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Error parsing XML: unbound prefix in custom view

I tried customizing a view from a library I'm using for my android app. The default xml code for this looks like this:
<it.gmariotti.cardslib.library.view.CardView
android:id="#+id/carddemo"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="12dp"/>
This works without any problems. But when I add a new line in the element, to use my custom layout, I get an error in the opening line, that you can see in the title of this question: This is the line I was adding in the end:
card:card_layout_resourceID="#layout/custom_layout" />
This is also exactly the code that was used as an example in the docs of the library.

You probably forgot to "define" a namespace for the custom attribute card. So add this to your root-view in your xml-file:
xmlns:card="http://schemas.android.com/apk/res-auto"

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

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?

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"

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.

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

xml file vs xml layout

I created two layout files.
Android XML Layout File which created this xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
</LinearLayout>
and an Android XML File
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
</LinearLayout>
From the looks of it they are both the same. When to use one over the other since they are both creating the same xml?

Yeah they are prograticaly the same, there isn't different type of xml. What matters, is the location of the xml (layout, values, ...).
In eclipse, there maybe is a small difference in the IDE view, i'm not sure that you have the Graphical Layout view if you create a Android XML file instead of a layout file (it's a detail because, you still can add this view ;) )
But results apk will be identical.

The reason why you would use an Android XML, is if you wanted to use the Layout Tool (the GUI layout editor). The Eclipse IDE will react to this file differently.
But, the code is the same, so the APK you create will also be the same - it doesn't matter in your final output.
I always use Android XML files, so they integrate into the ADT tools better.

the only difference as far as I can see is that in xml you can't use both "android:layout-width:" and "android:layout-height:" attributes which makes xml for example suitable for the use of menu and item bar and other uses.

Android Framework modification : Adding a custom layout

I am trying to add a new layout to the framework say my_layout.
I succeeded partially in that .
I defined my layout in an xml and put it inside frameworks/base/core/res/res/layout
I then mentioned the id in frameworks/base/core/res/res/values
public.xml and ids.xml
And after performing a complete build :
make framework
make update-api
make
i am able to access it as android.R.layout.my_layout
But now i want to access it through xml also :Eg: "#android:layout/my_layout"
In which place in the source do i mention this reference ?

Try to declare the id in your my_layout.xml file in your layout folder.
for example:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/home_layout"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >
...
Happy coding :)

using merge with layouts giving error for one file, no error on other. same code!

I am creating a few layouts with the root element being merge. inside the merge element i have a ScrollView containing a TextView. Here is the file:
<?xml version="1.0" encoding="utf-8"?>
<merge
xmlns:android="http://schemas.android.com/apk/res/android">
<ScrollView
android:id="#+id/scroll"
android:padding="6dp"
android:layout_below="#id/headingLayout"
android:layout_above="#id/tabsLayout"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<TextView
android:id="#+id/aboutTxt"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</ScrollView>
</merge>
I am getting 2 errors: there is no resource to match the layout_below and layout_above names i have put in.
I have another layout xml file including very similar components, some of which also reference #id/headingLayout and #id/tabsLayout. Both of these XML layout files do not contain the component with name headingLayout or tabsLayout
Why is it that one layout file has no errors about these references and the other layout file does? What the crap am i doing wrong, the app will build and run like i expect, until i add this merge to another layout?
I have even tried copying the xml from the working layout file, to find that it gives the same errors, something must be wrong with my new layout file. I have tried cleaning, rebuilding, opening/closing eclipse..

Wow, this is a strange fix i have found for this. the layout i have given in my question above(the one giving errors) is named about.xml. the layout with no errors which also includes merge and references to other xml components is named home.xml. the xml file including the referenced components is named base_layout.xml.
in the eclipse Package Explorer under the layout folder, items are listed alphabetically. so the home.xml file came after base_layout.xml, and could therefore reference the components inside it. Since about.xml came before base_layout.xml, about.xml could not reference anything inside base_layout.xml.
so i just renamed base_layout.xml to aaa_base_layout.xml so it would be first, and all errors go away.