angular-docs-cn/aio/content/guide/content-projection.md

Content projection

This topic describes how to use content projection to create flexible, reusable components.

To view or download the example code used in this topic, see the .

Content projection is a pattern in which you insert, or project, the content you want to use inside another component. For example, you could have a Card component that accepts content provided by another component.

The following sections describe common implementations of content projection in Angular, including:

• Single-slot content projection. With this type of content projection, a component accepts content from a single source.
• Multi-slot content projection. In this scenario, a component accepts content from multiple sources.
• Conditional content projection. Components that use conditional content projection render content only when specific conditions are met.

{@a single-slot }

Single-slot content projection

The most basic form of content projection is single-slot content projection. Single-slot content projection refers to creating a component into which you can project one component.

To create a component that uses single-slot content projection:

1. Create a component.

2. In the template for your component, add an ng-content element where you want the projected content to appear.

For example, the following component uses an ng-content element to display a message.

With the ng-content element in place, users of this component can now project their own message into the component. For example:

The ng-content element is a placeholder that does not create a real DOM element. Custom attributes applied to ng-content are ignored.

{@a multi-slot}

Multi-slot content projection

A component can have multiple slots. Each slot can specify a CSS selector that determines which content goes into that slot. This pattern is referred to as multi-slot content projection. With this pattern, you must specify where you want the projected content to appear. You accomplish this task by using the select attribute of ng-content.

To create a component that uses multi-slot content projection:

1. Create a component.

2. In the template for your component, add an ng-content element where you want the projected content to appear.

3. Add a select attribute to the ng-content elements. Angular supports selectors for any combination of tag name, attribute, CSS class, and the :not pseudo-class.

For example, the following component uses two ng-content elements.

Content that uses the question attribute is projected into the ng-content element with the select=[question] attribute.

ng-content without a select attribute

If your component includes an ng-content element without a select attribute, that instance receives all projected components that do not match any of the other ng-content elements.

In the preceding example, only the second ng-content element defines a select attribute. As a result, the first ng-content element receives any other content projected into the component.

{@a conditional }

Conditional content projection

If your component needs to conditionally render content, or render content multiple times, you should configure that component to accept an ng-template element that contains the content you want to conditionally render.

Using an ng-content element in these cases is not recommended, because when the consumer of a component supplies the content, that content is always initialized, even if the component does not define an ng-content element or if that ng-content element is inside of an ngIf statement.

With an ng-template element, you can have your component explicitly render content based on any condition you want, as many times as you want. Angular will not initialize the content of an ng-template element until that element is explicitly rendered.

The following steps demonstrate a typical implementation of conditional content projection using ng-template.

1. Create a component.

2. In the component that accepts an ng-template element, use an ng-container element to render that template, such as:

   This example uses the ngTemplateOutlet directive to render a given ng-template element, which you will define in a later step. You can apply an ngTemplateOutlet directive to any type of element. This example assigns the directive to an ng-container element because the component does not need to render a real DOM element.

3. Wrap the ng-container element in another element, such as a div element, and apply your conditional logic.

4. In the template where you want to project content, wrap the projected content in an ng-template element, such as:

   The ng-template element defines a block of content that a component can render based on its own logic. A component can get a reference to this template content, or TemplateRef, by using either the @ContentChild or @ContentChildren decorators. The preceding example creates a custom directive, appExampleZippyContent, as an API to mark the ng-template for the component's content. With the TemplateRef, the component can render the referenced content by using either the ngTemplateOutlet directive, or with ViewContainerRef.createEmbeddedView.

5. Create a directive with a selector that matches the custom attribute for your template. In this directive, inject a TemplateRef instance.

   In the previous step, you added an ng-template element with a custom attribute, appExampleZippyDirective. This code provides the logic that Angular will use when it encounters that custom attribute. In this case, that logic instructs Angular to instantiate a template reference.

6. In the component you want to project content into, use @ContentChild to get the template of the projected content.

   Prior to this step, your application has a component that instantiates a template when certain conditions are met. You've also created a directive that provides a reference to that template. In this last step, the @ContentChild decorator instructs Angular to instantiate the template in the designated component.

   In the case of multi-slot content projection, you can use @ContentChildren to get a QueryList of projected elements.

{@a ngprojectas }

Projecting content in more complex environments

As described in Multi-slot Content Projection, you typically use either an attribute, element, CSS Class, or some combination of all three to identify where to project your content. For example, in the following HTML template, a paragraph tag uses a custom attribute, question, to project content into the app-zippy-multislot component.

In some cases, you might want to project content as a different element. For example, the content you want to project might be a child of another element. You can accomplish this by using the ngProjectAs attribute.

For instance, consider the following HTML snippet:

This example uses an ng-container attribute to simulate projecting a component into a more complex structure.

Reminder!

The ng-container element is a logical construct that you can use to group other DOM elements; however, the ng-container itself is not rendered in the DOM tree.

In this example, the content we want to project resides inside another element. To project this content as intended, the template uses the ngProjectAs attribute. With ngProjectAs, the entire ng-container element is projected into a component using the question selector.