Ruby implementation of weather monitoring app from Head First Design Patterns, Chapter 2: The Observer Pattern

weather_monitoring_app.rb

require "observer"
require 'attr_extras'

module WeatherMonitoringApp
  module HeatIndex
    # rubocop:disable Metrics/AbcSize
    def self.calculate(t, rh)
      (
        16.923 +
        (0.185212 * t) +
        (5.37941 * rh) -
        (0.100254 * t * rh) +
        (0.00941695 * (t * t)) +
        (0.00728898 * (rh * rh)) +
        (0.000345372 * (t * t * rh)) -
        (0.000814971 * (t * rh * rh)) +
        (0.0000102102 * (t * t * rh * rh)) -
        (0.000038646 * (t * t * t)) +
        (0.0000291583 * (rh * rh * rh)) +
        (0.00000142721 * (t * t * t * rh)) +
        (0.000000197483 * (t * rh * rh * rh)) -
        (0.0000000218429 * (t * t * t * rh * rh)) +
        (0.000000000843296 * (t * t * rh * rh * rh))
      ) -
      (0.0000000000481975 * (t * t * t * rh * rh * rh))
    end
    # rubocop:enable Metrics/AbcSize
  end

  module DisplayElement
    def display
      raise NotImplementedError
    end
  end

  class WeatherData
    include Observable

    def set_measurements(temperature, humidity, pressure)
      self.temperature = temperature
      self.humidity = humidity
      self.pressure = pressure

      measurements_changed
    end

    def measurements_changed
      changed
      notify_observers(temperature, humidity, pressure)
    end

    private

    attr_accessor :temperature, :humidity, :pressure
  end

  class CurrentConditionsDisplay
    include DisplayElement

    def initialize(weather_data)
      self.weather_data = weather_data
      weather_data.add_observer(self)
    end

    def update(temperature, humidity, _pressure)
      self.temperature = temperature
      self.humidity = humidity
      display
    end

    def display
      puts "Current conditions: #{temperature}F degrees and #{humidity}\% humidity"
    end

    private

    attr_accessor :temperature, :humidity, :weather_data
  end

  class HeatIndexDisplay
    include DisplayElement

    def initialize(weather_data)
      self.weather_data = weather_data
      weather_data.add_observer(self)
    end

    def update(temperature, humidity, _pressure)
      self.temperature = temperature
      self.humidity = humidity
      display
    end

    def display
      puts "Head index is #{HeatIndex.calculate(temperature, humidity)}"
    end

    private

    attr_accessor :temperature, :humidity, :weather_data
  end

  class ForecastDisplay
    include DisplayElement

    def initialize(weather_data)
      self.current_pressure = 29.92
      self.weather_data = weather_data
      weather_data.add_observer(self)
    end

    def update(_temperature, _humidity, pressure)
      self.last_pressure = current_pressure
      self.current_pressure = pressure
      display
    end

    def display
      print "Forecast: "

      if current_pressure > last_pressure
        puts "Improving weather on the way!"
      elsif current_pressure == last_pressure
        puts "More of the same"
      elsif current_pressure < last_pressure
        puts "Watch out for cooler, rainy weather"
      end
    end

    private

    attr_accessor :last_pressure, :current_pressure, :weather_data
  end

  class WeatherStation
    def self.main
      weather_data = WeatherData.new

      CurrentConditionsDisplay.new(weather_data)
      HeatIndexDisplay.new(weather_data)
      ForecastDisplay.new(weather_data)

      weather_data.set_measurements(80, 65, 30.4)
      weather_data.set_measurements(82, 70, 29.2)
      weather_data.set_measurements(78, 90, 29.2)
    end
  end
end

WeatherMonitoringApp::WeatherStation.main

With attr_extras, perhaps you can try attr_implement 😄