How do you set a flash message in the template in Craft 3?

Question

How does one set a flash message in the templates? I can't find any documentation about this, only for plugins or getting the message.

My use-case is, I am checking if a user is allowed to access a certain page and if they cannot I want to set a flash message, then redirect them to another page. I prefer this than to keep them on the same page and is if else logic to show the message or the rest of the page.

Thanks

Answer 1

You can set flash messages using yii\web\Session::setFlash().

The session component is available to templates via craft.app.session.

{% do craft.app.session.setFlash('error', 'You’re not allowed to go there.') %}

For convenience, Craft’s SessionBehavior class adds shortcut setError() and setNotice() methods to the session component as well.

{% do craft.app.session.setError('You’re not allowed to go there.') %}

You can place that right before your {% redirect %} tag.

Answer 2

Edit: Brandon replied as I was writing this. Feel free to remove :)

You can do that using this in your conditional, just above your redirect tag

{% do craft.app.session.setFlash('error', 'no no no') %}
{% redirect "tosomewhere" %}

Then in the template you are redirecting to

{% set message = craft.app.session.getFlash('error', null, true) %}
{% if message %}
    {{ message }}
{% endif %}