Behavioral patterns (Observer, Strategy, Command) in Software Engineering - Mini Project: Build & Apply

Understanding Behavioral Patterns: Observer, Strategy, and Command

📖 Scenario: You are building a simple notification system for a weather app. The app needs to notify different parts of the system when the weather changes, allow users to choose different ways to get weather updates, and execute commands like turning notifications on or off.

🎯 Goal: Build a basic structure demonstrating the Observer, Strategy, and Command behavioral patterns using simple code examples and explanations.

📋 What You'll Learn

Create a list of observers representing parts of the system to be notified

Define a strategy variable to select the notification method

Implement a command structure to turn notifications on or off

Combine these patterns to simulate a weather update notification

💡 Why This Matters

🌍 Real World

Behavioral patterns like Observer, Strategy, and Command help organize code in apps that need flexible communication, different behaviors, and controlled actions, such as weather apps, messaging systems, or user interfaces.

💼 Career

Understanding these patterns is important for software developers to write clean, maintainable, and scalable code that adapts to changing requirements.

1

Set up the list of observers

Create a list called observers with these exact string entries: 'Display', 'Logger', and 'MobileApp' representing parts of the system that will receive weather updates.

Software Engineering

# Create a list called observers with 'Display', 'Logger', and 'MobileApp'
# Your code here

Hint

Use square brackets to create a list and separate items with commas.

2

Add a strategy variable for notification method

Create a variable called notification_method and set it to the string 'Email' to represent the chosen way to send weather updates.

Software Engineering

observers = ['Display', 'Logger', 'MobileApp']
# Create a variable notification_method and set it to 'Email'
# Your code here

Hint

Assign the string 'Email' directly to the variable notification_method.

3

Implement a command to toggle notifications

Create a variable called notifications_on and set it to True to represent that notifications are currently enabled.

Software Engineering

observers = ['Display', 'Logger', 'MobileApp']
notification_method = 'Email'
# Create a variable notifications_on and set it to True
# Your code here

Hint

Use the boolean value True to indicate notifications are enabled.

4

Combine patterns to simulate a weather update

Write a for loop using observer as the variable to iterate over observers. Inside the loop, write a comment that says # Notify {observer} via {notification_method}. Then add an if statement that checks if notifications_on is True and write a comment inside it that says # Send update.

Software Engineering

observers = ['Display', 'Logger', 'MobileApp']
notification_method = 'Email'
notifications_on = True
# Use a for loop to notify each observer
# Your code here

Hint

Use a for loop with the variable observer to go through observers. Use an if statement to check notifications_on.

Practice

1. Which behavioral pattern is used when an object needs to notify multiple other objects about a change in its state?

easy

A. Command

B. Strategy

C. Observer

D. Singleton

Solution

1. Step 1: Understand the role of Observer pattern

   The Observer pattern allows one object (the subject) to notify many dependent objects (observers) automatically when its state changes.

2. Step 2: Compare with other patterns

   Strategy selects algorithms at runtime, Command encapsulates requests, and Singleton restricts instance creation. Only Observer fits the notification role.

3. Final Answer:

   Observer -> Option C

4. Quick Check:

   Notification to many = Observer [OK]

Hint: Notification to many objects? Think Observer pattern [OK]

Common Mistakes:

• Confusing Strategy with Observer
• Thinking Command handles notifications
• Mixing Singleton with behavioral patterns

2. Which of the following is the correct way to define a Command pattern interface in object-oriented design?

easy

A. An interface with a method like execute() to perform the action

B. A class that holds multiple algorithms to choose from

C. A subject that notifies observers about changes

D. A singleton class that manages global state

Solution

1. Step 1: Identify Command pattern structure

   The Command pattern encapsulates a request as an object, typically by defining an interface with an execute() method to perform the action.

2. Step 2: Eliminate other options

   A class that holds multiple algorithms to choose from describes Strategy, C describes Observer, and D describes Singleton, none of which define a command interface.

3. Final Answer:

   An interface with a method like execute() to perform the action -> Option A

4. Quick Check:

   Command interface = execute() method [OK]

Hint: Command pattern always has an execute() method [OK]

Common Mistakes:

• Confusing Command with Strategy interface
• Thinking Command notifies observers
• Mixing Singleton with Command

3. Consider this code snippet using the Strategy pattern:

class Context:
    def __init__(self, strategy):
        self.strategy = strategy
    def execute(self, data):
        return self.strategy.do_algorithm(data)

class StrategyA:
    def do_algorithm(self, data):
        return sorted(data)

class StrategyB:
    def do_algorithm(self, data):
        return sorted(data, reverse=True)

context = Context(StrategyA())
result = context.execute([3, 1, 2])
print(result)

What will be the output?

medium

A. [3, 1, 2]

B. [1, 2, 3]

C. [3, 2, 1]

D. TypeError

Solution

1. Step 1: Understand the Strategy pattern usage

   The Context uses StrategyA, whose do_algorithm method sorts the list in ascending order.

2. Step 2: Trace the execution

   Calling context.execute([3, 1, 2]) calls StrategyA.do_algorithm which returns [1, 2, 3].

3. Final Answer:

   [1, 2, 3] -> Option B

4. Quick Check:

   StrategyA sorts ascending = [1, 2, 3] [OK]

Hint: StrategyA sorts ascending, so output is sorted list [OK]

Common Mistakes:

• Assuming StrategyB is used
• Confusing sorting order
• Expecting original list unchanged

4. In the Observer pattern, what is the main issue if observers are not properly detached when no longer needed?

medium

A. Memory leaks due to lingering references

B. Strategy selection becomes unpredictable

C. Commands fail to execute

D. Singleton instance is duplicated

Solution

1. Step 1: Understand observer detachment importance

   If observers are not detached, the subject keeps references to them, preventing garbage collection.

2. Step 2: Identify consequence

   This causes memory leaks because unused observers remain in memory unnecessarily.

3. Final Answer:

   Memory leaks due to lingering references -> Option A

4. Quick Check:

   Undetached observers cause memory leaks [OK]

Hint: Undetached observers cause memory leaks [OK]

Common Mistakes:

• Confusing with Strategy or Command issues
• Thinking it causes command execution failure
• Assuming Singleton duplication

5. You want to design a system where users can perform different operations (like save, load, delete) on files, and you want to queue these operations to execute later or undo them if needed. Which behavioral pattern best fits this requirement and why?

hard

A. Singleton pattern, because it ensures only one file manager exists

B. Observer pattern, because it notifies users about file changes

C. Strategy pattern, because it selects the best file operation algorithm

D. Command pattern, because it encapsulates requests as objects allowing queuing and undo

Solution

1. Step 1: Analyze the requirement for queuing and undo

   Queuing operations and supporting undo requires encapsulating each request as an object to store and manage them flexibly.

2. Step 2: Match with behavioral patterns

   The Command pattern encapsulates requests as objects, enabling queuing, logging, and undo functionality. Observer and Strategy do not support queuing commands, and Singleton only controls instance creation.

3. Final Answer:

   Command pattern, because it encapsulates requests as objects allowing queuing and undo -> Option D

4. Quick Check:

   Queue and undo operations = Command pattern [OK]

Hint: Queue and undo operations? Use Command pattern [OK]

Common Mistakes:

• Choosing Observer for queuing commands
• Confusing Strategy with Command
• Selecting Singleton for operation management