Comments in Android Layout xml

I would like to enter some comments into the layout XML files, how would I do that?

As other said, the comment in XML are like this

<!-- this is a comment -->

Notice that they can span on multiple lines

<!--
    This is a comment
    on multiple lines
-->

But they cannot be nested

<!-- This <!-- is a comment --> This is not -->

Also you cannot use them inside tags

<EditText <!--This is not valid--> android:layout_width="fill_parent" />

The World Wide Web Consortium (W3C) actually defined a comment interface. The definition says all the characters between the starting ' <!--' and ending '-->' form a part of comment content and no lexical check is done on the content of a comment.

More details are available on developer.android.com site.

So you can simply add your comment in between any starting and ending tag. In Eclipse IDE simply typing <!-- would auto complete the comment for you. You can then add your comment text in between.

For example:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    tools:context=".TicTacToe" >

 <!-- This is a comment -->

</LinearLayout>

Purpose of specifically mentioning in between is because you cannot use it inside a tag.

For example:

<TextView 
    android:text="@string/game_title"
    <!-- This is a comment -->
    android:layout_height="wrap_content"
    android:layout_width="fill_parent"/>

is wrong and will give following error

 Element type "TextView" must be followed by either attribute specifications, ">" or "/>".