Use of $implicit do not pass value when using content projection

While playing with the concept of ng-template (html to to render), ng-container (work with ng-template for rendering) and ng-content (content projection), I'm having trouble to make use of $implicit value in a particular situation.

When the context is provided within *ngTemplateOutlet using a ng-container and content projection,the implicit value is not present.

Cannot figure out why, any idea ?

See bellow the code :

<!-- app.component.html -->
<ng-template #inputTemplate let-labelDescription="inputLabel" let-defaultTagValue="inputValue">
  <label for="action">{{labelDescription}}</label><input id="idAction" value="{{defaultTagValue}}">
</ng-template>

<ng-template #buttonTemplate let-labelDescription="buttonLabel" let-defaultTagValue="buttonValue">
  <label for="action">label: {{labelDescription}}</label><h1><button id="idAction">my value : {{defaultTagValue}}</button></h1>
</ng-template>

<testing-componenet [id]="0" [description]="'send template button'">
  <ng-container *ngTemplateOutlet="buttonTemplate; context: { buttonLabel:'LABEL_BUTTON', $implicit: '..data..'}"></ng-container>
</testing-componenet>

// testing-component.component.ts
import { Component, Input } from '@angular/core';

@Component({
  selector: 'testing-component',
  standalone: true,
  imports: [],
  template: "<p> {{id}} </p><p> {{description}} </p> <ng-content></ng-content>"
})
export class TestingComponentComponent {
    @Input( {required:true} )
    id!: number ;
    @Input( {required:true} )
    description!:string;
}

Result :

Should be :

You have 2 options.

Since your second template expects a buttonLabel and buttonValue template variable

<!-- let-labelDescription="buttonLabel" let-defaultTagValue="buttonValue" -->

<ng-template #buttonTemplate let-labelDescription="buttonLabel" let-defaultTagValue="buttonValue">
  <label for="action">label: {{labelDescription}}</label><h1><button id="idAction">my value : {{defaultTagValue}}</button></h1>
</ng-template>

You should use the following ngTemplateOutlet snippet

<ng-container *ngTemplateOutlet="buttonTemplate; context: { buttonLabel:'LABEL_BUTTON', buttonValue: '..data..'}"></ng-container> <!-- not $implicit -->

---

On the other hand, you could use $implicit if you wanted to

<ng-container *ngTemplateOutlet="buttonTemplate; context: { buttonLabel:'LABEL_BUTTON', $implicit: '..data..'}"></ng-container>

but then you have to pass in the following template

<ng-template #buttonTemplate let-labelDescription="buttonLabel" let-defaultTagValue> <!-- Remove ="buttonValue" -->
  <label for="action">label: {{labelDescription}}</label><h1><button id="idAction">my value : {{defaultTagValue}}</button></h1>
</ng-template>

Making it more awesome

At the moment you're using <ng-content> which projects the <ng-container> and templateoutletdirective, and you're kinda lucky this works. A more slick way to do this is by writing a new directive

@Directive({ selector: 'testContent' })
export class TestContentDirective {
  constructor(testComponent: TestingComponentComponent, template: TemplateRef<any>) {
    testComponent.template = template;
  }
}

So now you can use your component like this:

@Component({
  selector: 'testing-component',
  template: `
    <p> {{id}} </p>
    <p> {{description}} </p>
    <ng-container *ngTemplateOutlet="template; context: { buttonLabel:'LABEL_BUTTON', buttonValue: '..data..'}"></ng-container>`,
})
export class TestingComponentComponent {
  @Input({ required: true }) id!: number;
  @Input({ required: true }) description!: string;
  template: TemplateRef<any> | null = null;
}

@Directive({ selector: '[testContent]' })
export class TestContentDirective {
  constructor(testComponent: TestingComponentComponent, template: TemplateRef<any>) {
    testComponent.template = template;
  }
}

@NgModule({
  declarations: [TestingComponentComponent, TestContentDirective],
  imports: [CommonModule],
  exports: [TestingComponentComponent, TestContentDirective]
})
export class TestModule {}

@Component({
  selector: 'app-root',
  standalone: true,
  imports: [TestModule],
  template: `
    <testing-component [id]="0" [description]="'send template button'">
      <ng-template testContent let-labelDescription="buttonLabel" let-defaultTagValue="buttonValue">
        <label for="action">label: {{labelDescription}}</label><h1><button id="idAction">my value : {{defaultTagValue}}</button></h1>
      </ng-template>
    </testing-component>
  `,
})
export class App {
  name = 'Angular';
}

bootstrapApplication(App);

To take things even further you can write it using the structural directive notation

<testing-component [id]="0" [description]="'send template button'">
  <div *testContent="let labelDescription=buttonLabel; let defaultTagValue=buttonValue">
    <label for="action">label: {{labelDescription}}</label><h1><button id="idAction">my value : {{defaultTagValue}}</button></h1>
  </div>
</testing-component>

Or you could also use a default template using the coalescing operator:

<ng-container *ngTemplateOutlet="template ?? defaultTemplate; ..."></ng-container>
<ng-template #defaultTemplate let-labelDescription="buttonLabel" let-defaultTagValue="buttonValue">
  <label for="action">label: Default description</label><h1><button id="idAction">my value: Default value</button></h1>
</ng-template>

So the conclusion is that you have 2 options. You can either use <ng-content select="[slot1]"> or you can use structural directives, assign the template to the main generic component, and project it while passing information from inside the generic component to wherever you're using it. I always tend to opt for the template approach, since it's more flexible and easier to pass data to the parent. I use it all the time. Here's an example component and here's how to use it.

Adding a template context guard

If you would take a look now at where you use your structural directive, you'd see that the template variables you receive have the any type

You can solve this by adding a ngTemplateContextGuard static field to your directive (DOCS):

@Directive({ selector: '[testContent]' })
export class TestContentDirective {
  constructor(testComponent: TestingComponentComponent, template: TemplateRef<any>) {
    console.log('received a template');
    testComponent.template = template;
  }
  
  public static ngTemplateContextGuard(
    dir: TestContentDirective,
    ctx: any
  ): ctx is BsTestTemplateContext {
    return true;
  }
}

export class BsTestTemplateContext {
  // public $implicit: TData = null!;
  buttonLabel!: string;
  buttonValue!: string;
}

Result:

StackBlitz, but they don't show tooltips in templates.