angular-cn/aio/content/guide/animations.md

# Animations

Motion is an important aspect in the design of modern web applications. Good
user interfaces transition smoothly between states with engaging animations
that call attention where it's needed. Well-designed animations can make a UI not only
more fun but also easier to use.

## Overview

Angular's animation system lets you build animations that run with the same kind of native
performance found in pure CSS animations. You can also tightly integrate your
animation logic with the rest of your application code, for ease of control.

<div class="alert is-helpful">

Angular animations are built on top of the standard [Web Animations API](https://w3c.github.io/web-animations/)
and run natively on [browsers that support it](http://caniuse.com/#feat=web-animation).

As of Angular 6, If the Web Animations API is not supported natively by the browser, then Angular will use CSS
keyframes as a fallback instead (automatically). This means that the polyfill is no longer required unless any
code uses [AnimationBuilder](/api/animations/AnimationBuilder). If your code does use AnimationBuilder, then
uncomment the `web-animations-js` polyfill from the `polyfills.ts` file generated by Angular CLI.
</div>

<div class="l-sub-section">

The examples in this page are available as a <live-example></live-example>.

</div>

## Setup

Before you can add animations to your application, you need
to import a few animation-specific modules and functions to the root application module.

<code-example path="animations/src/app/app.module.ts" region="animations-module" title="app.module.ts (animation module import excerpt)" linenums="false"></code-example>

#### Example basics

The animations examples in this guide animate a list of heroes.

A `Hero` class has a `name` property, a `state` property that indicates if the hero is active or not,
and a `toggleState()` method to switch between the states.

<code-example path="animations/src/app/hero.service.ts" region="hero" title="hero.service.ts (Hero class)" linenums="false"></code-example>

Across the top of the screen (`app.hero-team-builder.component.ts`)
are a series of buttons that add and remove heroes from the list (via the `HeroService`). 
The buttons trigger changes to the list that all of the example components see at the same time.

{@a example-transitioning-between-states}

## Transitioning between two states

<img src="generated/images/guide/animations/animation_basic_click.gif" alt="A simple transition animation" class="right">

You can build a simple animation that transitions an element between two states
driven by a model attribute.


Animations can be defined inside `@Component` metadata. 

<code-example path="animations/src/app/hero-list-basic.component.ts" region="imports" title="hero-list-basic.component.ts" linenums="false"></code-example>

With these, you can define an *animation trigger* called `heroState` in the component
metadata. It uses animations to transition between two states: `active` and `inactive`. When a
hero is active, the element appears in a slightly larger size and lighter color.

<code-example path="animations/src/app/hero-list-basic.component.ts" region="animationdef" title="hero-list-basic.component.ts (@Component excerpt)" linenums="false"></code-example>

<div class="alert is-helpful">

In this example, you are defining animation styles (color and transform) inline in the
animation metadata.

</div>

Now, using the `[@triggerName]` syntax, attach the animation that you just defined to
one or more elements in the component's template.

<code-example path="animations/src/app/hero-list-basic.component.ts" region="template" title="hero-list-basic.component.ts (excerpt)" linenums="false"></code-example>

Here, the animation trigger applies to every element repeated by an `ngFor`. Each of
the repeated elements animates independently. The value of the
attribute is bound to the expression `hero.state` and is always either `active` or `inactive`.

With this setup, an animated transition appears whenever a hero object changes state.
Here's the full component implementation:

<code-example path="animations/src/app/hero-list-basic.component.ts" title="hero-list-basic.component.ts"></code-example>

## States and transitions

Angular animations are defined as logical **states** and **transitions**
between states.

An animation state is a string value that you define in your application code. In the example
above, the states `'active'` and `'inactive'` are based on the logical state of
hero objects. The source of the state can be a simple object attribute, as it was in this case,
or it can be a value computed in a method. The important thing is that you can read it into the
component's template.

You can define *styles* for each animation state:

<code-example path="animations/src/app/hero-list-basic.component.ts" region="states" title="src/app/hero-list-basic.component.ts" linenums="false"></code-example>

These `state` definitions specify the *end styles* of each state.
They are applied to the element once it has transitioned to that state, and stay
*as long as it remains in that state*. In effect, you're defining what styles the element has in different states.

