Angular2: How is ngfor expanded

Question

I know the textbook rules on that <div *ngFor="let foo of foobars">{{foo.stuff}}</div> turns into <template ngFor let-foo="$implicit" [ngForOf]="foobars"><div>...</div></template>. My question is two-fold:

• HOW?
• What do I need to do to leverage this mechanism ("microsyntax") myself?

Ie turn <div *myDirective="item">{{item.stuff}}</div> into <template myDirective let-item="$implicit"><div>{{item.stuff}}</div></template>?

Since I read ngFor's source code top to bottom, I can only assume this dark magic is in the compiler somewhere, I've been up and down the angular github, but I can't put my finger on it. Help!

Answer 1

Yes, all magic happens in the compiler.

Let's take this template:

<div *ngFor="let foo of foobars">{{foo}}</div>

First it will be transformed to the following:

<div template="ngFor let foo of foobars>{{foo}}</div>

And then:

<template ngFor let-foo [ngForOf]="foobars"><div>{{foo}}</div></template>

In Angular2 rc.4 it looks like this

First is generated ast tree node (Abstract Syntax Tree node) and then all magic happens in the TemplateParseVisitor.visitElement(https://github.com/angular/angular/blob/2.0.0-rc.4/modules/%40angular/compiler/src/template_parser.ts#L284) specifically at the bottom (https://github.com/angular/angular/blob/2.0.0-rc.4/modules/%40angular/compiler/src/template_parser.ts#L394)

if (hasInlineTemplates) {
  var templateCssSelector = createElementCssSelector(TEMPLATE_ELEMENT, templateMatchableAttrs);
  var templateDirectiveMetas = this._parseDirectives(this.selectorMatcher, templateCssSelector);
  var templateDirectiveAsts = this._createDirectiveAsts(
      true, element.name, templateDirectiveMetas, templateElementOrDirectiveProps, [],
      element.sourceSpan, []);
  var templateElementProps: BoundElementPropertyAst[] = this._createElementPropertyAsts(
      element.name, templateElementOrDirectiveProps, templateDirectiveAsts);
  this._assertNoComponentsNorElementBindingsOnTemplate(
      templateDirectiveAsts, templateElementProps, element.sourceSpan);
  var templateProviderContext = new ProviderElementContext(
      this.providerViewContext, parent.providerContext, parent.isTemplateElement,
      templateDirectiveAsts, [], [], element.sourceSpan);
  templateProviderContext.afterElement();

  parsedElement = new EmbeddedTemplateAst(
      [], [], [], templateElementVars, templateProviderContext.transformedDirectiveAsts,
      templateProviderContext.transformProviders,
      templateProviderContext.transformedHasViewContainer, [parsedElement], ngContentIndex,
      element.sourceSpan);
}
return parsedElement;

This method returns EmbeddedTemplateAst. It's the same as:

<template ngFor let-foo [ngForOf]="foobars"><div>{{foo}}</div></template>

If you want to turn:

<div *myDirective="item">{{item.stuff}}</div>

into

<template myDirective let-item><div>{{item.stuff}}</div></template>

then you need to use the following syntax:

<div *myDirective="let item">{{item.stuff}}</div>

But in this case you won't pass context. Your custom structural directive might look like this:

@Directive({
  selector: '[myDirective]'
})
export class MyDirective {
  constructor(
    private _viewContainer: ViewContainerRef, 
    private _templateRef: TemplateRef<any>) {}

   @Input() set myDirective(prop: Object) {
    this._viewContainer.clear();
    this._viewContainer.createEmbeddedView(this._templateRef, prop); <== pass context
  }
} 

And you can use it like:

<div *myDirective="item">{{item.stuff}}</div>

               ||
               \/

<div template="myDirective:item">{{item.stuff}}</div>

               ||
               \/

<template [myDirective]="item">
   <div>{{item.stuff}}</div>
</template>

I hope this will help you understand how structural directives work.

Update:

Let's see how it works (plunker)

*dir="let foo v foobars" => [dirV]="foobars"

So you can write the following directive:

@Directive({
  selector: '[dir]'
})
export class MyDirective {
  @Input()
  dirV: any;

  @Input()
  dirK: any;

  ngAfterViewInit() {
    console.log(this.dirV, this.dirK);
  }
}

@Component({
  selector: 'my-app',
  template: `<h1>Angular 2 Systemjs start</h1>
  <div *dir="let foo v foobars k arr">{ foo }</div>
  `,
  directives: [MyDirective]
})
export class AppComponent {
  foobars = [1, 2, 3];
  arr = [3,4,5]
}

Here is the corresponding Plunker

See also

• https://angular.io/docs/ts/latest/guide/structural-directives.html#!#the-asterisk-effect
• https://teropa.info/blog/2016/03/06/writing-an-angular-2-template-directive.html
• https://www.bennadel.com/blog/3076-creating-an-index-loop-structural-directive-in-angular-2-beta-14.htm
• https://egghead.io/lessons/angular-2-write-a-structural-directive-in-angular-2

Live example you can find here https://alexzuza.github.io/enjoy-ng-parser/