angular-docs-cn/aio/content/guide/built-in-directives.md

# Built-in directives

Angular offers two kinds of built-in directives: [_attribute_ directives](guide/attribute-directives) and [_structural_ directives](guide/structural-directives).

<div class="alert is-helpful">

See the <live-example></live-example> for a working example containing the code snippets in this guide.

</div>

For more detail, including how to build your own custom directives, see [Attribute Directives](guide/attribute-directives) and [Structural Directives](guide/structural-directives).

<hr/>

{@a attribute-directives}

## Built-in attribute directives

Attribute directives listen to and modify the behavior of
other HTML elements, attributes, properties, and components.
You usually apply them to elements as if they were HTML attributes, hence the name.

Many NgModules such as the [`RouterModule`](guide/router "Routing and Navigation")
and the [`FormsModule`](guide/forms "Forms") define their own attribute directives.
The most common attribute directives are as follows:

* [`NgClass`](guide/built-in-directives#ngClass)&mdash;adds and removes a set of CSS classes.
* [`NgStyle`](guide/built-in-directives#ngStyle)&mdash;adds and removes a set of HTML styles.
* [`NgModel`](guide/built-in-directives#ngModel)&mdash;adds two-way data binding to an HTML form element.

<hr/>

{@a ngClass}

## `NgClass`

Add or remove several CSS classes simultaneously with `ngClass`.

<code-example path="built-in-directives/src/app/app.component.html" region="special-div" header="src/app/app.component.html"></code-example>

<div class="alert is-helpful">

To add or remove a *single* class, use [class binding](guide/attribute-binding#class-binding) rather than `NgClass`.

</div>

Consider a `setCurrentClasses()` component method that sets a component property,
`currentClasses`, with an object that adds or removes three classes based on the
`true`/`false` state of three other component properties. Each key of the object is a CSS class name; its value is `true` if the class should be added,
`false` if it should be removed.

<code-example path="built-in-directives/src/app/app.component.ts" region="setClasses" header="src/app/app.component.ts"></code-example>

Adding an `ngClass` property binding to `currentClasses` sets the element's classes accordingly:

<code-example path="built-in-directives/src/app/app.component.html" region="NgClass-1" header="src/app/app.component.html"></code-example>

<div class="alert is-helpful">

Remember that in this situation you'd call `setCurrentClasses()`,
both initially and when the dependent properties change.

</div>

<hr/>

{@a ngStyle}

## `NgStyle`

Use `NgStyle` to set many inline styles simultaneously and dynamically, based on the state of the component.

### Without `NgStyle`

For context, consider setting a *single* style value with [style binding](guide/attribute-binding#style-binding), without `NgStyle`.

<code-example path="built-in-directives/src/app/app.component.html" region="without-ng-style" header="src/app/app.component.html"></code-example>

However, to set *many* inline styles at the same time, use the `NgStyle` directive.

The following is a `setCurrentStyles()` method that sets a component
property, `currentStyles`, with an object that defines three styles,
based on the state of three other component properties:

<code-example path="built-in-directives/src/app/app.component.ts" region="setStyles" header="src/app/app.component.ts"></code-example>

Adding an `ngStyle` property binding to `currentStyles` sets the element's styles accordingly:

<code-example path="built-in-directives/src/app/app.component.html" region="NgStyle-2" header="src/app/app.component.html"></code-example>

<div class="alert is-helpful">

Remember to call `setCurrentStyles()`, both initially and when the dependent properties change.

</div>


<hr/>

{@a ngModel}

## `[(ngModel)]`: Two-way binding

The `NgModel` directive allows you to display a data property and
update that property when the user makes changes. Here's an example:

<code-example path="built-in-directives/src/app/app.component.html" header="src/app/app.component.html (NgModel example)" region="NgModel-1"></code-example>


### Import `FormsModule` to use `ngModel`

Before using the `ngModel` directive in a two-way data binding,
you must import the `FormsModule` and add it to the NgModule's `imports` list.
Learn more about the `FormsModule` and `ngModel` in [Forms](guide/forms#ngModel).

