When implementing views in Android, you oftentimes want to have values that are only meant to render nicely in the layout designer.
This is where the
tools: prefix comes in handy!
In the first text view you can see a text attribute (
tools:text) that is only visible in the layout designer. This way, you
can provide a sample text that will render nicely in the designer, but not accidentally expose sample texts in your actual application.
In the list view, the
tools:listitem directive specifies a layout which the list view items should use. This way, your list views are pre-filled with sample items in the designer. If you also use the
tools directives in your list item layout, the pre-filled items will use them as well.
You can use almost any
android attribute and replace it with
tools to have design-time attributes. In fact, if you provide attributes with both prefixes, the
tools prefix is valid during design-time, while your actual
android attribute is used during runtime.
You can find more information in the official documentation.