This guide goes into greater depth on special transition states such as `*` (wildcard) and `void`, and show how these special states are used for elements entering and leaving a view.
This chapter also explores multiple animation triggers, animation callbacks, and sequence-based animation using keyframes.
In Angular, transition states can be defined explicitly through the `state()` function, or using the predefined `*` (wildcard) and `void` states.
### Wildcard state
An asterisk `*` or *wildcard* matches any animation state. This is useful for defining transitions that apply regardless of the HTML element's start or end state.
For example, a transition of `open => *` applies when the element's state changes from open to anything else.
In the two-state button example, the wildcard isn't that useful because there are only two possible states, `open` and `closed`.
Wildcard states are better when an element in one particular state has multiple potential states that it can change to.
If the button can change from `open` to either `closed` or something like `inProgress`, using a wildcard state could reduce the amount of coding needed.
The `* => *` transition applies when any change between two states takes place.
Transitions are matched in the order in which they are defined. Thus, you can apply other transitions on top of the `* => *` (any-to-any) transition. For example, define style changes or animations that would apply just to `open => closed`, or just to `closed => open`, and then use `* => *` as a fallback for state pairings that aren't otherwise called out.
To do this, list the more specific transitions *before*`* => *`.
### Using wildcards with styles
Use the wildcard `*` with a style to tell the animation to use whatever the current style value is, and animate with that. Wildcard is a fallback value that's used if the state being animated isn't declared within the trigger.
You can use the `void` state to configure transitions for an element that is entering or leaving a page. See [Animating entering and leaving a view](#enter-leave-view).
### Combining wildcard and void states
You can combine wildcard and void states in a transition to trigger animations that enter and leave the page:
* A transition of `* => void` applies when the element leaves a view, regardless of what state it was in before it left.
* A transition of `void => *` applies when the element enters a view, regardless of what state it assumes when entering.
* The wildcard state `*` matches to *any* state, including `void`.
## Animating entering and leaving a view
This section shows how to animate elements entering or leaving a page.
In the component file, the `:enter` transition sets an initial opacity of 0, and then animates it to change that opacity to 1 as the element is inserted into the view.
Note that this example doesn't need to use `state()`.
## :increment and :decrement in transitions
The `transition()` function takes additional selector values, `:increment` and `:decrement`. Use these to kick off a transition when a numeric value has increased or decreased in value.
**Note:** The following example uses `query()` and `stagger()` methods, which is discussed in the [complex sequences](guide/complex-animation-sequences#complex-sequence) page.
If a trigger contains a boolean value as a binding value, then this value can be matched using a `transition()` expression that compares `true` and `false`, or `1` and `0`.
In the code snippet above, the HTML template binds a `<div>` element to a trigger named `openClose` with a status expression of `isOpen`, and with possible values of `true` and `false`. This is an alternative to the practice of creating two named states of `open` and `close`.
In the component code, in the `@Component` metadata under the `animations:` property, when the state evaluates to `true` (meaning "open" here), the associated HTML element's height is a wildcard style or default. In this case, use whatever height the element already had before the animation started. When the element is "closed," the element animates to a height of 0, which makes it invisible.
You can define more than one animation trigger for a component. You can attach animation triggers to different elements, and the parent-child relationships among the elements affect how and when the animations run.
### Parent-child animations
Each time an animation is triggered in Angular, the parent animation always get priority and child animations are blocked. In order for a child animation to run, the parent animation must query each of the elements containing child animations and then allow the animations to run using the [`animateChild()`](https://angular.io/api/animations/animateChild) function.
#### Disabling an animation on an HTML element
A special animation control binding called `@.disabled` can be placed on an HTML element to disable animations on that element, as well as any nested elements. When true, the `@.disabled` binding prevents all animations from rendering.
The code sample below shows how to use this feature.
When an element within an HTML template has animations disabled using the `@.disabled` host binding, animations are disabled on all inner elements as well.
You can't selectively disable multiple animations on a single element.
* A parent animation can use the [`query()`](https://angular.io/api/animations/query) function to collect inner elements located in disabled areas of the HTML template.
The animation `trigger()` function emits *callbacks* when it starts and when it finishes. The example below features a component that contains an `openClose` trigger.
In the HTML template, the animation event is passed back via `$event`, as `@trigger.start` and `@trigger.done`, where `trigger` is the name of the trigger being used.
In this example, the trigger `openClose` appears as follows.
A potential use for animation callbacks could be to cover for a slow API call, such as a database lookup.
For example, you could set up the **InProgress** button to have its own looping animation where it pulsates or does some other visual motion while the backend system operation finishes.
An animation can influence an end user to *perceive* the operation as faster, even when it isn't.
Thus, a simple animation can be a cost-effective way to keep users happy, rather than seeking to improve the speed of a server call and having to compensate for circumstances beyond your control, such as an unreliable network connection.
Callbacks can serve as a debugging tool, for example in conjunction with `console.warn()` to view the application's progress in a browser's Developer JavaScript Console.
The following code snippet creates console log output for the original example, a button with the two states of `open` and `closed`.
Keyframes include an *offset* that defines the point in the animation where each style change occurs.
Offsets are relative measures from zero to one, marking the beginning and end of the animation, respectively and should be applied to each of the keyframe's steps if used at least once.
Angular's animation support builds on top of web animations, so you can animate any property that the browser considers animatable.
This includes positions, sizes, transforms, colors, borders, and more. The W3C maintains a list of animatable properties on its [CSS Transitions](https://www.w3.org/TR/css-transitions-1/) page.
In these cases, you can use a special wildcard `*` property value under `style()`, so that the value of that particular style property is computed at runtime and then plugged into the animation.
The `keyframes()` function in Angular allows you to specify multiple interim styles within a single transition, with an optional offset to define the point in the animation where each style change occurs.
## More on Angular animations
You may also be interested in the following:
* [Introduction to Angular animations](guide/animations)