Remember to import the `FormsModule` to make `[(ngModel)]` available as follows:

<code-example path="built-in-directives/src/app/app.module.ts" header="src/app/app.module.ts (FormsModule import)" region="import-forms-module"></code-example>


You could achieve the same result with separate bindings to
the `<input>` element's  `value` property and `input` event:

<code-example path="built-in-directives/src/app/app.component.html" region="without-NgModel" header="src/app/app.component.html"></code-example>

To streamline the syntax, the `ngModel` directive hides the details behind its own `ngModel` input and `ngModelChange` output properties:

<code-example path="built-in-directives/src/app/app.component.html" region="NgModelChange" header="src/app/app.component.html"></code-example>

The `ngModel` data property sets the element's value property and the `ngModelChange` event property
listens for changes to the element's value.

### `NgModel` and value accessors

The details are specific to each kind of element and therefore the `NgModel` directive only works for an element
supported by a [ControlValueAccessor](api/forms/ControlValueAccessor)
that adapts an element to this protocol.
Angular provides *value accessors* for all of the basic HTML form elements and the
[Forms](guide/forms) guide shows how to bind to them.

You can't apply `[(ngModel)]` to a non-form native element or a
third-party custom component until you write a suitable value accessor. For more information, see
the API documentation on [DefaultValueAccessor](api/forms/DefaultValueAccessor).

You don't need a value accessor for an Angular component that
you write because you can name the value and event properties
to suit Angular's basic [two-way binding syntax](guide/two-way-binding)
and skip `NgModel` altogether.
The `sizer` in the
[Two-way Binding](guide/two-way-binding) section is an example of this technique.

Separate `ngModel` bindings are an improvement over binding to the
element's native properties, but you can streamline the binding with a
single declaration using the `[(ngModel)]` syntax:

<code-example path="built-in-directives/src/app/app.component.html" region="NgModel-1" header="src/app/app.component.html"></code-example>

This `[(ngModel)]` syntax can only _set_ a data-bound property.
If you need to do something more, you can write the expanded form;
for example, the following changes the `<input>` value to uppercase:

<code-example path="built-in-directives/src/app/app.component.html" region="uppercase" header="src/app/app.component.html"></code-example>

Here are all variations in action, including the uppercase version:

<div class="lightbox">
  <img src='generated/images/guide/built-in-directives/ng-model-anim.gif' alt="NgModel variations">
</div>

<hr/>

{@a structural-directives}

## Built-in _structural_ directives

Structural directives are responsible for HTML layout.
They shape or reshape the DOM's structure, typically by adding, removing, and manipulating
the host elements to which they are attached.

This section is an introduction to the common built-in structural directives:

* [`NgIf`](guide/built-in-directives#ngIf)&mdash;conditionally creates or destroys subviews from the template.
* [`NgFor`](guide/built-in-directives#ngFor)&mdash;repeat a node for each item in a list.
* [`NgSwitch`](guide/built-in-directives#ngSwitch)&mdash;a set of directives that switch among alternative views.

<div class="alert is-helpful">

The deep details of structural directives are covered in the
[Structural Directives](guide/structural-directives) guide,
which explains the following:

* Why you
[prefix the directive name with an asterisk (\*)](guide/structural-directives#the-asterisk--prefix).
* Using [`<ng-container>`](guide/structural-directives#ngcontainer "<ng-container>")
to group elements when there is no suitable host element for the directive.
* How to write your own structural directive.
* Why you [can only apply one structural directive](guide/structural-directives#one-per-element "one per host element") to an element.

</div>

<hr/>

{@a ngIf}

## NgIf

You can add or remove an element from the DOM by applying an `NgIf` directive to
a host element.
Bind the directive to a condition expression like `isActive` in this example.

<code-example path="built-in-directives/src/app/app.component.html" region="NgIf-1" header="src/app/app.component.html"></code-example>

<div class="alert is-helpful">

Don't forget the asterisk (`*`) in front of `ngIf`. For more information
on the asterisk, see the [asterisk (*) prefix](guide/structural-directives#the-asterisk--prefix) section of
[Structural Directives](guide/structural-directives).

</div>

When the `isActive` expression returns a truthy value, `NgIf` adds the
`ItemDetailComponent` to the DOM.
When the expression is falsy, `NgIf` removes the `ItemDetailComponent`
from the DOM, destroying that component and all of its sub-components.


