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

Built-in directives

Directives are classes that add additional behavior to elements in your Angular applications. With Angular's built-in directives, you can manage forms, lists, styles, and what users see.

See the for a working example containing the code snippets in this guide.

The different types of Angular directives are as follows:

1. Components—directives with a template. This type of directive is the most common directive type.
2. Attribute directives—directives that change the appearance or behavior of an element, component, or another directive.
3. Structural directives—directives that change the DOM layout by adding and removing DOM elements.

This guide covers built-in attribute directives and structural directives.

{@a attribute-directives}

Built-in attribute directives

Attribute directives listen to and modify the behavior of other HTML elements, attributes, properties, and components.

Many NgModules such as the RouterModule and the FormsModule define their own attribute directives. The most common attribute directives are as follows:

• NgClass—adds and removes a set of CSS classes.
• NgStyle—adds and removes a set of HTML styles.
• NgModel—adds two-way data binding to an HTML form element.

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

{@a ngClass}

Adding and removing classes with NgClass

You can add or remove multiple CSS classes simultaneously with ngClass.

To add or remove a single class, use class binding rather than NgClass.

Using NgClass with an expression

On the element you'd like to style, add [ngClass] and set it equal to an expression. In this case, isSpecial is a boolean set to true in app.component.ts. Because isSpecial is true, ngClass applies the class of special to the <div>.

Using NgClass with a method

1. To use NgClass with a method, add the method to the component class. In the following example, setCurrentClasses() sets the property currentClasses with an object that adds or removes three classes based on the true or false state of three other component properties.

Each key of the object is a CSS class name. If a key is true, ngClass adds the class. If a key is false, ngClass removes the class.

1. In the template, add the ngClass property binding to currentClasses to set the element's classes:

For this use case, Angular applies the classes on initialization and in case of changes. The full example calls setCurrentClasses() initially with ngOnInit() and when the dependent properties change through a button click. These steps are not necessary to implement ngClass. For more information, see the app.component.ts and app.component.html.

{@a ngstyle}

Setting inline styles with NgStyle

You can use NgStyle to set multiple inline styles simultaneously, based on the state of the component.

1. To use NgStyle, add a method to the component class.

In the following example, setCurrentStyles() sets the property currentStyles with an object that defines three styles, based on the state of three other component properties.

1. To set the element's styles, add an ngStyle property binding to currentStyles.

For this use case, Angular applies the styles upon initialization and in case of changes. To do this, the full example calls setCurrentStyles() initially with ngOnInit() and when the dependent properties change through a button click. However, these steps are not necessary to implement ngStyle on its own. See the app.component.ts and app.component.html for this optional implementation.

{@a ngModel}

Displaying and updating properties with ngModel

You can use the NgModel directive to display a data property and update that property when the user makes changes.

1. Import FormsModule and add it to the NgModule's imports list.

1. Add an [(ngModel)] binding on an HTML <form> element and set it equal to the property, here name.

This [(ngModel)] syntax can only set a data-bound property.

To customize your configuration, you can write the expanded form, which separates the property and event binding. Use property binding to set the property and event binding to respond to changes. The following example changes the <input> value to uppercase:

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

NgModel and value accessors

The NgModel directive works for an element supported by a ControlValueAccessor. Angular provides value accessors for all of the basic HTML form elements. For more information, see Forms.

To apply [(ngModel)] to a non-form native element or a third-party custom component, you have to write a value accessor. For more information, see the API documentation on DefaultValueAccessor.

When you write an Angular component, you don't need a value accessor or NgModel if you name the value and event properties according to Angular's two-way binding syntax.

{@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 introduces the most common built-in structural directives:

• NgIf—conditionally creates or disposes of subviews from the template.
• NgFor—repeat a node for each item in a list.
• NgSwitch—a set of directives that switch among alternative views.

For more information, see Structural Directives.

{@a ngIf}

Adding or removing an element with NgIf

You can add or remove an element by applying an NgIf directive to a host element.

When NgIf is false, Angular removes an element and its descendants from the DOM. Angular then disposes of their components, which frees up memory and resources.

To add or remove an element, bind *ngIf to a condition expression such as isActive in the following example.

