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Dynamic three column layout using Structural Directive in Angular 7 and Material UI based application


Recently I came across a requirement in which I had to implement a multi-column layout with Dynamic Data. At several places I had to present the data into two columns and at some other places in three or even four columns. But one thing was common which was my input. My input was always an array of some kind of objects.

The application I was writing was an Angular7 based dashboard application with lots of different ways of presenting data. I used Material UI for theming and fontaewsome for icons.

I had to present data in column layout similar to the following



In some other places I had to break data into 3 or 4 columns. Plus the layout was different at all places. For example as in the following screenshot.


There were several other places where I had to do the same but use different UI elements, colors and designs.

My only option for all such screen was to either write a recursive template for each of them or use a nested loop e.g a nested *ngFor all these screens. I found myself repeating a code like the following


<div class="col col-md-1" ngfor="let sublistIndex of [[0, list.length/2+1], [(list.length/2)+1, list.length]]">
<mat-accordion ngfor="let item of list.slice(sublistIndex[0],sublistIndex[1]);">
 <mat-expansion-panel>
   <mat-expansion-panel-header class="heading">
       {{item.title}}
   </mat-expansion-panel-header>
 </mat-expansion-panel>
</mat-accordion>
I had a different template for every place but the first line of code was essentially providing me indices to break the list into sublists. I always had to equally divide the list into sublists for all columns. In cases where number of items in list can be divided fully by number of columns, without any remainder I had to display equal number of items in all column and where there was a remainder the last column should hold extra items.

I had options to write code into my controller and repeat for every component or find a generic way to do this. I ended up with an Structural directive called *sublist. Following is the implementation of directive.

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

@Directive({
  selector: '[sublist][sublistOf][sublistNumberOfColumns]'
})
export class SublistDirective implements DoCheck {
  private list: any;
  private isDirty = false;
  private numberOfColumns = 1;

  @Input()
  set sublistOf(sublistOf: any) {
    this.list = sublistOf;
    this.isDirty = true;
  }

  @Input()
  set sublistNumberOfColumns (sublistNmberOfColumns: number) {
    this.numberOfColumns = sublistNmberOfColumns;
  }

  constructor(private container: ViewContainerRef,
              private template: TemplateRef) { }

  ngDoCheck(): void {
    if (this.isDirty) {
      this.isDirty = false;
      this.container.clear();
      const itemsInSublists = Math.floor(this.list.length / this.numberOfColumns);
      for (let i = 0, j = 0; i < this.numberOfColumns; i++) {
        const subList = i === this.numberOfColumns - 1 ? this.list.slice (j) : this.list.slice(j, itemsInSublists + j) ;
        this.container.createEmbeddedView(this.template, {$implicit: subList});
        j = j + itemsInSublists;
      }
    }
  }
}
So now I don't have to repeat my code anywhere and I just have to add one directive in my html anywhere, where I have to break the list to display in columns

<div class="col col-md-1" *sublist="let sublist of list; numberOfColumns: '2'">
<mat-accordion ngfor="let item of sublist">
      <mat-expansion-panel>
          <mat-expansion-panel-header class="heading">
              {{item.title}}
          </mat-expansion-panel-header>
        </mat-expansion-panel>
      </mat-accordion>
</div>

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