xmlns:custom="http://ift.tt/GEGVYd" seems not work?

I create a custom view com.way.customview.MagnetView,and use it in a layout XML:

<com.way.customview.MagnetView
                android:id="@+id/c_joke"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_margin="2dp"
                android:scaleType="matrix"
                android:src="@drawable/magnet_bg_robot_left" />

And I add a namespase in the head:

xmlns:custom="http://ift.tt/GEGVYd"

But the attribution android:src="@drawable/magnet_bg_robot_left" can't load backgroud image. Anyone could tell me why?