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 and disposes of the component and all of its sub-components.

For more information on NgIf and NgIfElse, see the NgIf API documentation.

Guarding against null

By default, NgIf prevents display of an element bound to a null value.

To use NgIf to guard a <div>, add *ngIf="yourProperty" to the <div>. In the following example, the currentCustomer name appears because there is a currentCustomer.

However, if the property is null, Angular does not display the <div>. In this example, Angular does not display the nullCustomer because it is null.

{@a ngFor}

Listing items with NgFor

You can use the NgFor directive to present a list of items.

1. Define a block of HTML that determines how Angular renders a single item.

2. To list your items, assign the short hand let item of items to *ngFor.

The string "let item of items" instructs Angular to do the following:

• Store each item in the items array in the local item looping variable
• Make each item available to the templated HTML for each iteration
• Translate "let item of items" into an <ng-template> around the host element
• Repeat the <ng-template> for each item in the list

For more information see the Structural directive shorthand section of Structural directives.

Repeating a component view

To repeat a component element, apply *ngFor to the selector. In the following example, the selector is <app-item-detail>.

You can reference a template input variable, such as item, in the following locations:

• within the ngFor host element
• within the host element 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.

For more information about template input variables, see Structural directive shorthand.

Getting the index of *ngFor

You can get the index of *ngFor in a template input variable and use it in the template.

In the *ngFor, add a semicolon and let i=index to the short hand. The following example gets the index in a variable named i and displays it with the item name.

The index property of the NgFor directive context returns the zero-based index of the item in 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 shorthand, see the Structural Directives guide.

{@a one-per-element}

Repeating elements when a condition is true

To repeat a block of HTML when a particular condition is true, put the *ngIf on a container element that wraps an *ngFor element. One or both elements can be an <ng-container> so you don't have to introduce extra levels of HTML.

Because structural directives add and remove nodes from the DOM, apply only one structural directive per element.

For more information about NgFor see the NgForOf API reference.

{@a ngfor-with-trackby}

Tracking items with *ngFor trackBy

By tracking changes to an item list, you can reduce the number of calls your application makes to the server. With the *ngFor trackBy property, Angular can change and re-render only those items that have changed, rather than reloading the entire list of items.

1. Add a method to the component that returns the value NgFor should track. In this example, the value to track is the item's id. If the browser has already rendered id, Angular keeps track of it and doesn't re-query the server for the same id.

1. In the short hand expression, set trackBy to the trackByItems() method.

Change ids creates new items with new item.ids. In the following illustration of the trackBy effect, Reset items creates new items with the same item.ids.

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

{@a ngcontainer}

Hosting a directive without a DOM element

The Angular <ng-container> is a grouping element that doesn't interfere with styles or layout because Angular doesn't put it in the DOM.

You can use <ng-container> when there's no single element to host the directive.

Here's a conditional paragraph using <ng-container>.

1. Import the ngModel directive from FormsModule.

2. Add FormsModule to the imports section of the relevant Angular module.

3. To conditionally exclude an <option>, wrap the <option> in an <ng-container>.

{@a ngSwitch}

Switching cases with NgSwitch

Like the JavaScript switch statement, NgSwitch displays one element from among several possible elements, based on a switch condition. Angular puts only the selected element into the DOM.

NgSwitch is a set of three directives:

• NgSwitch—an attribute directive that changes the behavior of its companion directives.
• NgSwitchCase—structural directive that 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—structural directive that adds its element to the DOM when there is no selected NgSwitchCase.

1. On an element, such as a <div>, add [ngSwitch] bound 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.

2. Bind to *ngSwitchCase and *ngSwitchDefault on the elements for the cases.

1. In the parent component, define currentItem so you can use it in the [ngSwitch] expression.

1. In each child component, add an item input property which is bound to the currentItem of the parent component. The following two snippets show the parent component and one of the child components. The other child components are identical to StoutItemComponent.

Switch directives also work with native HTML elements and web components. For example, you could replace the <app-best-item> switch case with a <div> as follows.

What's next

For information on how to build your own custom directives, see Attribute Directives and Structural Directives.