Android布局中的xmlns属性

Question

I'm well-versed in Java, but not that familiar with XML. In many of my layout/menu files, I have these lines:

xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"

Somtimes I have all of them, sometimes just some combination. Could somebody give an explanation towards these three lines? Under which scenarios should we include each one? So far I have just been adding them in whenever Android Studio prompts me, but I would like to understand the use cases. Thanks!

Answer 1

Could somebody give an explanation towards these three lines?

Each declare a different XML namespace ( android , app , and tools , respectively).

Under which scenarios should we include each one?

If your XML file has one or more attributes prefixed with android: , you need to declare the android namespace. Many XML files will have these, from framework-defined runtime attributes.

If your XML file has one or more attributes prefixed with app: , you need to declare the app namespace. Some layout and menu resource files will have these, from library-defined attributes.

If your XML file has one or more attributes prefixed with tools: , you need to declare the tools namespace. Few resource files will have these, from attributes used to help the build tools understand what to do.