Previously, an interceptor attempting to inject HttpClient directly
would receive a circular dependency error, as HttpClient was
constructed via a factory which injected the interceptor instances.
Users want to inject HttpClient into interceptors to make supporting
requests (ex: to retrieve an authentication token). Currently this is
only possible by injecting the Injector and using it to resolve
HttpClient at request time.
Either HttpClient or the user has to deal specially with the circular
dependency. This change moves that responsibility into HttpClient
itself. By utilizing a new class HttpInterceptingHandler which lazily
loads the set of interceptors at request time, it's possible to inject
HttpClient directly into interceptors as construction of HttpClient no
longer requires the interceptor chain to be constructed.
Fixes#18224.
PR Close#19809
HttpClient is an evolution of the existing Angular HTTP API, which exists
alongside of it in a separate package, @angular/common/http. This structure
ensures that existing codebases can slowly migrate to the new API.
The new API improves significantly on the ergonomics and features of the legacy
API. A partial list of new features includes:
* Typed, synchronous response body access, including support for JSON body types
* JSON is an assumed default and no longer needs to be explicitly parsed
* Interceptors allow middleware logic to be inserted into the pipeline
* Immutable request/response objects
* Progress events for both request upload and response download
* Post-request verification & flush based testing framework
