Ability to find out where router navigation came from

[ychaikin]

Which @angular/* package(s) are relevant/related to the feature request?

router

Description

The angular router lacks what the "referer" header does in HTTP, so to speak.

Proposed solution

Create previousRoute as part of the API.

Alternatives considered

None.

[atscott]

What is the use-case for this? The Angular router provides access to the previous successful navigation through Router.getCurrentNavigation().previousNavigation: https://angular.io/api/router/Navigation#previousNavigation

Of course, this is only available when a navigation is ongoing, but you could also save this value somewhere by subscribing to the events and filtering to NavigationEnd.

Is there a reason you can't use the existing APIs to accomplish your goal?

[ychaikin]

@atscott Thanks for the response.

The use case, which I have encountered quite a number of times, is when you want to do something inside of a component based on where the navigation was triggered from. By the time you are dealing with that type of logic, that information is not available in the current Router API.

In the most recent use case, I was using NGRX and triggering an "executeSearch" action from multiple places (some components, some not). The effect that handles that action is quite disconnected from the component and the "executeSearch" action can be triggered in many different ways, some because of navigation, some not. The solution to this is a bit of a "pretzel" where I have to create yet another effect that I listen to the router navigation event and then trigger some other chain of actions based on that. The whole chain is unnecessarily complicated simply because I can't look into this.router.previousUrl in the "executeSearch" action and change the logic based on that.

So, yes, of course, it is possible to get around this lack of "previous" and I can even create a state which keeps track of previously successful navigation and keep updating it. I am asking precisely that I shouldn't have to.

This is especially true since this comes up so often (not sure why I haven't ventured to create an issue for this until now).

It seems the API is great at telling you information while navigation is ongoing, thus getCurrentNavigation(), which allows you to look at previous navigation "mid-flight". This is great if you want to code to some functionality/decision that SHOULD be occurring mid-flight of navigation, not when navigation has already completed.

When it comes to information that's kept in the router "state", i.e., the Router, only the current url property is provided. previousUrl or some such would help a lot. I just feel like the HTTP "referer" header data has been lost here (i.e., the developer has to implement it).

Let me know what you think.

[angular-robot]

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[atscott]

@ychaikin Thanks for the thoughtful response. That's totally fair and given that previousSuccessfulNavigation is already available in the public API (but only during an ongoing navigation), I don't think it's that significant of a change to the public API surface to make the value accessible all the time. That is to say, when looking at information that is private implementation detail, I want to ensure that those details are stable enough to expose as public API. Since this is already exposed, there wouldn't be any additional support burden on maintaining the behavior/preventing breakages.

[ychaikin]

@atscott Thanks!

[angelnikolov]

@atscott @ychaikin I just stumbled upon this issue. We were basically using both this.router.getCurrentNavigation().previousNavigation and a custom state paramter accessed through this.router.getCurrentNavigation().extras.state['parameter'] to figure out the correct back route of our application.
I also found #48744 and from there I understand that in Angular 15 we can no longer use getCurrentNavigation() for nested routes, but should rather use location.getState() to extract the state parameters via the history API. However, how can we go about getting the previousNavigation now? That's not provided by the location service and we seem to be in a dead end with our migration.

Is using the private (this.router as any).navigationTransitions.lastSuccessfulNavigation a good resolution for this?
P.S Sorry for mentioning you both, just figured that you might have a lead on this considering you were looking at a similar issue above 🙇

[atscott]

@angelnikolov Ah, that's interesting. Exposing the lastSuccessfulNavigation would probably make upgrading easier. For me, that would bump the priority here. It's unfortunate that this didn't come up earlier since we just released v15.2 so this feature can't go in until v16 at the earliest now.

Is using the private (this.router as any).navigationTransitions.lastSuccessfulNavigation a good resolution for this?

I could never recommend accessing private API unless you're prepared to have it break at any time without a replacement. Is there a reason you couldn't just track this information in your own service by subscribing to NavigationEnd/ActivationEnd?