What is the meaning of xmlns:tools in Android XML layout?

Question

For example, in:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
...

Do I need to put it?

Answer 1

It defines the XML namespace of the document. You should put it, otherwise tags like <RelativeLayout> could be not recognied by the parser.

Namespaces are a way for XML documents to include tags from various vendors. By using xmlns attribute you declare, that, by default, you're using XML elements defined here: http://schemas.android.com/apk/res/android (note that this link is broken - this discussion explains why).

You also declare additional namespace, tools, which is not your default namespace, thus when referencing elements or attributes defined there, you must add tools prefix, on example:

tools:context=".SomeActivity"

Answer 2

In Android, the xmlns:tools attribute is used to define the XML namespace for the "tools" namespace prefix. It is commonly used in layout XML files to provide additional tools and information to aid in the development process. The tools namespace provides attributes that are used during the design and development of the app, but are ignored or stripped out during compilation and execution.

Here's an example of how the xmlns:tools attribute is typically used in an Android layout XML file:

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        tools:text="Hello World!"
        android:textColor="@color/black" />

</LinearLayout>

In the above example, the tools:context attribute is used to specify the activity class associated with the layout for design-time rendering and preview purposes. The tools:text attribute is used to provide placeholder text that is only displayed in the Android Studio layout editor, but not at runtime.

By utilizing the tools namespace, developers can customize the appearance and behavior of their layouts specifically for design and testing purposes without affecting the final production code.

Answer 3

In fact, when you do :

<RelativeLayout android:id> </RelativeLayout>

Instead of calling android:id, the xml will call http://schemas.android.com/apk/res/android:id . It just the page that declare all the attribute and views that you can use in your xml.

Here is an explanation. http://www.w3schools.com/xml/xml_namespaces.asp

Answer 4

Following is a useful link from Android dev portal: https://developer.android.com/studio/write/tool-attributes.html

It says

Android Studio supports a variety of XML attributes in the tools namespace that enable design-time features (such as which layout to show in a fragment) or compile-time behaviors (such as which shrinking mode to apply to your XML resources). When you build your app, the build tools remove these attributes so there is no effect on your APK size or runtime behavior.

i.e. tools namespace helps designing UI and all attributes with prefix 'tools' will be removed at build time.