CSS Animationen nutzen

CSS Animationen ermöglichen animierte Übergänge von einem CSS Style zum nächsten. Die Animationen bestehen aus zwei Komponenten: Einem Style, der die Animation beschreibt, sowie einem Set von Keyframes, dass Start, Ende und Zwischenpositionen der Animation festlegt.

Es gibt drei zentrale Vorteile von CSS Animationen gegenüber skriptbasierten Animationstechniken:

1. Sie sind einfach zu nutzen für simple Animationen; man kann sie ohne Javascript-Wissen kreieren. 
2. Die Animationen laufen performant, sogar unter mässiger Systemauslastung. Im Gegensatz dazu fallen selbst simple Javascript-Animationen durch schlechte Performance auf. Die Engine kann einzelne Frames überspringen und kennt weitere Techniken um die Ausführung so sauber wie möglich zu gestalten. 
3. Da der Browser die Animation kontrolliert, kann er selbstständig die Performance optimieren, zum Beispiel durch das Drosseln der Freqenz von Animationen in aktuell nicht sichtbaren Browser-Tabs.

Konfigureren der Animation

Um eine CSS-Animation zu kreieren, fügt man dem zu animierenden Element die animation-Eigenschaft oder seine Sub-Eigenschaften zu. So lässt sich das Timing und die Dauer der Animation bestimmen sowie weitere Details des Animationsablaufes. Man legt damit nicht die eigentliche Darstellung der Animation fest, die mittels @keyframes definiert wird. Siehe Definieren der Animationssequenz mittels Keyframes weiter unten.

Die Sub-Eigenschaften der animation-Eigenschaft sind:

animation-delay

Setzt den Abstand zwischen dem Zeitpunkt, an dem die Animation vollständig geladen ist und dem Start der Animationssequenz.

animation-direction

Legt fest, ob die Animation nach einem Durchgang rückwärts abgespielt wird oder sich immer gleich wiederholt. 

animation-duration

Legt die Dauer der Animation für einen kompletten Durchgang fest.

animation-iteration-count

Konfiguriert wie oft die Animation wiederholt wird; mittels infinite wird die Animation unendlich wiederholt.

animation-name

Spezifiziert den Namen der @keyframes-Regel, welche die einzelnen Keyframes beschreibt.

animation-play-state

Ermöglicht das Pausieren und Wiederufnehmen einer Animationssequenz.

animation-timing-function

Konfiguriert das Timing der Animation. Konkret werden die Übergänge zwischen den einzelnen Keyframes mittels Beschleunigungskurven festgelegt.

animation-fill-mode

Legt fest, was für Styles vor und nach dem Ausführen der Animation auf das animierte Element angewendet werden.

Definieren der Animationssequenz mittels Keyframes

Once you've configured the animation's timing, you need to define the appearance of the animation. This is done by establishing two or more keyframes using the @keyframes at-rule. Each keyframe describes how the animated element should render at a given time during the animation sequence.

Since the timing of the animation is defined in the CSS style that configures the animation, keyframes use a percentage to indicate the time during the animation sequence at which they take place. 0% indicates the first moment of the animation sequence, while 100% indicates the final state of the animation. Because these two times are so important, they have special aliases: from and to. Both are optional. If from/0% or to/100% is not specified, the browser starts or finishes the animation using the computed values of all attributes.

You can optionally include additional keyframes that describe intermediate steps along the way from the starting point to the ending point of the animation.

Beispiele

Making text slide across the browser window

This simple example styles the <p> element so that the text slides in from off the right edge of the browser window.

Note that animations like this can cause the page to become wider than the browser window. To avoid this problem put the element to be animated in a container, and set overflow:hidden on the container.

p {
  animation-duration: 3s;
  animation-name: slidein;
}

@keyframes slidein {
  from {
    margin-left: 100%;
    width: 300%; 
  }

  to {
    margin-left: 0%;
    width: 100%;
  }
}

In this example the style for the <p> element specifies that the animation should take 3 seconds to execute from start to finish, using the animation-duration property, and that the name of the @keyframes at-rule defining the keyframes for the animation sequence is named "slidein".

If we wanted any custom styling on the <p> element to appear in browsers that don't support CSS animations, we would include it here as well; however, in this case we don't want any custom styling other than the animation effect.

The keyframes are defined using the @keyframes at-rule. In this case, we have just two keyframes. The first occurs at 0% (using the alias from). Here, we configure the left margin of the element to be at 100% (that is, at the far right edge of the containing element), and the width of the element to be 300% (or three times the width of the containing element). This causes the first frame of the animation to have the header drawn off the right edge of the browser window.

The second (and final) keyframe occurs at 100% (using the alias to). The left margin is set to 0% and the width of the element is set to 100%. This causes the header to finish its animation flush against the left edge of the content area.

<p>The Caterpillar and Alice looked at each other for some time in silence:
at last the Caterpillar took the hookah out of its mouth, and addressed
her in a languid, sleepy voice.</p>

Adding another keyframe

Let's add another keyframe to the previous example's animation. Let's say we want the header's font size to increase as it moves from right to left for a while, then to decrease back to its original size. That's as simple as adding this keyframe:

75% {
  font-size: 300%;
  margin-left: 25%;
  width: 150%;
}

p {
  animation-duration: 3s;
  animation-name: slidein;
}

@keyframes slidein {
  from {
    margin-left: 100%;
    width: 300%; 
  }

  to {
    margin-left: 0%;
    width: 100%;
  }
}

<p>The Caterpillar and Alice looked at each other for some time in silence:
at last the Caterpillar took the hookah out of its mouth, and addressed
her in a languid, sleepy voice.</p>

This tells the browser that 75% of the way through the animation sequence, the header should have its left margin at 25% and the width should be 150%.

