How do I redirect users after they submit a form?

Question

How do I redirect users after they submit a form? I tried the following code, but it doesn't work. It says the route doesn't exist.

public  function submitForm(array &$form, FormStateInterface $form_state){
   $url = Url::fromRoute('/mon-profil/obtenir_document'); // == /node/1
   $form_state->setRedirectUrl($url);
}

Answer 1

You are using the wrong method: Url::fromRoute() is expecting the route name, as Kevin said.
If you really need to use a path alias, which would allow you to change the redirect without editing your code (which would keep referring the same path alias), you need to use Url::fromUserInput() that accepts a path starting with /, which also include a path alias, if you remember to add a slash at the beginning.

As side note, if you want to redirect users using a route, you don't first need to get a Url object: Use FormState::setRedirect(), which accepts a route name.
Using a route (for example the one used by Kevin's example), your code would become the following one.

public  function submitForm(array &$form, FormStateInterface $form_state){
   $form_state->setRedirect('entity.node.canonical', ['node' => 1]);
}

In my example, I hard-coded the node ID, but real code would probably get it in a different way. For example, the module could create a node, store its node ID using the state API, and use the state API to retrieve it.

Answer 2

You're getting that error because fromRoute() takes the route name as the argument, not a literal path.

Assuming you know the node id you want to redirect to:

$url = Url::fromRoute('entity.node.canonical', array('node' => $node_id));

You don't ever want to redirect to internal paths like that (in your question) because they are subject to change.