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# Animations

AngularJS provides animation hooks for common directives such as ngRepeat, ngSwitch, and ngView, as well as custom directives via the `$animate` service. These animation hooks are set in place to trigger animations during the life cycle of various directives and when triggered, will attempt to perform a CSS Transition, CSS Keyframe Animation or a JavaScript callback Animation (depending on whether an animation is placed on the given directive). Animations can be placed using vanilla CSS by following the naming conventions set in place by AngularJS or with JavaScript code, defined as a factory.

Note that we have used non-prefixed CSS transition properties in our examples as the major browsers now support non-prefixed properties. If you intend to support older browsers or certain mobile browsers then you will need to include prefixed versions of the transition properties. Take a look at http://caniuse.com/#feat=css-transitions for what browsers require prefixes, and https://github.com/postcss/autoprefixer for a tool that can automatically generate the prefixes for you.

Animations are not available unless you include the `ngAnimate` module as a dependency of your application.

Below is a quick example of animations being enabled for `ngShow` and `ngHide`:

    <div ng-init="checked = true">
      <label>
        <input type="checkbox" ng-model="checked" />
        Is visible
      </label>
      <div class="content-area sample-show-hide" ng-show="checked">
        Content...
      </div>
    </div>

    .content-area {
      border: 1px solid black;
      margin-top: 10px;
      padding: 10px;
    }

    .sample-show-hide {
      transition: all linear 0.5s;
    }
    .sample-show-hide.ng-hide {
      opacity: 0;
    }

## Installation

See the API docs for `ngAnimate` for instructions on installing the module.

You may also want to setup a separate CSS file for defining CSS-based animations.

## How they work

Animations in AngularJS are completely based on CSS classes. As long as you have a CSS class attached to a HTML element within your application, you can apply animations to it. Lets say for example that we have an HTML template with a repeater like so:

    <div ng-repeat="item in items" class="repeated-item">
      {{ item.id }}
    </div>

As you can see, the `repeated-item` class is present on the element that will be repeated and this class will be used as a reference within our application's CSS and/or JavaScript animation code to tell AngularJS to perform an animation.

As `ngRepeat` does its thing, each time a new item is added into the list, `ngRepeat` will add an `ng-enter` class to the element that is being added. When removed it will apply an `ng-leave` class and when moved around it will apply an `ng-move` class.

Taking a look at the following CSS code, we can see some transition and keyframe animation code set up for each of those events that occur when `ngRepeat` triggers them:

    /*
      We are using CSS transitions for when the enter and move events
      are triggered for the element that has the `repeated-item` class
    */
    .repeated-item.ng-enter, .repeated-item.ng-move {
      transition: all 0.5s linear;
      opacity: 0;
    }

    /*
      `.ng-enter-active` and `.ng-move-active` are where the transition destination
      properties are set so that the animation knows what to animate
    */
    .repeated-item.ng-enter.ng-enter-active,
    .repeated-item.ng-move.ng-move-active {
      opacity: 1;
    }

    /*
      We are using CSS keyframe animations for when the `leave` event
      is triggered for the element that has the `repeated-item` class
    */
    .repeated-item.ng-leave {
      animation: 0.5s my_animation;
    }

    @keyframes my_animation {
      from { opacity: 1; }
      to   { opacity: 0; }
    }

The same approach to animation can be used using JavaScript code (for simplicity, we rely on jQuery to perform animations here):

    myModule.animation('.repeated-item', function() {
      return {
        enter: function(element, done) {
          // Initialize the element's opacity
          element.css('opacity', 0);

          // Animate the element's opacity
          // (`element.animate()` is provided by jQuery)
          element.animate({opacity: 1}, done);

          // Optional `onDone`/`onCancel` callback function
          // to handle any post-animation cleanup operations
          return function(isCancelled) {
            if (isCancelled) {
              // Abort the animation if cancelled
              // (`element.stop()` is provided by jQuery)
              element.stop();
            }
          };
        },
        leave: function(element, done) {
          // Initialize the element's opacity
          element.css('opacity', 1);

          // Animate the element's opacity
          // (`element.animate()` is provided by jQuery)
          element.animate({opacity: 0}, done);

          // Optional `onDone`/`onCancel` callback function
          // to handle any post-animation cleanup operations
          return function(isCancelled) {
            if (isCancelled) {
              // Abort the animation if cancelled
              // (`element.stop()` is provided by jQuery)
              element.stop();
            }
          };
        },

        // We can also capture the following animation events:
        move: function(element, done) {},
        addClass: function(element, className, done) {},
        removeClass: function(element, className, done) {}
      }
    });

With these generated CSS class names present on the element at the time, AngularJS automatically figures out whether to perform a CSS and/or JavaScript animation. Note that you can't have both CSS and JavaScript animations based on the same CSS class. See here for more details.

