Drag ‘n’ Drop with Angular Materials Table Component

Image: https://material.angular.io/

Blog Post #007

Duncan Faulkner – February 2020

The Angular Material mat-table component probably has the most examples on the Angular Materials website. But I think we can squeeze one more in.

So here is a Drag ‘n’ Drop example.

I needed the ability to drag ‘n’ drop rows in a mat-table for a project I was working on and I thought this would make a good example of how easy it was to implement.

This example requires all the usual setup of an Angular application, rather than fill this post with all the steps to setup an Angular project. I have written a post here that goes through the steps. I will also create a another separate post on adding Angular Material, but for now here is how to add Angular Material to your Angular Project.

Project set up? Let’s add Angular Materials, from the terminal type:

ng add @angular/material

This will add the Angular Material dependencies to the project, next create a directory called shared and then add a new file called material.module.ts in this directory.

Then add the material imports, for this we just need MatTableModule.

// Other Material imports here
import { MatTableModule } from '@angular/material/table';

const MATERIALMODULES = [MatTableModule];

@NgModule({
     imports: [...MATERIALMODULES],
     declarations: [],
     exports: [...MATERIALMODULES]
});
export class MaterialModule {}

Then import material.module.ts file into the app.module.ts file.

// default modules add when app.module.ts was created
import { BrowserModule } from "@angular/platform-browser";
import { NgModule } from "@angular/core";
import { AppRoutingModule } from "./app-routing.module";
import { AppComponent } from "./app.component";
import { BrowserAnimationsModule } from "@angular/platform-browser/animations";
import { HttpClientModule } from "@angular/common/http";

// Add these modules to import
import { MaterialModule } from "./shared/materials.module";
import { DragDropModule } from "@angular/cdk/drag-drop";

const MATERIALMODULE = [MaterialModule];
const IMPORTMODULES = [
  BrowserModule,
  AppRoutingModule,
  MaterialModule,
  BrowserAnimationsModule,
  HttpClientModule,
  DragDropModule
];
const APPCOMPONENT = [AppComponent];

@NgModule({
  declarations: [...APPCOMPONENT],
  imports: [...IMPORTMODULES],
  exports: [...MATERIALMODULE],
  providers: [],
  bootstrap: [...APPCOMPONENT]
})
export class AppModule {}

The MaterialModule is the Angular Material file we just created a few moments ago and the DragDropModule is required to implement the drag and drop from the Angular Material CDK (more on this later).

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In the app.component.html file remove any html and replace it with the following.

<table mat-table [dataSource]="dataSource" cdkDropList [cdkDropListData]="dataSource"
    (cdkDropListDropped)="drop($event)" class="mat-elevation-z8">

    <ng-container matColumnDef="drag">
        <th mat-header-cell *matHeaderCellDef> Drag </th>
        <td mat-cell *matCellDef="let element">
            <mat-icon cdkDragHandle svgIcon="dragVertical"></mat-icon>
        </td>
    </ng-container>

    <!-- Position Column -->
    <ng-container matColumnDef="position">
        <th mat-header-cell *matHeaderCellDef> No. </th>
        <td mat-cell *matCellDef="let element"> {{element.position}} </td>
    </ng-container>

    <!-- Name Column -->
    <ng-container matColumnDef="name">
        <th mat-header-cell *matHeaderCellDef> Name </th>
        <td mat-cell *matCellDef="let element"> {{element.name}} </td>
    </ng-container>

    <!-- Weight Column -->
    <ng-container matColumnDef="weight">
        <th mat-header-cell *matHeaderCellDef> Weight </th>
        <td mat-cell *matCellDef="let element"> {{element.weight}} </td>
    </ng-container>

    <!-- Symbol Column -->
    <ng-container matColumnDef="symbol">
        <th mat-header-cell *matHeaderCellDef> Symbol </th>
        <td mat-cell *matCellDef="let element"> {{element.symbol}} </td>
    </ng-container>

    <tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
    <tr mat-row *matRowDef="let row; columns: displayedColumns;" cdkDragLockAxis="y" cdkDrag [cdkDragData]="row">
    </tr>
</table>

A mat-table component takes a datasource, but we also need to add cdkDropList, a cdkDropListData and cdkDropListDropped to the table. The cdkDropList is a container that wraps a set of draggable items. The cdkDropListData is the datasource to attach to this container. And the cdkDropListDropped emits an event when a user drops an item into the container.

The last <tr> section in the mat-table, needs the collection of columns this is the array containing the column names, and row refers to current row.

The cdkDragLockAxis restricts the dragged element to either vertical or horizontal, this stops the user dragging the item all over the screen if it doesn’t make sense to, for example if you are only dragging a list of items to sort them then locking the axis to the “y axis” would keep this within the boundary of the list. The cdkDrag refers to the item being dragged and cdkDragData is the data attached to this drag instance.

That’s the HTML section, now for the code section.

import { Component } from "@angular/core";
import { IconService } from "./services/icon.service";
import { CdkDragDrop, moveItemInArray } from "@angular/cdk/drag-drop";

export interface PeriodicElement {
  name: string;
  position: number;
  weight: number;
  symbol: string;
}

const ELEMENT_DATA: PeriodicElement[] = [
  { position: 1, name: "Hydrogen", weight: 1.0079, symbol: "H" },
  { position: 2, name: "Helium", weight: 4.0026, symbol: "He" },
  { position: 3, name: "Lithium", weight: 6.941, symbol: "Li" },
  { position: 4, name: "Beryllium", weight: 9.0122, symbol: "Be" },
  { position: 5, name: "Boron", weight: 10.811, symbol: "B" },
  { position: 6, name: "Carbon", weight: 12.0107, symbol: "C" },
  { position: 7, name: "Nitrogen", weight: 14.0067, symbol: "N" },
  { position: 8, name: "Oxygen", weight: 15.9994, symbol: "O" },
  { position: 9, name: "Fluorine", weight: 18.9984, symbol: "F" },
  { position: 10, name: "Neon", weight: 20.1797, symbol: "Ne" }
];

/**
 * @title Basic use of `<table mat-table>`
 */
@Component({
  selector: "app-root",
  styleUrls: ["app.component.scss"],
  templateUrl: "app.component.html"
})
export class AppComponent {
  constructor(private iconService: IconService) {
    this.iconService.registerIcons();
  }

  displayedColumns: string[] = ["drag", "position", "name", "weight", "symbol"];
  dataSource = ELEMENT_DATA;

  drop(event: CdkDragDrop<PeriodicElement[]>) {
    moveItemInArray(this.dataSource, event.previousIndex, event.currentIndex);
    console.log(event.container.data);
  }
}

The drop event takes a generic CdkDragDrop<> in this case it’s a array of type PeriodicElement, this refers to the row that is being dropped. The moveItemInArray takes a datasource, the previousIndex and the currentIndex. The dataSource is the array (in this example the ELEMENT_DATA), the previousIndex is where the item came (position 10) from and the currentIndex (position 1) is where the item is being dropped. This method does all the hard work of the sorting etc… we just need to write the code to persist this and update the view (I’ve not done that in this example). If you check the console.log at this point then we can see that the data has been reordered.

And that’s all there is to add Drag n Drop to a mat-table.

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