[Angular] Create a simple *ngFor

In this post, we are going to create our own structure directive *ngFor.

What it should looks like in HTML?

    <div>
      <ul>
        <li *myFor="let item of items; let i = index;">
          {{ i }} Member: {{ item.name | json }}
        </li>
        <template myFor [myForOf]="items" let-item let-i="index">
          <li>{{ i }} Member: {{ item.name | json }}</li>
        </template>
      </ul>
    </div>

So here, we have a '*myFor' directive. It also use 'myForOf' direcitve. And a implicit value 'item'. 

Notice that:

        <li *myFor="let item of items; let i = index;">
          {{ i }} Member: {{ item.name | json }}
        </li>

and

        <template myFor [myForOf]="items" let-item let-i="index">
          <li>{{ i }} Member: {{ item.name | json }}</li>
        </template>

They have the same effect.

Now let's create myFor directive:

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

@Directive({
  selector: '[myFor][myForOf]'
})
export class MyForDirective {
  @Input()
  set myForOf(collection) {
    this.view.clear();
    collection.forEach((item, index) => {
      this.view.createEmbeddedView(
        this.template,
        {
          $implicit: item,
          index
        }
      )
    });
  }

  constructor(
    private view: ViewContainerRef,
    private template: TemplateRef<any>
  ) {
    // this.template will point to the host element
  }
}

It is good to clear the view every time we generate new embedded view:

this.view.clear();

Also we gave implicit 'value' and 'index'.

      this.view.createEmbeddedView(
        this.template,
        {
          $implicit: item, // let-item
          index // let-i="index"
        }
      )