Angular provides several methods to insert and display content within components. Two key approaches include using the <ng-content> directive and dynamically creating instances of TemplateRef. This article will discuss <ng-content>, its limitations, and situations where you might prefer it over TemplateRef.

Content Projection with<ng-content>

In Angular, content projection is a way to inject HTML content from a parent component into a child component. This mechanism is otherwise known as transclusion. The <ng-content> tag is Angular's built-in directive for achieving content projection. It acts as a placeholder that marks the spot (or spots) where the projected content should appear within the template of the component.

How<ng-content>Works

<ng-content> is very straightforward and easy to use and is great for simple wrapper components that do not require much complexity in terms of where or how the projected content is rendered.

A perfect example of when to use ng-content is with a component like a card. Here, we can set up our container's style and content placeholders. Then, we project content from the parent into the card component for it to be displayed.

<!-- parent.component.html -->
<app-card>
  <p>This content will be projected into the app-card component.</p>
</app-card>

<!-- card.component.html -->
<div class="card-container">
  <ng-content></ng-content>
</div>

In this example, the <p> tag from our parent component is projected into the <ng-content> placeholder in the app-card component's template. The <ng-content> element acts as a placeholder and doesn't produce a DOM element itself, so the resulting HTML appears as follows:

<div class="card-container">
  <p>This content will be projected into the app-card component.</p>
</div>

That's it.

In this example, since the ng-content directive doesn't have a select attribute, the <ng-content> placeholder in the card component acts as the default slot. We can also project content into several slots.

Projecting multiple slots

We can use multiple <ng-content> placeholders in our component to project more than one piece of content. This is done using the select attribute, which supports CSS selectors to choose specific elements from the parent to inject into the component's content.

Example:

<!-- parent.component.html -->
<app-card>
  <h1 title>Home</h1>
  <p description>This content will be projected into the app-card component.</p>
</app-card>

<!-- card.component.html -->
<div class="card-container">
  <ng-content select="[title]"></ng-content>
  <hr />
  <ng-content select="[description]"></ng-content>
</div>

The above template configuration will result in the following HTML output:

<div class="card-container">
  <h1 title>Home</h1>
  <hr />
  <p description>This content will be projected into the app-card component.</p>
</div>

Limitations with<ng-content>

1. Static Projection Points

ng-content slots are defined at compile-time and cannot be dynamically created or moved at runtime. This means that the structure of content projection within a component is fixed once the component is compiled, limiting the ability to dynamically alter the projection points based on runtime conditions or user interactions.

2. Single Projection per Slot

Each piece of content intended for projection can only be projected into a single location. If you need the same content to appear in multiple places within the host component, ng-content alone cannot accomplish this without duplicating the content in the parent component.

3. Limited Contextual Awareness

The projected content is inserted into the host component as-is and does not have direct access to the context of the container component.

When content is projected into a component using ng-content, it essentially remains in the context of its originating component. This means the projected content has direct access to the data and methods of its parent component (the component where it was defined), not the component into which it is being projected (the host component).

This contrasts with approaches like TemplateRef and ngTemplateOutlet, which can provide more flexibility in passing context to the projected content.

4. Lack of Control Over Instantiated Content

Once content is projected using ng-content, the receiving component has limited control over the projected content. For instance, it cannot easily manipulate, replace, or dynamically interact with the content in the same way it could with content instantiated through a TemplateRef and ViewContainerRef.

5. No Direct Support for Conditional Projection or Repetition

ng-content does not directly support conditional content projection or repeating content without additional structural directives in the parent component. If your use case requires projecting content conditionally or repeating projected content based on a collection, you would need to manage this logic outside of the ng-content directive, typically in the parent component.

While ng-content is excellent for straightforward scenarios where static content needs to be injected into a component's template, its limitations become apparent in more complex or dynamic use cases. For scenarios requiring more dynamic content manipulation, conditional rendering of projected content, or the need to project content into multiple locations dynamically, alternative approaches involving TemplateRef, ngTemplateOutlet, or programmatic view manipulation might be more appropriate.

When to choose ng-content overTemplateRef

Choosing to project content with <ng-content> over TemplateRef in Angular is particularly advantageous for scenarios that prioritize simplicity and static content projection. <ng-content> shines in situations where the structure of the projected content is fixed and does not require dynamic manipulation post-compilation. It is ideal for creating generic wrapper components, such as modals, tabs, or cards, where the content to be projected is known beforehand and doesn't change based on runtime conditions.

While <ng-content> is ideal for simple projection needs, TemplateRef provides a more robust method for dynamic and context-sensitive content rendering. By becoming proficient in both techniques, you can fully utilize Angular's templating features to develop flexible and reusable components.