'extend' property of Qml State

The property 'State.extend' holds the base state. When a state extends another state, it inherits all the changes of the base state and you need only specify the difference in your extended state. The usage of this property can largely reduce the code size: See the example below:

Without using extend (bad)

states: [
State { name: "album"
 PropertyChanges { target: but_album; state: 'Selected' }
 PropertyChanges { target: but_radio; state: 'Unselected' }
 PropertyChanges { target: but_rss; state: 'Unselected' }
},
State { name: "radio"
 PropertyChanges { target: but_album; state: 'Unselected' }
 PropertyChanges { target: but_radio; state: 'Selected' }
 PropertyChanges { target: but_rss; state: 'Unselected' }
},
State { name: "rss"
 PropertyChanges { target: but_album; state: 'Unselected' }
 PropertyChanges { target: but_radio; state: 'Unselected' }
 PropertyChanges { target: but_rss; state: 'Selected' }
}
]

Using extend (good)

states: [
State { name: "default"
 PropertyChanges { target: but_album; state: 'UnSelected' }
 PropertyChanges { target: but_radio; state: 'Unselected' }
 PropertyChanges { target: but_rss; state: 'Unselected' }
},
State { name: "album"; extend: 'default'
 PropertyChanges { target: but_album; state: 'Selected' }
},
State { name: "radio"; extend: 'default'
 PropertyChanges { target: but_radio; state: 'Selected' }
},
State { name: "rss"; extend: 'default'
 PropertyChanges { target: but_rss; state: 'Selected' }
}
] 

Reference:
http://doc.qt.nokia.com/4.7-snapshot/qml-state.html#extend-prop