3

I'm using Angular 5.2.9.

I was wondering when should I use Renderer2 over ngStyle ? Which is the best solution ?

1:<div #div>FOO BAR</div>

  @ViewChild('div') div: ElementRef;

  constructor(private renderer: Renderer2) {}

  ngAfterViewInit() {
      this.renderer.setStyle(this.div.nativeElement, 'background', 'blue');
  }

2:<div [ngStyle]="styleValue">FOO BAR</div>

  styleValue: any = {};

  ngAfterViewInit() {
      this.styleValue = {background: 'blue'};
  }

I know that it is easier to use "ngStyle" in a ngFor, eg: 

<div ngFor="let elem of array" [ngStyle]="styleValue">

Otherwise you should do for this case: <div ngFor="let elem of array" #div>FOO BAR</div>

  @ViewChildren('div') divs: QueryList<ElementRef>;

  constructor(private renderer: Renderer2) {}

  ngAfterViewInit() {
     this.divs.change.subscribe(() => {
        this.toFlipArray.forEach((div) => {
            this.renderer.setStyle(this.div.nativeElement, 'background', 'blue');
        })
     }
  }

This seems much more longer in a ngFor to use Renderer2 and I have even not killed the subscription.

Is there a difference in performance ? Maybe somewhere else ?

HDJEMAI
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bossno
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1 Answers1

2

Both ngStyle and renderer.setStyle are used to dynamically style a component

But renderer.setStyle looks to take precedence over [ngStyle] even if ngStyle looks to be a type of embedded style.

Demo example:

https://stackblitz.com/edit/angular-jtdk4z?file=src%2Fapp%2Fapp.component.html

When looking to the internal implementation of ngStyle:

https://github.com/angular/angular/blob/master/packages/common/src/directives/ng_style.ts

it looks like it itself is implemented using renderer.setStyle

@Directive({selector: '[ngStyle]'})
export class NgStyle implements DoCheck {
  private _ngStyle: {[key: string]: string};
  private _differ: KeyValueDiffer<string, string|number>;

  constructor(
      private _differs: KeyValueDiffers, private _ngEl: ElementRef, private _renderer: Renderer2) {}

  @Input()
  set ngStyle(v: {[key: string]: string}) {
    this._ngStyle = v;
    if (!this._differ && v) {
      this._differ = this._differs.find(v).create();
    }
  }

  ngDoCheck() {
    if (this._differ) {
      const changes = this._differ.diff(this._ngStyle);
      if (changes) {
        this._applyChanges(changes);
      }
    }
  }

  private _applyChanges(changes: KeyValueChanges<string, string|number>): void {
    changes.forEachRemovedItem((record) => this._setStyle(record.key, null));
    changes.forEachAddedItem((record) => this._setStyle(record.key, record.currentValue));
    changes.forEachChangedItem((record) => this._setStyle(record.key, record.currentValue));
  }

  private _setStyle(nameAndUnit: string, value: string|number|null|undefined): void {
    const [name, unit] = nameAndUnit.split('.');
    value = value != null && unit ? `${value}${unit}` : value;

    if (value != null) {
      this._renderer.setStyle(this._ngEl.nativeElement, name, value as string);
    } else {
      this._renderer.removeStyle(this._ngEl.nativeElement, name);
    }
}
HDJEMAI
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  • if ngStyle is using Renderer then can we assume it's also using Renderer2? – Jeb50 Nov 24 '19 at 23:29
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    Yes, look at what is injected in the constructor in the code: ``private _renderer: Renderer2``. you see that Renderer2 is injected. – HDJEMAI Nov 25 '19 at 00:27
  • awesome, so they are the same thing; and no worry of [ngStyle] would expose vulnerability. – Jeb50 Nov 25 '19 at 00:31
  • [Renderer2, provides an API that can safely be used](https://www.syncfusion.com/blogs/post/top-5-best-practices-angular-app-security.aspx). With a quick search I found that link, but you can search more to find best practices for Angular App Security. So ``[ngStyle]`` should not expose vulnerability. – HDJEMAI Nov 25 '19 at 00:48