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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">Dependency injection</h1><p>Dependency injection is an object-oriented design pattern that allows for the decoupling of dependencies between objects making code more maintainable, testable and modular.</p><p>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.</p><p>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.</p><h2 id="example">Example</h2><p>In the example to follow we will have two classes:</p><ul><li><code>ConsoleLogger</code><ul><li>This will simply log something and be a dependency for the class below</li></ul></li></ul><ul><li><code>UserService</code><ul><li>This will depend on <code>ConsoleLogger</code></li></ul></li></ul><p>First we define an interface for the dependency:</p><pre><code class="ts language-ts">interface ILogger {
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log(message: string): void;
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}</code></pre><p>So this is a class that has a single method <code>log</code> which receives a <code>message</code> string as an argument and returns a side-effect.</p><p>Now we’ll implement this class:</p><pre><code class="ts language-ts">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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}</code></pre><p>Now we create a class that depends on it:</p><pre><code class="ts language-ts">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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}</code></pre><p>We can see that the constructor references the <code>Logger</code> interface. Thus we inject the dependency when instantiating <code>UserService</code>:</p><pre><code class="ts language-ts">// 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");</code></pre></div></article><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">OOP</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html> |