Understanding Dependency Injection in Dynamically Loaded Components in Angular

Explore how Angular's Dependency Injection behaves in dynamically loaded components and avoid common pitfalls with lazy-loaded modules.
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Understanding Dependency Injection in Dynamically Loaded Components in Angular

Angular’s powerful Dependency Injection (DI) system is essential for managing services and components in a modular way. However, when dealing with dynamically loaded components and services, especially in scenarios involving lazy-loaded modules, developers can encounter unexpected behaviors. This guide dives into a common problem and its solution to enhance your understanding of Angular’s DI system.

The Problem: Unexpected Behavior of Services

Imagine you have a service in your Angular application that’s injected into multiple components, one of which is lazy-loaded, and the other is loaded dynamically. You might expect that since the service is annotated with providedIn: 'root', it should be a singleton, and any instance of the service should share the same state. However, you observe differing values for an internal property of the service, indicating that multiple instances are being created.

Example Service

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In this case, after initializing both components, you see two different numbers printed from the service’s constructor. Why does this happen?

The Solution: Understanding Lazy Loaded Modules

Why Different Random Numbers?

The key reason for this behavior is that the module defining SomeService is initialized twice—once for each component using it. When using Angular's lazy-loaded modules, the service does not behave as a singleton across the entire application as one might expect.

Key Points to Consider:

Lazy-loaded modules aren't cached: Angular's lazy-loading mechanism does not cache modules (i.e., it does not "remember" that a particular lazy-loaded module has been loaded already). Each request for that module results in a fresh instance unless you implement caching yourself.

Scope of Injection: Each dynamically loaded component creates its injector as a leaf within the current injector hierarchy. As a result, the instances of services defined in the lazy-loaded module get created again.

Why Is the Module Initiated Twice?

When you call methods like loadChildren or moduleFactory.create(this.injector), Angular does not cache the instances of lazy-loaded modules. Instead, each time this method runs, it creates a new instance of the module and its injector. This leads to repeated initializations of services if those services are defined within the module.

Caching the Lazy Loaded Module

To prevent this issue and ensure that you only create a module once, consider implementing a caching layer in your service loader. Here’s how you can do that:

Create a Cache Map: Use a Map to store instances of modules that have already been created.

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Check the Cache Before Creating a Module: When attempting to load a module, first check the cache. If it exists, return the cached instance instead of creating a new one.

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By implementing this caching mechanism, you can effectively ensure that each lazy-loaded module is initialized only once, thus preserving the singleton nature of services like SomeService across different parts of your application.

Conclusion: Take Control of Angular’s Dependency Injection

Understanding how Angular’s Dependency Injection works, especially in relation to lazy-loaded modules and dynamically loaded components, is crucial for building robust applications. By recognizing that lazy-loaded modules do not share services as singletons due to their unique injector instances, you can take proactive steps to manage services effectively.

Ensure you implement caching in your dynamic loading service to avoid unintentional multip

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