Flash messages
Rather than blindly redirecting users without information, we can utilize flash messages in Yii2 to display one-time useful pieces of information to the user, such as what action they need to perform in order to complete another action (such as them having to log in to view the secure page).
In Yii1, user-specified flash messages can be tied directly to the user component. In Yii2, they're solely managed by the session object. In this section, we'll show how to use flash messages by example by enhancing our login view. We'll also take advantage of several of the other widgets and helpers we've covered in previous chapters.
As shown in the previous section, when a user is a guest and they try to access a secure page, we simply redirect them back to the login page without any information. To provide good user experience, we can set a flash message before redirecting the user and then display that flash message in our login view. As an example, the behaviors() method of our controller...
