用 position="replace" 代替父视图相关定义，就可以在父视图调用子类的方法
<xpath expr="//field[@name='standard_price']" position="replace">
<group name='cost_prices' colspan="2" col="4">
<field name="standard_price" nolabel="1" attrs="{'readonly':[('cost_method','=','average')]}"/>
<field name="cost_price_extra" groups="product.group_product_variant"/>
</group>
</xpath>
<xpath expr="//button[@name='open_ui']" position="replace">
<button name="open_ui" type="object" string="Start Selling" attrs="{'invisible' : [('pos_state', 'not in', ('opened',))]}" class="oe_highlight" invisible="True"/>
</xpath>
