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</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><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:
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:
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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