## Class and `ngClass` animation hooks

AngularJS also pays attention to CSS class changes on elements by triggering the add and remove hooks. This means that if a CSS class is added to or removed from an element then an animation can be executed in between, before the CSS class addition or removal is finalized. (Keep in mind that AngularJS will only be able to capture class changes if an interpolated expression or the ng-class directive is used on the element.)

The example below shows how to perform animations during class changes:

    <p>
      <button ng-click="myCssVar='css-class'">Set</button>
      <button ng-click="myCssVar=''">Clear</button>
      <br>
      <span ng-class="myCssVar">CSS-Animated Text</span>
    </p>

    .css-class-add, .css-class-remove {
      transition: all 0.5s cubic-bezier(0.250, 0.460, 0.450, 0.940);
    }

    .css-class,
    .css-class-add.css-class-add-active {
      color: red;
      font-size: 3em;
    }

    .css-class-remove.css-class-remove-active {
      font-size: 1em;
      color: black;
    }

Although the CSS is a little different than what we saw before, the idea is the same.

## Which directives support animations?

A handful of common AngularJS directives support and trigger animation hooks whenever any major event occurs during their life cycle. The table below explains in detail which animation events are triggered:

Directive | Supported Animations
--- | ---
ngRepeat | enter, leave, and move
ngIf | enter and leave
ngSwitch | enter and leave
ngInclude | enter and leave
ngView | enter and leave
ngMessage / ngMessageExp | enter and leave
ngClass / {{class}​} | add and remove
ngClassEven / ngClassOdd | add and remove
ngHide | add and remove (the `ng-hide` class)
ngShow | add and remove (the `ng-hide` class)
ngModel | add and remove (various classes)
form / ngForm | add and remove (various classes)
ngMessages | add and remove (the `ng-active`/`ng-inactive` classes)

For a full breakdown of the steps involved during each animation event, refer to the API docs.

## How do I use animations in my own directives?

Animations within custom directives can also be established by injecting `$animate` directly into your directive and making calls to it when needed.

    myModule.directive('my-directive', ['$animate', function($animate) {
      return function(scope, element) {
        element.on('click', function() {
          if (element.hasClass('clicked')) {
            $animate.removeClass(element, 'clicked');
          } else {
            $animate.addClass(element, 'clicked');
          }
        });
      };
    }]);

## Animations on app bootstrap / page load

By default, animations are disabled when the AngularJS app bootstraps. If you are using the `ngApp` directive, this happens in the `DOMContentLoaded` event, so immediately after the page has been loaded. Animations are disabled, so that UI and content are instantly visible. Otherwise, with many animations on the page, the loading process may become too visually overwhelming, and the performance may suffer.

Internally, `ngAnimate` waits until all template downloads that are started right after bootstrap have finished. Then, it waits for the currently running $digest and one more after that, to finish. This ensures that the whole app has been compiled fully before animations are attempted.

If you do want your animations to play when the app bootstraps, you can enable animations globally in your main module's run function:

    myModule.run(function($animate) {
      $animate.enabled(true);
    });

## How to (selectively) enable, disable and skip animations

There are several different ways to disable animations, both globally and for specific animations. Disabling specific animations can help to speed up the render performance, for example for large `ngRepeat` lists that don't actually have animations. Because `ngAnimate` checks at runtime if animations are present, performance will take a hit even if an element has no animation.

### During the config: $animateProvider.customFilter()

This function can be called during the config phase of an app. It takes a filter function as the only argument, which will then be used to "filter" animations (based on the animated element, the event type, and the animation options). Only when the filter function returns `true`, will the animation be performed. This allows great flexibility - you can easily create complex rules, such as allowing specific events only or enabling animations on specific subtrees of the DOM, and dynamically modify them, for example disabling animations at certain points in time or under certain circumstances.

    app.config(function($animateProvider) {
      $animateProvider.customFilter(function(node, event, options) {
        // Example: Only animate `enter` and `leave` operations.
        return event === 'enter' || event === 'leave';
      });
    });

The `customFilter` approach generally gives a big speed boost compared to other strategies, because the matching is done before other animation disabling strategies are checked.

Best Practice: Keep the filtering function as lean as possible, because it will be called for each DOM action (e.g. insertion, removal, class change) performed by "animation-aware" directives. See here for a list of built-in directives that support animations. Performing computationally expensive or time-consuming operations on each call of the filtering function can make your animations sluggish.

