QML warning: depends on non-NOTIFYable properties

Below code:

Item
{
    visible: {
        if (type) return true;
        else return false;
    }
}

If "type" property is not notifiable, you will see this message:

QmlExpression: Expression qrc:/qml/default/element/Button.qml:172:26 depends on non-NOTIFYable properties:
    Button_QMLTYPE_12::type


Usually this happens when "type" is user-defined property and it doesn't have NOTIFY in Q_PROPERTY.

The solution is obvious:

1. Add NOTIFY to the property "type"
2. Add CONSTANT in the last of Q_PROPERTY

For #1, it's 

Q_PROPERTY(int type READ getType WRITE setType NOTIFY typeChanged)

For #2, it's

Q_PROPERTY(int type READ getType WRITE setType CONSTANT)