.transition-timing-function() Mixin

[ahmadnassri]

added .transition-timing-function(@timing-function) mixin, defaults to @timing-function.

useful for customizing CSS transitions.

reference: transition-timing-function on MDN

[BBosman]

I think the plan is to deprecate a lot of those mixins with the switch to autoprefixer (#12670), so I'm not sure if adding a new one to the mix makes sense at this time.

[ahmadnassri]

@BBosman I'm all for killing vendor prefixes ... and thus these mixins.

That said, for folks using bootstrap LESS files directly, we may not necessarily run through an autoprefixer task (although, I probably should) so relying on a grunt task in the main build of Bootstrap may not satisfy the need for people who are using the sources directly and generating our own CSS.

I will keep this PR open for the choice of the community whether this is useful or not, but I will also go back to my project and use an auto-prefixer as well as CSScomb because that is certainly the right way to do it.

[crimeminister]

+1

[mdo]

Yeah I'm torn here. Clearly the route we want to go for maintaining Bootstrap is to abandon use of the mixins. However, we still need to include them for folks who just use Less or Sass.

I guess our docs should just make that clearer when we implement autoprefixer?

[ahmadnassri]

@mdo I believe the community will support the move, so long as it is clearly documented (the why and the how) of relying on autoprefixer.

again, will leave this PR open for the maintainers to decide...

[BBosman]

Makes sense.

But is there a specific reason to call the mixin transition-timing-function instead of animation-timing-function?

To me, it would me more logical to have the name match the properties it sets.

[ahmadnassri]

@BBosman you sir, are correct, the mistake is the CSS property it's setting should be transition-timing-function ... I was hasty in making a PR. sending a fix now.