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Re: [css3-transitions] suppression of transition starting, inheritance

From: Giovanni Campagna <scampa.giovanni@gmail.com>
Date: Thu, 11 Jun 2009 16:38:35 +0200
Message-ID: <65307430906110738r36695c8cw6346417dd29c7608@mail.gmail.com>
To: "L. David Baron" <dbaron@dbaron.org>
Cc: www-style@w3.org
A shorter method to describe processing model 2 is "Transitions
affects the Used Values, not the Computed Values. This means that
values in the middle of a Transition are never inherited.". It would
not break scripts anyway, because those query the CSS2.0 Computed
Value (now Used Value)

2009/6/11 L. David Baron <dbaron@dbaron.org>:
> http://dev.w3.org/csswg/css3-transitions/ doesn't currently have
> a processing model that explains exactly what makes a transition
> start.
>
> I think one rule for what should start a transition is that any
> change to the computed value of a CSS property that is not part of
> an animation (where part of an animation includes changes caused by
> a transition or changes caused by other animation technology,
> including both the first and last steps of such animation).  I think
> the spec ought to say something like that.
>
> However, even that description doesn't define how this interacts
> with inheritance.  To explain what I mean by this, I'd like to walk
> through two ways of processing the dynamic change in this example
> when the outermost div (a) is hovered (changing its color from blue
> to purple):
>  <style type="text/css">
>    #a { color: blue; }
>    #a:hover { color: fuchsia; }
>    #b { transition: 3s color; }
>    #c { transition: 10s color; }
>  </style>
>  <div id="a">
>    <div id="b">
>      Text in B.
>      <div id="c">
>        Text in C.
>      </div>
>    </div>
>  </div>
>
> In the first method of processing, the implementation takes the
> following steps:
>  1) When div#a enters the :hover state, it computes the style
>     change for the entire document tree.
>  2) It compares the old and new styles resulting from that change,
>     and determines that the style changes on both div#b and div#c
>     require starting transitions.
>  3) The color of "Text in B" then transitions over 3 seconds, while
>     the color of "Text in C" transitions over 10 seconds.
>
> This processing model has a serious deficiency:  if we swap the
> times so that the inner element has the shorter time, then that
> inner element transitions twice: after completing its transition, it
> picks up inheriting the end part of the transition of its ancestor.
>
> If we try to further change the model to prevent that problem, we're
> likely to end up with two additional problems:
>  a) a complex model for suppressing inheritance of transitions (I
>    expect it would be hard to define this, especially in the
>    presence of multiple transitions occurring that happen at
>    different times)
>  b) a discontinuity between transition-duration: 1ms and
>    transition-duration: 0, because transition-duration: 0 (combined
>    with transition-delay: 0) is the default that we use to indicate
>    that no transitions need to happen.  (And I don't see how to fix
>    this one other than by changing the the initial value of
>    transition-property to none, and having separate
>    transition-duration-inherited and
>    transition-duration-not-inherited for properties that inherit by
>    default or don't (which follow the same inheritance as the
>    properties they apply to), so I don't think this is a realistic
>    approach.)
>
> In the second method of processing, the implementation instead does
> the following:
>  1) When div#a enters the :hover state, it starts computing the
>     style change for the document tree.  The order it does this in
>     doesn't strictly matter, but because of inheritance it has to
>     happen ancestors-before-descendants.
>  2) When it finishes computing the style change for div#b but
>     before it starts computing the style change for any of its
>     descendants, it compares the old and new styles resulting from
>     the style change on div#b and recognizes that it needs to start
>     a transition.
>  3) It immediately resets the computed style for div#b back to the
>     value before the transition, before computing the style change
>     for any of the descendants of div#b.  (Note that I've chosen
>     the wording very carefully here.  In most cases, the style at
>     the start of the transition is the same as it was before the
>     transition began, but this is not always the case -- for
>     example, with negative transition-delay.  In this case, we
>     actually want to use the value before the transition in this
>     step so that we don't trigger the start of transitions on
>     descendant elements.)
>  4) While computing the style changes for descendants of div#b,
>     styles that haven't changed as a result of the fixup in (3)
>     don't trigger the start of additional transitions.
>  5) After the entire document tree has had style recomputed,
>     immediately initiate the transitions that started by
>     transitioning to the style at the start of the transition.  (As
>     I mentioned in (3), this style is only going to be different in
>     the case of negative transition-delay.)   These style changes
>     should be considered as caused by an animation, and thus don't
>     trigger the start of new transitions.
> This processing model has the disadvantage that a long-duration
> transition can be hidden by a shorter-duration transition on an
> ancestor.  So, again, there is a discontinuity, but this time
> between the ancestor having transition-duration: 0 and
> transition-duration: 1ms.
>
>
> I wrote a test (basically the above example, plus some borders to
> make things more visible) to see what's happening in existing
> implementations:
> http://lists.w3.org/Archives/Public/www-archive/2009Jun/att-0073/transition.html
> and it looks like my somewhat-out-of-date version of WebKit does
> neither of these:  when transition in to :hover, it starts the inner
> transition when the outer one completes, but when transitioning out
> of hover, it gives the result expected with the first model.
>
> -David
>
> --
> L. David Baron                                 http://dbaron.org/
> Mozilla Corporation                       http://www.mozilla.com/
>
>
Received on Thursday, 11 June 2009 14:39:08 GMT

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