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158.326 Software Construction - Tutorial 5

Note: To implement this task, create C# project of Windows Forms App using Microsoft Visual Studio 2022. Alternately, you can also choose to create an Console App – this is optional

In this tutorial you will be implementing the Observer pattern covered in the lecture 5, using the Weather Station case study found in chapter 2 of “Head First Design Patterns” . Below is the class diagram for this case study, implemented using the Observer pattern.

We will implement the Observer pattern for this case study as a Windows Forms application that comprises of 4 forms. From the diagram below, WeatherData, CurrentConditionDisplay, ForecastDisplay and StatisticsDisplay   classes will be a separate Form class each. Ignore the ThirdPartyDisplay class in the diagram.

We will build this application incrementally, one display form at a time.

Image from “Head First Design Patterns” by Eric Freeman, Elisabeth Robson, Bert Bates, Kathy Sierra

Exercise 1

Create C# projects of Windows Forms App (or Console App if you don’t have access to Windows computer) using Microsoft Visual Studio 2022. Define the 3 interfaces from the diagram. First create the WeatherData form that looks something like the image below left, in which one can enter the temperature, humidity and pressure values.    Choose yourself what type of a user event you would like to be triggered that will read the text box values and store them into variables in your form (floats or ints). Next, implement the interface methods in the form and any other methods you might need.

Next create the CurrentConditionsDisplay form that looks something like the form above right, whose functionality is to display values from the WeatherData form when it is registered as an observer. Implement the interfaces’ methods and the button click events. For the Update() method in the Observer interface, use the push technique to update the observer objects as shown in the book. That means that the Update() method will be taking the temperature, humidity and pressure values as arguments and passing them to all the observers. Define the constructors in both the CurrentConditionsDisplay and WeatherData forms as shown in the class as well as in the supplementary lecture slides in order to create a multi-form application.

Once you have connected everything up and launched your application, whatever is typed into the WeatherData form should be reflected in the CurrentConditions form when the observer form registers as an observer with the subject, as shown in the images below.

Exercise 2

Extend the application by adding the ForecastDisplay form as pictured below.

Implement very naïve logic which sets the “Expected conditions” label as “Sunshine” if the pressure in the

WeatherData form is above 1000, otherwise as “Rain” .

Exercise 3

Extend the application further by adding the StatisticsDisplay form as pictured below.

This form will keep track of all the temperature entries during the periods when it was registered as an observer, and will provide simple statistics such as the average, maximum and the minimum temperatures experienced.

Exercise 4

You have now seen how the Observer patterns works. There are, however, different ways of implementing it. You have implemented the Observer pattern using the push technique, whereby it is the Subject who decides what data to update the observers with. As an exercise, now modify your code to implement the pull technique. Start by adding a  new  method  to  the  Observer  interface  called  UpdatePull()  which  unlike  the  Update()  method,  takes  no arguments. Modify your program in such a way that enables the observers to fetch the data they specifically need from the Subject.