How to control animation speed (requestAnimationFr

问题:

I change the text color with requestAnimationFrame(animate); function:

requestAnimationFrame(animate);

function animate(time){
  ... // change text color here
  if (offset_s < offset_e) {requestAnimationFrame(animate);}
}

offset_s and offset_s indicates start and end positions of the text for color change. In some cases the animation should last for 2 seconds, but in order cases - for 5 seconds, but offset_e - offset_s could be the same in these two cases. What can I do to control the speed of animation based on given time in seconds/milliseconds?

回答1:

From the tags of the question i can only see that you animate something drawn on canvas and thats why u cannot use css-animation or jquery-animation.

You have to control the length of the animation by calculating the time difference.

u can do it similar to this example

function start_animate(duration) {
  var requestID;
  var startTime =null; 
  var time ;   

  var animate = function(time) {


   time = new Date().getTime(); //millisecond-timstamp

   if (startTime === null) {
        startTime = time;
   }
   var progress = time - startTime;

   if (progress < duration ) {

       if(offset_s < offset_e){
         // change text color here

       }
       requestID= requestAnimationFrame(animate);
   } 
   else{
      cancelAnimationFrame(requestID);
   }
   requestID=requestAnimationFrame(animate);
  }
  animate();
}

trigger your animation and call start_animate(2000) //duration in millisecond 1000=1 sec

回答2:

You should separate concerns clearly.
Have a single requestAnimationFrame running, which computes the current animation time and calls every update and draw related functions.
Then your animations would be handled by a function (or class instance if you go OOP) that deals with the current animation time.

Just some direction for the code :

var animationTime = -1;
var _lastAnimationTime = -1;

function launchAnimation() {
    requestAnimationFrame(_launchAnimation);
}    

function _launchAnimation(time) {
    animationTime = 0;
    _lastAnimationTime = time;
    requestAnimationFrame(animate);
}

function animate(time){
  requestAnimationFrame(animate);
  var dt = time - _lastAnimationTime ;
  _lastAnimationTime = time;
  animationTime += dt;

  // here call every draw / update functions
  //  ...
  animationHandler.update(animationTime);
  animationHandler.draw(context);
}

To start your 'engine', just call launchAnimation then you'll have a valid animationTime and dt to deal with.

I'd make animationHandler an instance of an AnimationHandler class, that allows to add/remove/update/draw animations.

回答3:

I suggest to use setInterval function in JavaScript.

requestAnimationFrame really needs some 'ugly' calculations. Don't believe me? Scroll up, you will see...

So, to make setInterval function as handy as rAF(requestAnimationFrame) store the function inside of variable. Here is an example:

var gameLoop = setInterval(function() {
  update();
  draw();
  if (gameOver)
    clearInterval(gameLoop);
}, 1000/FPS);

given way, you can control your FPS and pick correct velocity for your objects.