### During the config: $animateProvider.classNameFilter()

This function too can be called during the config phase of an app. It takes a regex as the only argument, which will then be matched against the classes of any element that is about to be animated. The regex allows a lot of flexibility - you can either allow animations for specific classes only (useful when you are working with 3rd party animations), or exclude specific classes from getting animated.

    app.config(function($animateProvider) {
      $animateProvider.classNameFilter(/animate-/);
    });

    /* prefixed with `animate-` */
    .animate-fade-add.animate-fade-add-active {
      transition: all 1s linear;
      opacity: 0;
    }

The `classNameFilter` approach generally gives a big speed boost compared to other strategies, because the matching is done before other animation disabling strategies are checked. However, that also means it is not possible to override class name matching with the two following strategies. It's of course still possible to enable / disable animations by changing an element's class name at runtime.

### At runtime: $animate.enabled()

This function can be used to enable / disable animations in two different ways:

With a single `boolean` argument, it enables / disables animations globally: `$animate.enabled(false)` disables all animations in your app.

When the first argument is a native DOM or jqLite/jQuery element, the function enables / disables animations on this element and all its children: `$animate.enabled(myElement, false)`. You can still use it to re-enable animations for a child element, even if you have disabled them on a parent element. And compared to the `classNameFilter`, you can change the animation status at runtime instead of during the config phase.

Note however that the `$animate.enabled()` state for individual elements does not overwrite disabling rules that have been set in the classNameFilter.

### Via CSS styles: overwriting styles in the `ng-animate` CSS class

Whenever an animation is started, `ngAnimate` applies the `ng-animate` class to the element for the whole duration of the animation. By applying CSS transition / animation styling to that class, you can skip an animation:

    .my-class {
      transition: transform 2s;
    }

    .my-class:hover {
      transform: translateX(50px);
    }

    my-class.ng-animate {
      transition: 0s;
    }

By setting `transition: 0s`, `ngAnimate` will ignore the existing transition styles, and not try to animate them (Javascript animations will still execute, though). This can be used to prevent issues with existing animations interfering with `ngAnimate`.

## Preventing flicker before an animation starts

When nesting elements with structural animations, such as `ngIf`, into elements that have class-based animations such as `ngClass`, it sometimes happens that before the actual animation starts, there is a brief flicker or flash of content where the animated element is briefly visible.

To prevent this, you can apply styles to the `ng-[event]-prepare` class, which is added as soon as an animation is initialized, but removed before the actual animation starts (after waiting for a `$digest`). This class is only added for structural animations (`enter`, `move`, and `leave`).

Here's an example where you might see flickering:

    <div ng-class="{red: myProp}">
      <div ng-class="{blue: myProp}">
        <div class="message" ng-if="myProp"></div>
      </div>
    </div>

It is possible that during the `enter` event, the `.message` div will be briefly visible before it starts animating. In that case, you can add styles to the CSS that make sure the element stays hidden before the animation starts:

    .message.ng-enter-prepare {
      opacity: 0;
    }

    /* Other animation styles ... */

## Preventing collisions with existing animations and third-party libraries

By default, any `ngAnimate`-enabled directives will assume that `transition` / `animation` styles on the element are part of an `ngAnimate` animation. This can lead to problems when the styles are actually for animations that are independent of `ngAnimate`.

For example, an element acts as a loading spinner. It has an infinite css animation on it, and also an `ngIf` directive, for which no animations are defined:

    .spinner {
      animation: rotating 2s linear infinite;
    }

    @keyframes rotating {
      from { transform: rotate(0deg); }
      to { transform: rotate(360deg); }
    }

Now, when the `ngIf` expression changes, `ngAnimate` will see the spinner animation and use it to animate the `enter`/`leave` event, which doesn't work because the animation is infinite. The element will still be added / removed after a timeout, but there will be a noticeable delay.

This might also happen because some third-party frameworks place animation duration defaults across many element or className selectors in order to make their code small and reusable.

You can prevent this unwanted behavior by adding CSS to the `.ng-animate` class, that is added for the whole duration of each animation. Simply overwrite the transition / animation duration. In the case of the spinner, this would be:

    .spinner.ng-animate {
      animation: 0s none;
      transition: 0s none;
    }

If you do have CSS transitions / animations defined for the animation events, make sure they have a higher priority than any styles that are not related to `ngAnimate`.

You can also use one of the other strategies to disable animations.

## Enable animations outside of the application DOM tree: $animate.pin()

Before animating, `ngAnimate` checks if the animated element is inside the application DOM tree. If not, no animation is run. Usually, this is not a problem since most apps use the `html` or `body` elements as their root.

Problems arise when the application is bootstrapped on a different element, and animations are attempted on elements that are outside the application tree, e.g. when libraries append popup or modal elements to the body tag.

You can use `$animate.pin(element, parentHost)` to associate an element with another element that belongs to your application. Simply call it before the element is added to the DOM / before the animation starts, with the element you want to animate, and the element which should be its assumed parent.

## More about animations

For a full breakdown of each method available on `$animate`, see the API documentation.

To see a complete demo, see the animation step in the phonecat tutorial.