Making it repeat

To make the animation repeat itself, simply use the animation-iteration-count property to indicate how many times to repeat the animation. In this case, let's use infinite to have the animation repeat indefinitely:

p {
  animation-duration: 3s;
  animation-name: slidein;
  animation-iteration-count: infinite;
}

@keyframes slidein {
  from {
    margin-left: 100%;
    width: 300%; 
  }

  to {
    margin-left: 0%;
    width: 100%;
  }
}

<p>The Caterpillar and Alice looked at each other for some time in silence:
at last the Caterpillar took the hookah out of its mouth, and addressed
her in a languid, sleepy voice.</p>

Making it move back and forth

That made it repeat, but it's very odd having it jump back to the start each time it begins animating. What we really want is for it to move back and forth across the screen. That's easily accomplished by setting animation-direction to alternate:

p {
  animation-duration: 3s;
  animation-name: slidein;
  animation-iteration-count: infinite;
  animation-direction: alternate;
}

@keyframes slidein {
  from {
    margin-left: 100%;
    width: 300%; 
  }

  to {
    margin-left: 0%;
    width: 100%;
  }
}

<p>The Caterpillar and Alice looked at each other for some time in silence:
at last the Caterpillar took the hookah out of its mouth, and addressed
her in a languid, sleepy voice.</p>

Using animation events

You can get additional control over animations -- as well as useful information about them -- by making use of animation events. These events, represented by the AnimationEvent object, can be used to detect when animations start, finish, and begin a new iteration. Each event includes the time at which it occurred as well as the name of the animation that triggered the event.

We'll modify the sliding text example to output some information about each animation event when it occurs, so we can get a look at how they work.

Adding the CSS

We start with creating the CSS for the animation. This animation will last for 3 seconds, be called "slidein", repeat 3 times, and alternate direction each time. In the @keyframes, the width and margin-left are manipulated to make the element slide across the screen.

.slidein {
  -moz-animation-duration: 3s;
  -webkit-animation-duration: 3s;
  animation-duration: 3s;
  -moz-animation-name: slidein;
  -webkit-animation-name: slidein;
  animation-name: slidein;
  -moz-animation-iteration-count: 3;
  -webkit-animation-iteration-count: 3;
  animation-iteration-count: 3;
  -moz-animation-direction: alternate;
  -webkit-animation-direction: alternate;
  animation-direction: alternate;
}
    
@-moz-keyframes slidein {
  from {
    margin-left:100%;
    width:300%
  }
      
  to {
    margin-left:0%;
    width:100%;
  }
}

@-webkit-keyframes slidein {
  from {
    margin-left:100%;
    width:300%
  }
  
  to {
   margin-left:0%;
   width:100%;
 }
}

@keyframes slidein {
  from {
    margin-left:100%;
    width:300%
  }
  
  to {
   margin-left:0%;
   width:100%;
 }
}

Adding the animation event listeners

We'll use JavaScript code to listen for all three possible animation events. This code configures our event listeners; we call it when the document is first loaded in order to set things up.

var e = document.getElementById("watchme");
e.addEventListener("animationstart", listener, false);
e.addEventListener("animationend", listener, false);
e.addEventListener("animationiteration", listener, false);

e.className = "slidein";

This is pretty standard code; you can get details on how it works in the documentation for element.addEventListener(). The last thing this code does is set the class on the element we'll be animating to "slidein"; we do this to start the animation.

Why? Because the animationstart event fires as soon as the animation starts, and in our case, that happens before our code runs. So we'll start the animation ourselves by setting the class of the element to the style that gets animated after the fact.

Receiving the events

The events get delivered to the listener() function, which is shown below.

function listener(e) {
  var l = document.createElement("li");
  switch(e.type) {
    case "animationstart":
      l.innerHTML = "Started: elapsed time is " + e.elapsedTime;
      break;
    case "animationend":
      l.innerHTML = "Ended: elapsed time is " + e.elapsedTime;
      break;
    case "animationiteration":
      l.innerHTML = "New loop started at time " + e.elapsedTime;
      break;
  }
  document.getElementById("output").appendChild(l);
}

This code, too, is very simple. It simply looks at the event.type to determine which kind of animation event occurred, then adds an appropriate note to the <ul> (unordered list) we're using to log these events.

The output, when all is said and done, looks something like this:

• Started: elapsed time is 0
• New loop started at time 3.01200008392334
• New loop started at time 6.00600004196167
• Ended: elapsed time is 9.234000205993652

Note that the times are very close to, but not exactly, those expected given the timing established when the animation was configured. Note also that after the final iteration of the animation, the animationiteration event isn't sent; instead, the animationend event is sent.

The HTML

Just for the sake of completeness, here's the HTML that displays the page content, including the list into which the script inserts information about the received events:

<h1 id="watchme">Watch me move</h1>
<p>
  This example shows how to use CSS animations to make <code>H1</code>
  elements move across the page.
</p>
<p>
  In addition, we output some text each time an animation event fires,
  so you can see them in action.
</p>
<ul id="output">
</ul>
</body>

See also

• AnimationEvent
• Detecting CSS animation support
• Using CSS transitions