### Show/hide vs. `NgIf`

Hiding an element is different from removing it with `NgIf`.
For comparison, the following example shows how to control
the visibility of an element with a
[class](guide/attribute-binding#class-binding) or [style](guide/attribute-binding#style-binding) binding.

<code-example path="built-in-directives/src/app/app.component.html" region="NgIf-3" header="src/app/app.component.html"></code-example>

When you hide an element, that element and all of its descendants remain in the DOM.
All components for those elements stay in memory and
Angular may continue to check for changes.
You could be holding onto considerable computing resources and degrading performance
unnecessarily.

`NgIf` works differently. When `NgIf` is `false`, Angular removes the element and its descendants from the DOM.
It destroys their components, freeing up resources, which
results in a better user experience.

If you are hiding large component trees, consider `NgIf` as a more
efficient alternative to showing/hiding.

<div class="alert is-helpful">

For more information on `NgIf` and `ngIfElse`, see the [API documentation about NgIf](api/common/NgIf).

</div>

### Guard against null

Another advantage of `ngIf` is that you can use it to guard against null. Show/hide
is best suited for very simple use cases, so when you need a guard, opt instead for `ngIf`. Angular will throw an error if a nested expression tries to access a property of `null`.

The following shows `NgIf` guarding two `<div>`s.
The `currentCustomer` name appears only when there is a `currentCustomer`.
The `nullCustomer` will not be displayed as long as it is `null`.

<code-example path="built-in-directives/src/app/app.component.html" region="NgIf-2" header="src/app/app.component.html"></code-example>

<code-example path="built-in-directives/src/app/app.component.html" region="NgIf-2b" header="src/app/app.component.html"></code-example>

<div class="alert is-helpful">

See also the
[safe navigation operator](guide/template-expression-operators#safe-navigation-operator "Safe navigation operator (?.)") below.

</div>
<hr/>

{@a ngFor}
## `NgFor`

`NgFor` is a repeater directive&mdash;a way to present a list of items.
You define a block of HTML that defines how a single item should be displayed
and then you tell Angular to use that block as a template for rendering each item in the list.
The text assigned to `*ngFor` is the instruction that guides the repeater process.

The following example shows `NgFor` applied to a simple `<div>`.

<code-example path="built-in-directives/src/app/app.component.html" region="NgFor-1" header="src/app/app.component.html"></code-example>

<div class="alert is-helpful">

Don't forget the asterisk (`*`) in front of `ngFor`. For more information
on the asterisk, see the [asterisk (*) prefix](guide/structural-directives#the-asterisk--prefix) section of
[Structural Directives](guide/structural-directives).

</div>

You can also apply an `NgFor` to a component element, as in the following example.

<code-example path="built-in-directives/src/app/app.component.html" region="NgFor-2" header="src/app/app.component.html"></code-example>

{@a microsyntax}

<div class="callout is-critical">
<header>*ngFor microsyntax</header>

The string assigned to `*ngFor` is not a [template expression](guide/interpolation). Rather,
it's a *microsyntax*&mdash;a little language of its own that Angular interprets.
The string `"let item of items"` means:

> *Take each item in the `items` array, store it in the local `item` looping variable, and
make it available to the templated HTML for each iteration.*

Angular translates this instruction into an `<ng-template>` around the host element,
then uses this template repeatedly to create a new set of elements and bindings for each `item`
in the list.
For more information about microsyntax, see the [Structural Directives](guide/structural-directives#microsyntax) guide.

</div>


{@a template-input-variable}

{@a template-input-variables}

### Template input variables

The `let` keyword before `item` creates a template input variable called `item`.
The `ngFor` directive iterates over the `items` array returned by the parent component's `items` property
and sets `item` to the current item from the array during each iteration.

Reference `item` within the `ngFor` host element
as well as within its descendants to access the item's properties.
The following example references `item` first in an interpolation
and then passes in a binding to the `item` property of the `<app-item-detail>` component.

<code-example path="built-in-directives/src/app/app.component.html" region="NgFor-1-2" header="src/app/app.component.html"></code-example>

For more information about template input variables, see
[Structural Directives](guide/structural-directives#template-input-variable).

