Mat Stepper needs a beforeSelectionChange event

[gmurph52]

Feature Description

There should be an event that is triggered when you attempt to navigate to a different step via the mat stepper header.

Currently, there is only a selectionChange event that doesn't get triggered until after the next navigation takes place.

Use Case

In my scenario, when a user attempts to navigate to the next step, I need to first call an endpoint to validate the data they entered. Once I have validated that the data is correct, I can proceed to the next step with stepper.next(). This works as expected if the user clicks on my next button with a custom (click) event, but if they click on the header to navigate, there is no way to call the endpoint before navigation.

Using selectionChange does not work, because there is no way to prevent navigation after the StepperSelectionEvent has happened.

Alternatively, instead of a beforeSelectionChange event, if there were a way to prevent the next navigation when subscribing to the selectionChange event, that would also work. Or if there was a way to prevent navigation from the header, but still capture clicks, so that stepper.next could manually be called.

[crisbeto]

For what it's worth, you should be able to get this to work by doing your validation in an async validator.