Resolution modifiers : Angular dependency injection resolution mechanism of the service starts from component and stops either when a service is found or NUllInjector is reached. This is the default resolution and it can be changed using Resolution Modifier.
Table of Contents
Resolution modifiers fall into three categories.
@Optional()
@Self()
@SkipSelf()
@Host()
@Optional() allows Angular to consider a service you inject to be optional and does not throws the error when the search for the service fails.
@Self is used, Angular will only look for a value that is bound on the component injector for the element that this Directive/Component exists on.
@Host is used, Angular will look for a value that is bound on either the component injector for the element that this Directive/Component exists on, or on the injector of the parent component. Angular calls this parent component the “host”.
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Source code available for download
The source code is available on Stackblitz of the Angular Dependency Injection – Resolution modifiers – @Self or @Optional @Host
Demo Application URL: URL or download the project sources from https://stackblitz.com/edit/inject-and-injectable-qwnzso