicationController
  layout false
  skip_before_filter :preload_json, :check_xhr, :redirect_to_login_if_required

  def manifest
    render json: default_manifest.to_json
  end

  def opensearch
    render file: "#{Rails.root}/app/views/metadata/opensearch.xml"
  end

  private

  def default_manifest
    manifest = {
      name: SiteSetting.title,
      short_name: SiteSetting.title,
      display: 'standalone',
      orientation: 'portrait',
      start_url: "#{Discourse.base_uri}/",
      background_color: "##{ColorScheme.hex_for_name('secondary')}",
      theme_color: "##{ColorScheme.hex_for_name('header_background')}",
      icons: [
        {
          src: SiteSetting.apple_touch_icon_url,
          sizes: "144x144",
          type: "image/png"
        }
      ]
    }

    if SiteSetting.native_app_install_banner
      manifest = manifest.merge({
        prefer_related_applications: true,
        related_applications: [
          {
            platform: "play",
            id: "com.discourse"
          }
        ]
      })
    end

    manifest
  end
end