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