The Directive Composition API in Angular

The Directive Composition API in Angular provides a way to create custom directives that can be used to manipulate the behavior and appearance of elements in your templates. It allows you to define custom logic and use it to interact with elements in your views.

Here's a step-by-step guide on how to use the Directive Composition API in Angular:

Import the necessary classes:
In order to use the Directive Composition API, you'll first need to import the Directive and ViewContainerRef classes from the @angular/core module. These classes will be used to define your custom directive and manipulate elements in the view.

import { Directive, ViewContainerRef } from '@angular/core';

Create a directive class:
Next, create a class for your custom directive and use the @Directive decorator to provide metadata about the directive. The selector property in the metadata should be a CSS selector that you'll use to apply the directive to elements in your templates.

@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {

  constructor(private viewContainer: ViewContainerRef) { }

}

Implement the directive logic:
In the directive class, you can implement the logic for the directive using the ViewContainerRef to manipulate elements in the view. For example, you can create an embedded view and use it to wrap the content of an element in your template with a yellow background.

@Directive({
  selector: '[appHighlight]'
})
export class HighlightDirective {

  constructor(private viewContainer: ViewContainerRef) { 
    this.viewContainer.createEmbeddedView(this.template);
  }

  private get template() {
    return `
      <ng-template>
        <div style="background-color: yellow;">
          <ng-content></ng-content>
        </div>
      </ng-template>
    `;
  }

}

Register the directive in your module:
Finally, you'll need to register the directive in your module's declarations array. This will make the directive available for use in your templates.

@NgModule({
  declarations: [
    HighlightDirective
  ],
  ...
})
export class AppModule { }

Use the directive in your templates:
You can now use the directive in your templates by applying the selector (in this case appHighlight) to any element. When the directive is applied, the content of the element will be wrapped in a yellow background.

<p appHighlight>This text will be highlighted</p>

This is just a basic example of how to use the Directive Composition API in Angular. You can use it to create more complex custom directives as needed to fit your specific use case. The ability to manipulate elements in the view and interact with templates gives you a lot of flexibility and power when building Angular applications.