Observer and Strategy Patterns

Applying Observer and Strategy Patterns for Smart Home Security System and Climate Control

This unit focuses on two design patterns: Observer and Strategy . We'll use these patterns to enable responsive security and flexible climate control in our smart home system.

Quick Summary

Observer Pattern : Purpose : Allows an object (observer) to watch another (subject) and be notified of changes. Steps : Define a Subject class with registration, unregistration, and notification methods (e.g., for a SecuritySystem). Implement observer interfaces or abstract classes that react to changes (e.g., SecurityApp, SecurityLight). Strategy Pattern : Purpose : Defines a family of algorithms and makes them interchangeable. Steps : Define a strategy interface or abstract class with a method for the desired behavior (e.g., TemperatureControlStrategy for controlling temperature). Implement specific strategies with concrete behaviors (e.g., HeatingStrategy, CoolingStrategy). Create a context to use and switch strategies (e.g., a ClimateControl system).

Implementing the Observer Pattern for the Security System

Here is the complete code to implement the Observer pattern using TypeScript : TypeScript // Define the Observer interface interface Observer {update(subject: Subject): void;} // Define the Subject class class Subject {private observers: Observer[] = []; public registerObserver(observer: Observer): void {this.observers.push(observer);} public unregisterObserver(observer: Observer): void {this.observers = this.observers.filter(obs => obs !== observer);} protected notifyObservers(): void {for (const observer of this.observers) {observer.update(this);}}} // Define the SecuritySystem class class SecuritySystem extends Subject {private state: string | null = null; public setState(state: string): void {this.state = state; this.notifyObservers();} public getState(): string | null {return this.state;}} // Implement the SecurityApp observer class SecurityApp implements Observer {public update(subject: Subject): void {if (subject instanceof SecuritySystem) {console.log(`SecurityApp notified. New state: ${subject.getState()}`);}}} // Implement the SecurityLight observer class SecurityLight implements Observer {public update(subject: Subject): void {if (subject instanceof SecuritySystem) {console.log(`SecurityLight activated! State: ${subject.getState()}`);}}} // Example usage const securitySystem = new SecuritySystem(); const app = new SecurityApp(); const light = new SecurityLight(); securitySystem.registerObserver(app); securitySystem.registerObserver(light); securitySystem.setState("Intrusion Detected"); // Expected Output: // SecurityApp notified. New state: Intrusion Detected // SecurityLight activated! State: Intrusion Detected The securitySystem instance notifies both SecurityApp and SecurityLight when the state changes to "Intrusion Detected."

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Implementing the Strategy Pattern for Climate Control

Here is the complete code to implement the Strategy pattern using TypeScript:

TypeScript

// Define the TemperatureControlStrategy interface
interface TemperatureControlStrategy {
    controlTemperature(temperature: number): void;
}

// Implement the HeatingStrategy
class HeatingStrategy implements TemperatureControlStrategy {
    public controlTemperature(temperature: number): void {
        console.log(`Heating to ${temperature} degrees.`);
    }
}

// Implement the CoolingStrategy
class CoolingStrategy implements TemperatureControlStrategy {
    public controlTemperature(temperature: number): void {
        console.log(`Cooling to ${temperature} degrees.`);
    }
}

// Define the ClimateControl context
class ClimateControl {
    private strategy: TemperatureControlStrategy;

    constructor(strategy: TemperatureControlStrategy) {
        this.strategy = strategy;
    }

    public setStrategy(strategy: TemperatureControlStrategy): void {
        this.strategy = strategy;
    }
    
    public controlTemperature(temperature: number): void {
        this.strategy.controlTemperature(temperature);
    }
}

// Example usage
const climateControl = new ClimateControl(new HeatingStrategy());
climateControl.controlTemperature(22); // Expected Output: Heating to 22 degrees.

climateControl.setStrategy(new CoolingStrategy());
climateControl.controlTemperature(18); // Expected Output: Cooling to 18 degrees.

// Define the TemperatureControlStrategy interface
interface TemperatureControlStrategy {
    controlTemperature(temperature: number): void;
}

// Implement the HeatingStrategy
class HeatingStrategy implements TemperatureControlStrategy {
    public controlTemperature(temperature: number): void {
        console.log(`Heating to ${temperature} degrees.`);
    }
}

// Implement the CoolingStrategy
class CoolingStrategy implements TemperatureControlStrategy {
    public controlTemperature(temperature: number): void {
        console.log(`Cooling to ${temperature} degrees.`);
    }
}

// Define the ClimateControl context
class ClimateControl {
    private strategy: TemperatureControlStrategy;

    constructor(strategy: TemperatureControlStrategy) {
        this.strategy = strategy;
    }

    public setStrategy(strategy: TemperatureControlStrategy): void {
        this.strategy = strategy;
    }
    
    public controlTemperature(temperature: number): void {
        this.strategy.controlTemperature(temperature);
    }
}

// Example usage
const climateControl = new ClimateControl(new HeatingStrategy());
climateControl.controlTemperature(22); // Expected Output: Heating to 22 degrees.

climateControl.setStrategy(new CoolingStrategy());
climateControl.controlTemperature(18); // Expected Output: Cooling to 18 degrees.