feat: add Dependency Inversion principle implementation

Author: angelo.yanve

04-SOLID/04x05_DIP/README.md

@@ -0,0 +1,6 @@
+## Example
+
+In this example we are going to show a great implementation of the Dependency Inversion Principle (DIP) with the example of [EmailService](../04x02_OCP/README.md).
+
+We are going to create a `EmailService` class that will be the high-level module and we will create a `SendGridService` and `MailChimpService` classes that will be the low-level modules.\
+We will use the `MailService` class to send an email and we will use the `SendGridService` and `MailChimpService` classes to send the email respecting the DIP.

04-SOLID/04x05_DIP/bad.ts

@@ -0,0 +1,43 @@
+interface IMail {
+    from: string
+    to: string
+    subject: string
+    body: string
+    //... other fields
+}
+  
+interface ShortEmailTransmissionResult {
+    success: boolean
+    message: string
+}
+
+interface IMailService {
+    sendMail(email: IMail): Promise<ShortEmailTransmissionResult>
+}
+
+
+class _SendGridService implements IMailService {
+    sendMail(email: IMail): Promise<ShortEmailTransmissionResult> {
+      // send mail
+      return Promise.resolve({success: true, message: 'Mail sent successfully'});
+    }
+}
+
+class _MailChimpService implements IMailService {
+    sendMail(email: IMail): Promise<ShortEmailTransmissionResult> {
+      // send mail
+      return Promise.resolve({success: true, message: 'Mail sent successfully'});
+    }
+}
+
+class _EmailService {
+    // we're limiting ourselves to a particlar concrete class.
+    private emailService: _SendGridService; // <- concretion
+    constructor (emailService: _SendGridService) { // <- concretion
+      this.emailService = emailService;
+    }
+    
+    send_newMail(email: IMail): Promise<ShortEmailTransmissionResult> {
+      return this.emailService.sendMail(email);
+    }
+}

04-SOLID/04x05_DIP/good.ts

@@ -0,0 +1,45 @@
+interface IMail {
+    from: string
+    to: string
+    subject: string
+    body: string
+    //... other fields
+}
+  
+interface ShortEmailTransmissionResult {
+    success: boolean
+    message: string
+}
+
+interface IMailService {
+    sendMail(email: IMail): Promise<ShortEmailTransmissionResult>
+}
+
+
+class SendGridService implements IMailService {
+    sendMail(email: IMail): Promise<ShortEmailTransmissionResult> {
+      // send mail
+      return Promise.resolve({success: true, message: 'Mail sent successfully'});
+    }
+}
+
+class MailChimpService implements IMailService {
+    sendMail(email: IMail): Promise<ShortEmailTransmissionResult> {
+      // send mail
+      return Promise.resolve({success: true, message: 'Mail sent successfully'});
+    }
+}
+
+//? IMPORTANT: We're no longer limiting ourselves to a particlar concrete class.
+
+class EmailService {
+    private emailService: IMailService; // <- abstraction
+    constructor (emailService: IMailService) { // <- abstraction
+      this.emailService = emailService;
+    }
+
+    sendMail(email: IMail): Promise<ShortEmailTransmissionResult> {
+      return this.emailService.sendMail(email);
+    }
+
+  }

04-SOLID/04x05_DIP/use-case.ts

@@ -0,0 +1,6 @@
+
+
+const mailChimpService = new MailChimpService();
+const sendGridService = new SendGridService();
+
+const emailService = new EmailService(mailChimpService); //or new EmailService(sendGridService); 

04-SOLID/README.md

@@ -37,4 +37,12 @@ This principle is about **inheritance** and **polymorphism**. It states that if
 
 This principle is about **interfaces**. It states it’s better to have `smaller`, `focused` **interfaces** rather than large, monolithic ones.
 
-**Example:** go to files[InterfaceSegregationPrinciple](./04x04_ISP)
+**Example:** go to files[InterfaceSegregationPrinciple](./04x04_ISP)
+
+## Dependency Inversion Principle (DIP)
+***High-level modules should not depend on low-level modules. Both should depend on abstractions.*** 
+
+- **High-level modules** are `interfaces` and `abstract classes` that define the policy of the application. 
+- **Low-level modules** are the concrete `classes` that implement the policy.
+
+**Example:** go to files [DependencyInversionPrinciple](./04x05_DIP)