After you define states, you can define *transitions* between the states. Each transition
controls the timing of switching between one set of styles and the next:

<code-example path="animations/src/app/hero-list-basic.component.ts" region="transitions" title="src/app/hero-list-basic.component.ts" linenums="false"></code-example>

<figure>
  <img src="generated/images/guide/animations/ng_animate_transitions_inactive_active.png" alt="In Angular animations you define states and transitions between states" width="400">
</figure>

If several transitions have the same timing configuration, you can combine
them into the same `transition` definition:

<code-example path="animations/src/app/hero-list-combined-transitions.component.ts" region="transitions" title="src/app/hero-list-combined-transitions.component.ts" linenums="false"></code-example>

When both directions of a transition have the same timing, as in the previous
example, you can use the shorthand syntax `<=>`:

<code-example path="animations/src/app/hero-list-twoway.component.ts" region="transitions" title="src/app/hero-list-twoway.component.ts" linenums="false"></code-example>

You can also apply a style during an animation but not keep it around
after the animation finishes. You can define such styles inline, in the `transition`. In this example,
the element receives one set of styles immediately and is then animated to the next.
When the transition finishes, none of these styles are kept because they're not
defined in a `state`.

<code-example path="animations/src/app/hero-list-inline-styles.component.ts" region="transitions" title="src/app/hero-list-inline-styles.component.ts" linenums="false"></code-example>

### The wildcard state `*`

The `*` ("wildcard") state matches *any* animation state. This is useful for defining styles and
transitions that apply regardless of which state the animation is in. For example:

* The `active => *` transition applies when the element's state changes from `active` to anything else.
* The `* => *` transition applies when *any* change between two states takes place.

<figure>
  <img src="generated/images/guide/animations/ng_animate_transitions_inactive_active_wildcards.png" alt="The wildcard state can be used to match many different transitions at once" width="400">
</figure>

### The `void` state

The special state called `void` can apply to any animation. It applies
when the element is *not* attached to a view, perhaps because it has not yet been
added or because it has been removed. The `void` state is useful for defining enter and
leave animations.

For example the `* => void` transition applies when the element leaves the view,
regardless of what state it was in before it left.

<figure>
  <img src="generated/images/guide/animations/ng_animate_transitions_void_in.png" alt="The void state can be used for enter and leave transitions" width="400">
</figure>

The wildcard state `*` also matches `void`.

## Example: Entering and leaving

<img src="generated/images/guide/animations/animation_enter_leave.gif" alt="Enter and leave animations" class="right" width="250">

Using the `void` and `*` states you can define transitions that animate the
entering and leaving of elements:

* Enter: `void => *`
* Leave: `* => void`

For example, in the `animations` array below there are two transitions that use
the `void => *` and `* => void` syntax to animate the element in and out of the view.

<code-example path="animations/src/app/hero-list-enter-leave.component.ts" region="animationdef" title="hero-list-enter-leave.component.ts (excerpt)" linenums="false"></code-example>

Note that in this case the styles are applied to the void state directly in the
transition definitions, and not in a separate `state(void)` definition. Thus, the transforms
are different on enter and leave: the element enters from the left
and leaves to the right.

<div class="l-sub-section">

These two common animations have their own aliases:

<code-example language="typescript">
  transition(':enter', [ ... ]); // void => *
  transition(':leave', [ ... ]); // * => void
</code-example>

</div>

## Example: Entering and leaving from different states

<img src="generated/images/guide/animations/animation_enter_leave_states.gif" alt="Enter and leave animations combined with state animations" class="right" width="200">

You can also combine this animation with the earlier state transition animation by
using the hero state as the animation state. This lets you configure
different transitions for entering and leaving based on what the state of the hero
is:

* Inactive hero enter: `void => inactive`
* Active hero enter: `void => active`
* Inactive hero leave: `inactive => void`
* Active hero leave: `active => void`

This gives you fine-grained control over each transition:

<figure>
  <img src="generated/images/guide/animations/ng_animate_transitions_inactive_active_void.png" alt="This example transitions between active, inactive, and void states" width="400">
</figure>

<code-example path="animations/src/app/hero-list-enter-leave-states.component.ts" region="animationdef" title="hero-list-enter-leave.component.ts (excerpt)" linenums="false"></code-example>

## Animatable properties and units

Since Angular's animation support builds on top of Web Animations, you can animate any property
that the browser considers *animatable*. This includes positions, sizes, transforms, colors,
borders, and many others. The W3C maintains
[a list of animatable properties](https://www.w3.org/TR/css3-transitions/#animatable-properties)
on its [CSS Transitions page](https://www.w3.org/TR/css3-transitions).

For positional properties that have a numeric value, you can define a unit by providing
the value as a string with the appropriate suffix:

* `'50px'`
* `'3em'`
* `'100%'`

If you don't provide a unit when specifying dimension, Angular assumes the default of `px`:

* `50` is the same as saying `'50px'`

## Automatic property calculation

<img src="generated/images/guide/animations/animation_auto.gif" alt="Animation with automated height calculation" class="right" width="220">

Sometimes you don't know the value of a dimensional style property until runtime.
For example, elements often have widths and heights that
depend on their content and the screen size. These properties are often tricky
to animate with CSS.

