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

Built-in directives

Angular offers two kinds of built-in directives: attribute directives and structural directives.

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

For more detail, including how to build your own custom directives, see 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. You usually apply them to elements as if they were HTML attributes, hence the name.

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.

{@a ngClass}

NgClass

Add or remove several CSS classes simultaneously with ngClass.

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

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.

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

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

{@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, without NgStyle.

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:

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

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

{@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:

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.

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

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

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

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 that adapts an element to this protocol. Angular provides value accessors for all of the basic HTML form elements and the 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.

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 and skip NgModel altogether. The sizer in the 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:

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:

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

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

The deep details of structural directives are covered in the Structural Directives guide, which explains the following:

• Why you prefix the directive name with an asterisk (*).
• Using <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 to an element.

{@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.

Don't forget the asterisk (*) in front of ngIf. For more information on the asterisk, see the asterisk (*) prefix section of Structural Directives.

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 or style binding.

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.

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

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.

{@a ngFor}

NgFor

NgFor is a repeater directive—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>.

Don't forget the asterisk (*) in front of ngFor. For more information on the asterisk, see the asterisk (*) prefix section of Structural Directives.

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

{@a microsyntax}

*ngFor microsyntax

The string assigned to *ngFor is not a template expression. Rather, it's a microsyntax—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.

{@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.

For more information about template input variables, see Structural Directives.

*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.

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.

{@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.

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

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

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

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.

{@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.

NgSwitch is actually a set of three, cooperating directives: NgSwitch, NgSwitchCase, and NgSwitchDefault as in the following example.

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 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.