How does android recognized the different attributes of element in xml?

Question

I am sure everyone who has work with android before is familiar with the xml namespace declaration xmlns:android = xmlns:android="http://schemas.android.com/apk/res/android. Here is where my question lies.

If the namespace declaration above is just primarily for unique identification then where exactly did all the attributes like android:layout_width or android:layout_height come from?

The only way I can make sense of this is if xmlns:android="http://schemas.android.com/apk/res/android is a schema which contains predefined attributes as a result, you are allowed to modify all these attributes. If that is the case then where is this schema stored?

I have taken a look at the link above to see if my post is potentially a duplicate post but it is not. The other post just basically tells you what xmlns:android="http://schemas.android.com/apk/res/android does. However, I already know what the function of that name space declaration is. My concern is how does android knows what attributes are there for each elements when the namespace above reference nothing? or at least I think that the namespace above is referenced to nothing.

Answer 1

The namespace doesn't reference nothing. It's necessary to know which attributes to pull.

Here is a snippet of the Android Source Code for a ViewGroup.

<declare-styleable name="ViewGroup_Layout">
    <!-- Specifies the basic width of the view.  This is a required attribute
         for any view inside of a containing layout manager.  Its value may
         be a dimension (such as "12dip") for a constant width or one of
         the special constants. -->
    <attr name="layout_width" format="dimension">
        <!-- The view should be as big as its parent (minus padding).
             This constant is deprecated starting from API Level 8 and
             is replaced by {@code match_parent}. -->
        <enum name="fill_parent" value="-1" />
        <!-- The view should be as big as its parent (minus padding).
             Introduced in API Level 8. -->
        <enum name="match_parent" value="-1" />
        <!-- The view should be only big enough to enclose its content (plus padding). -->
        <enum name="wrap_content" value="-2" />
    </attr>
    <!-- Specifies the basic height of the view.  This is a required attribute
         for any view inside of a containing layout manager.  Its value may
         be a dimension (such as "12dip") for a constant height or one of
         the special constants. -->
    <attr name="layout_height" format="dimension">
        <!-- The view should be as big as its parent (minus padding).
             This constant is deprecated starting from API Level 8 and
             is replaced by {@code match_parent}. -->
        <enum name="fill_parent" value="-1" />
        <!-- The view should be as big as its parent (minus padding).
             Introduced in API Level 8. -->
        <enum name="match_parent" value="-1" />
        <!-- The view should be only big enough to enclose its content (plus padding). -->
        <enum name="wrap_content" value="-2" />
    </attr>
</declare-styleable>

https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/res/res/values/attrs.xml#3005

how does android knows what attributes are there for each elements

No different than defining custom attributes for your custom View class, it pulls from a res/values/attrs.xml that is located within the SDK.

Referencing a section there.

Once you define the custom attributes, you can use them in layout XML files just like built-in attributes. The only difference is that your custom attributes belong to a different namespace. Instead of belonging to the http://schemas.android.com/apk/res/android namespace, they belong to http://schemas.android.com/apk/res/[your package name]

So, android is just the namespace for the built-in attributes.