add: notes on dependency injection

Software_Engineering/Dependency_injection.md

@@ -0,0 +1,71 @@
+---
+categories:
+  - Software Engineering
+tags: [OOP]
+---
+
+# Dependency injection
+
+Dependency injection is an object-oriented design pattern that allows for the decoupling of dependencies between objects making code more maintainable, testable and modular.
+
+The basic idea is to resist having any class (C) depend on the specific implementation details of any other class to which it sustains a dependency (D). If we can abstract the implementation of D, this makes it easier to swap-out and change C's dependencies later on without having to re-write C as a result of changes to D.
+
+This is where interfaces become very helpful because they are schematic representations of the main methods in a given class (basically their names, params and return value). As long as this is kept consistent in D when changes are made, you avoid conflicts when changes or refactorings are made to D.
+
+## Example
+
+In the example to follow we will have two classes:
+
+- `ConsoleLogger`
+  - This will simply log something and be a dependency for the class below
+
+* `UserService`
+  - This will depend on `ConsoleLogger`
+
+First we define an interface for the dependency:
+
+```ts
+interface ILogger {
+  log(message: string): void;
+}
+```
+
+So this is a class that has a single method `log` which receives a `message` string as an argument and returns a side-effect.
+
+Now we'll implement this class:
+
+```ts
+class ConsoleLogger implements ILogger {
+  log(message: string): void {
+    console.log(message);
+  }
+}
+```
+
+Now we create a class that depends on it:
+
+```ts
+class UserService {
+  private logger: ILogger;
+
+  constructor(logger: ILogger) {
+    this.logger = logger;
+  }
+
+  createUser(username: string): void {
+    // ... (some logic to create a user)
+    this.logger.log(`User created: ${username}`);
+  }
+}
+```
+
+We can see that the constructor references the `Logger` interface. Thus we inject the dependency when instantiating `UserService`:
+
+```ts
+// First implement our earlier class that matches the `Logger` in its shape:
+const logger: ILogger = new ConsoleLogger();
+
+// Then pass it as a dependency:
+const userService: UserService = new UserService(logger);
+userService.createUser("John Doe");
+```

_meta/Topic_Log.md

@@ -54,11 +54,11 @@
 
 ## Git
 
-- [ ] What is rebasing?
-- [ ] What is `git switch`
+- [x] What is rebasing?
+- [x] What is `git switch`
 - [x] What is cherry-picking
 - [x] Tagging (also in relation to Git flow)
-- [ ] How can you rollback without a hard-reset, i.e. how can you keep the future state (from the point of view of the rolled-back branch) accessible?
+- [x] How can you rollback without a hard-reset, i.e. how can you keep the future state (from the point of view of the rolled-back branch) accessible?
 - [ ] Difference between restore and reset
 
 ## JavaScript
@@ -75,5 +75,6 @@ Look into these new features:
 
 ## TypeScript
 
-- Records
-- Mapped types
+- ~~Records~~
+- ~~Mapped types~~
+- SOLID principles OOP