CSS3动画的Javascript降级解决方案

Question

I've been creating a number of small thick client JavaScript apps for an iPad app, which loads the relevant app in a UIWebview. I am now making them cross browser and need to incorporate some fallbacks for CSS animations and transitions using JavaScript.

My webkit specific implementation uses CSS classes for all animations/transitions, for which the start and end states are known at design time, using add/remove class in javascript and utilising the relevant webkitTransitionEnd/webkitAnimationEnd events.

For 'dynamic' transitions I have a simple 'animate' function which just applies styling properties to relevant elements.

I would like to keep the internal API for appling transitions as similar as possible to the current implementation by simple adding/removing classes etc. I generally have a CSS and js file (both minified) for an app.

So a few questions/points that I would appreciate any input on:

1. IE7/8 issues - IE9.js

2. Dynamically adding vendor specific prefixes - so far found 'jquery.css3finalize'.

3. Transitioning to a class: 'jquery.animateToClass' - seems to search stylesheet(s) every time a class is applied - should relevant classes be cached on further lookups? Is this slow/resource hungry?

4. For '@keyframe' animations: I'd like to have a javascript object 'representation' of keyframes of each CSS3 animation. Therefore passing a class to the 'doAnimationWithClass' function would use normal css3 animation if available in the browser but if not it would pass the 'object version' to a function that would chain the each key frame transition using css3 transitions (if available) or jQuery.animate (or equivalent), ultimately arriving at the same result.

So for instance:

CSS:

@-webkit-keyframes an-animation {
  0% { opacity: 0; }
  50% { opacity: 1; }
  100% { opacity: 0; }
}

.an-animation {
  -webkit-animation-name: an-animation;
  -webkit-animation-duration: 1s;
  -webkit-animation-timing-function: linear;
  -webkit-animation-iteration-count: 2;
}

JS:

var animations = {
    an-animation: {
      keyframes: [
      { 
        duration: '',
        timing: 'linear',
        props: {
          opacity: 0
        }
      },
      { 
        duration: '0.5s',
        timing: 'linear',
        props: {
          opacity: 1
        }
      },
      { 
        duration: '0.5s',
        timing: 'linear',
        props: {
          opacity: 0
        }
      }
    ]
  }
};

var animationClasses = [
  an-animation: {
    name: 'an-animation';
    duration: '1s';
    timing: 'linear';
    count: 2;
  }
];

function doAnimationWithClass(className) {
  if (modernizer.cssanimations) {
    //normal implementation
  }
  else {
    //lookup class in animationclasses
    //lookup relevant animation object in animationHash
    //pass to animation chaining function
  }
}

The creation of animationHash etc could be done client side, or simply created at design time (with a batch/build file).

Any thoughts or pointers to a library that already does this in a more sensible way would be appreciated.

Answer 1

Yes, that's right. You need to transfer keyframes setting to js object. I think css animation and keyframe is a better way to write animation. JS animation way is hardly to maintain. Here is my workaround solution. And I also write a small tool for translating css keyframes to js object (css keyframes to js object) .

var myFrame = {
  '0%': {
    left: 0,
    top: 0
  },
  '25%': {
    left: 100,
    top: 100
  },
  '50%': {
    left: 0,
    top: 300
  },
  '100%': {
    left: 0,
    top: 0
  }
};

var keyframes = {
  set: function($el, frames, duration) {
    var animate;
    animate = function() {
      var spendTime;
      spendTime = 0;
      $.each(frames, function(idx, val) {
        var stepDuration, stepPercentage;
        stepPercentage = idx.replace('%', '') / 100;
        stepDuration = duration * stepPercentage - spendTime;
        $el.animate(val, stepDuration);
        return spendTime += stepDuration;
      });
      return setTimeout(animate, duration);
    };
    return animate();
  }
};

keyframes.set($('#mydiv'), myFrame, 2000); 

• demo: http://jsbin.com/uzodut/4/watch

Answer 2

Blackbingo, you answer served to me perfectly. I added the easing option, to implement a jquery fallback for ie8 in a parallax starfield (see CSS parallax background of stars ) like this:

var animations = {
    'STAR-MOVE': {
        '0%': {
            'background-position-x': '5%',
            'background-position-y': '5%'
        },
        '100%': {
            'background-position-x': '1300%',
            'background-position-y': '600%'
        }
    }
};

var keyframes = {
  set: function($el, frames, duration, easing) {
    var animate;
    animate = function() {
      var spendTime;
      spendTime = 0;
      $.each(frames, function(idx, val) {
        var stepDuration, stepPercentage;
        stepPercentage = idx.replace('%', '') / 100;
        stepDuration = duration * stepPercentage - spendTime;
        $el.animate(val, stepDuration, easing);
        return spendTime += stepDuration;
      });
      return setTimeout(animate, duration);
    };
    return animate();
  }
};

keyframes.set($('.midground'), animations['STAR-MOVE'], 150000,'linear');
keyframes.set($('.foreground'), animations['STAR-MOVE'], 100000,'linear');

Answer 3

以下是一些重要的指示http://addyosmani.com/blog/css3transitions-jquery/