Which is the correct use case of Full Modal Windows

Question

I don't know when I should use a modal and when not.

For example, a list of things to buy, user click in a button "new", should I open a modal or a new page as expected for web apps?

The information behind the modal is not useful for the fill of the form inside it. The user can cancel it anytime (I can click add and think "screw it, not now").

Answer 1

By definition a modal window is used to break a users flow and alert them to something or grab the users attention.

It sounds like you do not want to break the users flow.

Webopedia has a pretty good explanation about why a modal would be used:

A modal window is commonly used when the author wants to retain the user's focus on the information in the modal as it is impossible for the user to interact with the other windows of the same process.

Wikipedia lists a few use cases:

• Drawing attention to vital pieces of information. This use has been criticised as ineffective because users are bombarded with too many dialog boxes, and habituate to simply clicking "Close", "Cancel", or "OK" without reading or understanding the message.
• Blocking the application flow until information required to continue is entered, as for example a password in a login process. Another example are file dialogs to open and save files in an application.
• Collecting application configuration options in a centralized dialog. In such cases, typically the changes are applied upon closing the dialog, and access to the application is disabled while the edits are being made.
• Warning that the effects of the current action are not reversible. This is a frequent interaction pattern for modal dialogs, but it is also criticised by usability experts as being ineffective for its intended use (protection against errors in destructive actions) due to habituation. Actually making the action reversible (providing an "undo" option) is recommended instead.