Name of the method in this View's context to invoke when the view is clicked. This name must correspond to a public method that takes exactly one parameter of type View. For instance, if you specify android:onClick="sayHello", you must declare a public void sayHello(View v) method of your context (typically, your Activity).

Must be a string value, using '\\;' to escape characters such as '\\n' or '\\uxxxx' for a unicode character.

This may also be a reference to a resource (in the form "@[package:]type:name") or theme attribute (in the form "?[package:][type:]name") containing a value of this type.

[Android Documentation]