[Bug Report] Trigger mechanism of span-method is unreasonable

[moonose]

Element UI version

2.9.1

OS/Browsers version

Google Chrome 74.0.3729.169 64bit

Vue version

2.6.10

Reproduction Link

https://codepen.io/moonose/pen/zVxpmK

Steps to reproduce

1.bind span-method to a table
2.input some data in the input box

What is Expected?

span-method will not be triggered.
I think the reasonable mechanism of span-method should be like this:
. the bound table data changed, span-method will be triggered
. other data changed, span-method will not be triggered

What is actually happening?

span-method will be triggered

[island205]

span-method is called by table-body's render function when data changing, so it should be used as stateless at present.