How Can You Inject Services Outside of Components in Angular 2 (Beta)?

Service Injection Outside of Components in Angular 2 (Beta)

Understanding Injection Beyond Components

In Angular 2 (Beta), dependency injection enables developers to pass around services seamlessly within components. However, it is often necessary to inject services outside of component classes.

Injecting Services via @Injectable Decorator

The key to injecting services outside of components lies in using the @Injectable decorator on the service class. This decorator signifies that the constructor parameters of the class can be injected.

Hierarchical Injection System

Angular 2 employs a hierarchical injector system that maps to the component tree. Injectors for services are not explicitly defined; instead, they are implicitly associated with components. Hierarchical linking ensures that child injectors have access to providers defined in parent injectors.

Sample Application and Injector Relationships

Consider the following sample application with an AppComponent (main component), ChildComponent, Service1 (used by ChildComponent), and Service2 (used by Service1):

<br>Application<pre class="brush:php;toolbar:false"> |

AppComponent

 |

ChildComponent
getData() --- Service1 --- Service2

This application has three injectors: application, AppComponent, and ChildComponent. Service1 injection for ChildComponent and Service2 injection for Service1 involve the following injector hierarchy:

1. ChildComponent injector
2. AppComponent injector
3. Application injector

Dynamic Dependency Resolution

Injectors dynamically resolve dependencies based on the injector hierarchy. Missing providers are automatically searched for in parent injectors, cascading up to the application injector.

Provider Configuration at Multiple Levels

Providers can be configured at each injector level (application, component). This allows for flexible dependency sharing:

• Application-level providers create instances shared throughout the application.
• Component-level providers create instances shared within the component tree and involved services.

Conclusion

By utilizing the @Injectable decorator and understanding the injector hierarchy, developers can effectively inject services outside of components in Angular 2 (Beta), enabling more efficient dependency management and code organization.

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