### `*ngFor` with `index`

The `index` property of the `NgFor` directive context
returns the zero-based index of the item in each iteration.
You can capture the `index` in a template input variable and use it in the template.

The next example captures the `index` in a variable named `i` and displays it with the item name.

<code-example path="built-in-directives/src/app/app.component.html" region="NgFor-3" header="src/app/app.component.html"></code-example>

<div class="alert is-helpful">

`NgFor` is implemented by the `NgForOf` directive. Read more about the other `NgForOf` context values such as `last`, `even`,
and `odd` in the [NgForOf API reference](api/common/NgForOf).

</div>

{@a trackBy}
### *ngFor with `trackBy`

If you use `NgFor` with large lists, a small change to one item, such as removing or adding an item, can trigger a cascade of DOM manipulations. For example, re-querying the server could reset a list with all new item objects, even when those items were previously displayed. In this case, Angular sees only a fresh list of new object references and has no choice but to replace the old DOM elements with all new DOM elements.

You can make this more efficient with `trackBy`.
Add a method to the component that returns the value `NgFor` should track.
In this case, that value is the hero's `id`. If the `id` has already been rendered,
Angular keeps track of it and doesn't re-query the server for the same `id`.

<code-example path="built-in-directives/src/app/app.component.ts" region="trackByItems" header="src/app/app.component.ts"></code-example>

In the microsyntax expression, set `trackBy` to the `trackByItems()` method.

<code-example path="built-in-directives/src/app/app.component.html" region="trackBy" header="src/app/app.component.html"></code-example>

Here is an illustration of the `trackBy` effect.
"Reset items" creates new items with the same `item.id`s.
"Change ids" creates new items with new `item.id`s.

* With no `trackBy`, both buttons trigger complete DOM element replacement.
* With `trackBy`, only changing the `id` triggers element replacement.

<div class="lightbox">
  <img src="generated/images/guide/built-in-directives/ngfor-trackby.gif" alt="Animation of trackBy">
</div>


<div class="alert is-helpful">

Built-in directives use only public APIs; that is,
they do not have special access to any private APIs that other directives can't access.

</div>

<hr/>

{@a ngSwitch}
## The `NgSwitch` directives

NgSwitch is like the JavaScript `switch` statement.
It displays one element from among several possible elements, based on a switch condition.
Angular puts only the selected element into the DOM.
<!-- API Flagged -->
`NgSwitch` is actually a set of three, cooperating directives:
`NgSwitch`, `NgSwitchCase`, and `NgSwitchDefault` as in the following example.

 <code-example path="built-in-directives/src/app/app.component.html" region="NgSwitch" header="src/app/app.component.html"></code-example>

<div class="lightbox">
  <img src="generated/images/guide/built-in-directives/ngswitch.gif" alt="Animation of NgSwitch">
</div>

`NgSwitch` is the controller directive. Bind it to an expression that returns
the *switch value*, such as `feature`. Though the `feature` value in this
example is a string, the switch value can be of any type.

**Bind to `[ngSwitch]`**. You'll get an error if you try to set `*ngSwitch` because
`NgSwitch` is an *attribute* directive, not a *structural* directive.
Rather than touching the DOM directly, it changes the behavior of its companion directives.

**Bind to `*ngSwitchCase` and `*ngSwitchDefault`**.
The `NgSwitchCase` and `NgSwitchDefault` directives are _structural_ directives
because they add or remove elements from the DOM.

* `NgSwitchCase` adds its element to the DOM when its bound value equals the switch value and removes
its bound value when it doesn't equal the switch value.

* `NgSwitchDefault` adds its element to the DOM when there is no selected `NgSwitchCase`.

The switch directives are particularly useful for adding and removing *component elements*.
This example switches among four `item` components defined in the `item-switch.components.ts` file.
Each component has an `item` [input property](guide/inputs-outputs#input "Input property")
which is bound to the `currentItem` of the parent component.

Switch directives work as well with native elements and web components too.
For example, you could replace the `<app-best-item>` switch case with the following.

<code-example path="built-in-directives/src/app/app.component.html" region="NgSwitch-div" header="src/app/app.component.html"></code-example>