Using xlink:href in SVG can cause pointless style recalculations

[tomvandezande]

Edited by @evancz

The runtime behavior of justCirc and symAndUse is very different in the following code.

• With symAndUse the browser is recalculating styles on every animation frame.
• With justCirc it does not do this.

You can observe this by profiling the example with the Chrome timeline

import Html exposing (Html)
import Svg exposing (..)
import Svg.Attributes exposing (..)
import VirtualDom exposing (attributeNS)
import AnimationFrame
import Time exposing (Time)


-- PROBLEM

view : Model -> Html Msg
view model =
    let
        justCirc = [ circle [cx "50", cy "50", r "40", strokeWidth "8", stroke "red", fill "red"] [] ]

        symAndUse = 
            [ symbol [id "sym01"] justCirc
            , use [xlinkHref "#sym01", x "0", y "0", width "100", height "100"] []
            ]
    in
        svg [ viewBox "0 0 200 200", width "600px" ]
        --justCirc
        symAndUse


-- SETUP

main = Html.program { init = init, view = view, update = update, subscriptions = subscriptions }

type alias Model = Time

init : (Model, Cmd Msg)
init =
  (0, Cmd.none)

type Msg = Tick Time

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of
    Tick newTime -> (0, Cmd.none)

subscriptions : Model -> Sub Msg
subscriptions model =
  AnimationFrame.times Tick

Original Comment:

SSCCE here

If you profile this example using the Chrome timeline, you can see that the browser is recalculating styles on every animation frame. Since the DOM isn't changing at all, this shouldn't be the case? If you were to replace the last line symAndUse with justCirc, it does work as expected (no calculating restyles).

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[anka-213]

I believe the reason for this issue is that for all namespaced attributes the value is an object,

https://github.com/elm/virtual-dom/blob/5a5bcf48720bc7d53461b3cd42a9f19f119c5503/src/Elm/Kernel/VirtualDom.js#L263-L270

which we then try to compare by reference.

https://github.com/elm/virtual-dom/blob/5a5bcf48720bc7d53461b3cd42a9f19f119c5503/src/Elm/Kernel/VirtualDom.js#L911-L913

This is bound to fail and force a patch every time we do a comparison on namespaced attributes.

---

It seems like Firefox is kinder when you use setAttributeNS with the old value, while Chrome forces a recalculation and will even try to re-fetch images from the network (see evancz/elm-playground#5 for an example).

[anka-213]

Adding a condition like this

 if (xValue === yValue && xKey !== 'value' && xKey !== 'checked' 
 	|| category === 'a__1_EVENT' && _VirtualDom_equalEvents(xValue, yValue)
        || category === 'a__1_ATTR_NS' && xValue.__namespace === yValue.__namespace && xValue.__value === yValue.__value)

to the if-clause in _VirtualDom_diffFacts should solve the problem.