Merge pull request #9 from dancergraham/feature/observer

Feature/observer

Author: dancergraham

chapter02_observer/readme.md

@@ -1,7 +1,10 @@
 # Chapter 2: Observer design pattern
 
-> **Observer**: defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically.
+> **Observer**: defines a one-to-many dependency between objects so that when one object changes state, all of its
+> dependents are notified and updated automatically.
 
-Very useful design pattern, often used in user interfaces and as part of the popular Model-View-Controller design pattern used, for instance, in Django.
-
-I find the name contradictory: shouldn't it be called the 'Subject' design pattern as the subject is the key actor ?
+A very useful design pattern, often used in user interfaces 
+and as part of the popular Model-View-Controller (MVC) pattern used, 
+for instance, in Django.
+As I wrote out the code I found it very appealing that I did not need 
+to change the subject at all to add new observers.

chapter02_observer/weather.py

@@ -112,12 +112,39 @@ def display(self):
             print("Watch out for cooler, rainy weather")
 
 
+class HeatIndexDisplay(DisplayElement, Observer):
+    def __init__(self, weather_data):
+        self.heat_index = 0.
+        self._weather_data = weather_data
+        weather_data.register_observer(self)
+
+    def update(self, temp, humidity, pressure):
+        self.heat_index = self._compute_heat_index(temp, humidity)
+        self.display()
+
+    def display(self):
+        print(f"Heat index is {self.heat_index:.5f}")
+
+    @classmethod
+    def _compute_heat_index(cls, t, rh):
+        index = ((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)))
+        return index
+
+
 def weather_station():
     weather_data = WeatherData()
 
     current_display = CurrentConditionsDisplay(weather_data)
     statistics_display = StatisticsDisplay(weather_data)
     forecast_display = ForecastDisplay(weather_data)
+    heat_index_display = HeatIndexDisplay(weather_data)
 
     weather_data.set_measurements(80, 65, 30.4)
     weather_data.set_measurements(82, 70, 29.2)