Should clicking a drop-down menu in a native app a second time close it?

1. How long should dropdown be?
2. When should you use a dropdown?
3. Under what circumstances is a menu used instead of a radio button group?

How long should dropdown be?

Avoid dropdowns for more than 10 options or less than 5

There are three main issues with dropdowns that have such a large number of options: no overview – seeing more than 20 uncategorized options can be confusing and intimidating to users, making it difficult for them to find the input they need.

When should you use a dropdown?

The use case for drop-down menus

They also help conserve screen space. Offering drop-down menus can help users avoid scrolling and can quickly get them access to your site's content. For large websites, drop-down menus can save users time by allowing them to jump down a level or two to get to the content they seek.

Under what circumstances is a menu used instead of a radio button group?

Rule#1: When You Want to Emphasize Options

You want the user to read all options. The options are unfamiliar to the user and there is less or no chance that he can predict them.