In these cases, you can use a special `*` property value so that the value of the
property is computed at runtime and then plugged into the animation.

In this example, the leave animation takes whatever height the element has before it
leaves and animates from that height to zero:

<code-example path="animations/src/app/hero-list-auto.component.ts" region="animationdef" title="src/app/hero-list-auto.component.ts" linenums="false"></code-example>

## Animation timing

There are three timing properties you can tune for every animated transition:
the duration, the delay, and the easing function. They are all combined into
a single transition *timing string*.

### Duration

The duration controls how long the animation takes to run from start to finish.
You can define a duration in three ways:

* As a plain number, in milliseconds: `100`
* In a string, as milliseconds: `'100ms'`
* In a string, as seconds: `'0.1s'`

### Delay

The delay controls the length of time between the animation trigger and the beginning
of the transition. You can define one by adding it to the same string
following the duration. It also has the same format options as the duration:

* Wait for 100ms and then run for 200ms: `'0.2s 100ms'`

### Easing

The [easing function](http://easings.net/) controls how the animation accelerates
and decelerates during its runtime. For example, an `ease-in` function causes
the animation to begin relatively slowly but pick up speed as it progresses. You
can control the easing by adding it as a *third* value in the string after the duration
and the delay (or as the *second* value when there is no delay):

* Wait for 100ms and then run for 200ms, with easing: `'0.2s 100ms ease-out'`
* Run for 200ms, with easing: `'0.2s ease-in-out'`

<img src="generated/images/guide/animations/animation_timings.gif" alt="Animations with specific timings" class="right" width="220">

### Example

Here are a couple of custom timings in action. Both enter and leave last for
200 milliseconds, that is `0.2s`, but they have different easings. The leave begins after a
slight delay of 10 milliseconds as specified in `'0.2s 10 ease-out'`:

<code-example path="animations/src/app/hero-list-timings.component.ts" region="animationdef" title="hero-list-timings.component.ts (excerpt)" linenums="false"></code-example>

## Multi-step animations with keyframes

<img src="generated/images/guide/animations/animation_multistep.gif" alt="Animations with some bounce implemented with keyframes" class="right" width="220">

Animation *keyframes* go beyond a simple transition to a more intricate animation
that goes through one or more intermediate styles when transitioning between two sets of styles.

For each keyframe, you specify an *offset* that defines at which point
in the animation that keyframe applies. The offset is a number between zero,
which marks the beginning of the animation, and one, which marks the end.

This example adds some "bounce" to the enter and leave animations with
keyframes:

<code-example path="animations/src/app/hero-list-multistep.component.ts" region="animationdef" title="hero-list-multistep.component.ts (excerpt)" linenums="false"></code-example>

Note that the offsets are *not* defined in terms of absolute time. They are relative
measures from zero to one. The final timeline of the animation is based on the combination
of keyframe offsets, duration, delay, and easing.

Defining offsets for keyframes is optional. If you omit them, offsets with even
spacing are automatically assigned. For example, three keyframes without predefined
offsets receive offsets `0`, `0.5`, and `1`.

## Parallel animation groups

<img src="generated/images/guide/animations/animation_groups.gif" alt="Parallel animations with different timings, implemented with groups" class="right" width="220px">

You've seen how to animate multiple style properties at the same time:
just put all of them into the same `style()` definition.

But you may also want to configure different *timings* for animations that happen
in parallel. For example, you may want to animate two CSS properties but use a
different easing function for each one.

For this you can use animation *groups*. In this example, using groups both on
enter and leave allows for two different timing configurations. Both
are applied to the same element in parallel, but run independently of each other:

<code-example path="animations/src/app/hero-list-groups.component.ts" region="animationdef" title="hero-list-groups.component.ts (excerpt)" linenums="false"></code-example>

One group animates the element transform and width; the other group animates the opacity.

## Animation callbacks

A callback is fired when an animation is started and also when it is done.

In the keyframes example, you have a `trigger` called `@flyInOut`. You can hook
those callbacks like this:

<code-example path="animations/src/app/hero-list-multistep.component.ts" region="template" title="hero-list-multistep.component.ts (excerpt)" linenums="false"></code-example>

The callbacks receive an `AnimationEvent` that contains useful properties such as
`fromState`, `toState` and `totalTime`.

Those callbacks will fire whether or not an animation is picked up.