Mastering Angular MatTree with FlatTreeControl and CdkVirtualScrollViewport: Preserving Expanded Node State

Welcome to this comprehensive guide on using Angular MatTree with FlatTreeControl and CdkVirtualScrollViewport. In this article, we’ll dive deep into the world of Angular Material and explore how to create a tree component that preserves expanded node state.

Why Use Angular MatTree?

Angular MatTree is a powerful component for creating hierarchical data structures, such as trees, in your Angular applications. It provides a flexible and customizable way to display and interact with complex data. With MatTree, you can easily create trees with multiple levels of depth, and users can expand and collapse nodes as needed.

What is FlatTreeControl?

FlatTreeControl is a utility class provided by Angular Material that helps you to control the expansion state of nodes in your tree. It provides methods for expanding and collapsing nodes, as well as for getting the current expansion state of the tree.

What is CdkVirtualScrollViewport?

CdkVirtualScrollViewport is a directive provided by Angular CDK (Component Dev Kit) that enables virtual scrolling in your Angular application. Virtual scrolling is a technique that allows you to display a large dataset in a scrolling container while only rendering the visible items, which improves performance and reduces memory consumption.

Setting Up the Project

Before we dive into the implementation, let’s set up a new Angular project.

ng new angular-mattree-example
cd angular-mattree-example
ng add @angular/material

This will create a new Angular project with the Material library installed.

Creating the Tree Component

Next, let’s create a new component for our tree.

ng generate component tree

In the `tree.component.ts` file, add the following code:

import { Component } from '@angular/core';
import { FlatTreeControl } from '@angular/cdk/tree';
import { CdkVirtualScrollViewport } from '@angular/cdk/scrolling';

interface TreeNode {
  name: string;
  children?: TreeNode[];
}

@Component({
  selector: 'app-tree',
  template: `
    <cdk-virtual-scroll-viewport [itemSize]="20">
      <mat-tree [dataSource]="treeControl.dataNodes" [treeControl]="treeControl">
        <mat-tree-node *matTreeNodeDef="let node">
          <mat-icon matListItemAvatar>folder</mat-icon>
          <mat-list-item>{{ node.name }}</mat-list-item>
        </mat-tree-node>
      </mat-tree>
    </cdk-virtual-scroll-viewport>
  `,
})
export class TreeComponent {
  treeControl: FlatTreeControl<TreeNode>
  dataSource: TreeNode[] = [
    {
      name: 'Node 1',
      children: [
        {
          name: 'Node 1.1',
          children: [
            { name: 'Node 1.1.1' },
            { name: 'Node 1.1.2' },
          ],
        },
        { name: 'Node 1.2' },
      ],
    },
    {
      name: 'Node 2',
      children: [
        { name: 'Node 2.1' },
        { name: 'Node 2.2' },
      ],
    },
  ];

  constructor() {
    this.treeControl = new FlatTreeControl<TreeNode>(node => node.children, node => node.children.length > 0);
  }
}

In this code, we’re creating a tree component that uses the `FlatTreeControl` to manage the expansion state of the nodes. We’re also using the `CdkVirtualScrollViewport` to enable virtual scrolling.

Preserving Expanded Node State

Now that we have our tree component set up, let’s talk about how to preserve the expanded node state. When the user expands or collapses a node, we want to remember that state so that when the user navigates away and comes back, the node is still in the same expanded state.

To achieve this, we’ll use a service to store the expansion state of each node.

ng generate service tree-state

In the `tree-state.service.ts` file, add the following code:

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class TreeStateService {
  private state: { [id: string]: boolean } = {};

  getExpandedState(node: TreeNode): boolean {
    return this.state[node.name] || false;
  }

  setExpandedState(node: TreeNode, state: boolean): void {
    this.state[node.name] = state;
  }
}

This service provides methods for getting and setting the expansion state of each node. We’ll use this service in our tree component to store and retrieve the expansion state.

Updating the Tree Component

Let’s update our tree component to use the `TreeStateService` to store and retrieve the expansion state.

import { Component } from '@angular/core';
import { FlatTreeControl } from '@angular/cdk/tree';
import { CdkVirtualScrollViewport } from '@angular/cdk/scrolling';
import { TreeStateService } from './tree-state.service';

interface TreeNode {
  name: string;
  children?: TreeNode[];
}

@Component({
  selector: 'app-tree',
  template: `
    <cdk-virtual-scroll-viewport [itemSize]="20">
      <mat-tree [dataSource]="treeControl.dataNodes" [treeControl]="treeControl">
        <mat-tree-node *matTreeNodeDef="let node">
          <mat-icon matListItemAvatar>folder</mat-icon>
          <mat-list-item>{{ node.name }}</mat-list-item>
          <mat-icon matTreeNodeToggle>
            <mat-icon>{{ treeState.getExpandedState(node) ? 'expand_more' : 'chevron_right' }}</mat-icon>
          </mat-icon>
        </mat-tree-node>
      </mat-tree>
    </cdk-virtual-scroll-viewport>
  `,
})
export class TreeComponent {
  treeControl: FlatTreeControl<TreeNode>
  dataSource: TreeNode[] = [
    {
      name: 'Node 1',
      children: [
        {
          name: 'Node 1.1',
          children: [
            { name: 'Node 1.1.1' },
            { name: 'Node 1.1.2' },
          ],
        },
        { name: 'Node 1.2' },
      ],
    },
    {
      name: 'Node 2',
      children: [
        { name: 'Node 2.1' },
        { name: 'Node 2.2' },
      ],
    },
  ];

  constructor(private treeState: TreeStateService) {
    this.treeControl = new FlatTreeControl<TreeNode>(node => node.children, node => node.children.length > 0);
  }

  onTreeNodeToggle(node: TreeNode): void {
    this.treeState.setExpandedState(node, !this.treeState.getExpandedState(node));
    this.treeControl.expandDescendants(node);
  }
}

In this updated code, we’re using the `TreeStateService` to get and set the expansion state of each node. We’re also using the `onTreeNodeToggle` method to update the expansion state when the user toggles a node.

Conclusion

And that’s it! With this guide, you should now have a comprehensive understanding of how to use Angular MatTree with FlatTreeControl and CdkVirtualScrollViewport to create a tree component that preserves expanded node state. Remember to store the expansion state in a service to preserve it across route changes.

Best Practices

Here are some best practices to keep in mind when working with Angular MatTree:

• Use a service to store the expansion state to preserve it across route changes.
• Use the `FlatTreeControl` to manage the expansion state of the nodes.
• Use the `CdkVirtualScrollViewport` to enable virtual scrolling and improve performance.
• Use the `mat-tree-node` directive to define the template for each node.
• Use the `mat-icon` directive to display icons for each node.

Troubleshooting

If you’re experiencing issues with your tree component, here are some common problems and solutions:

Problem

Solution

Nodes are not expanding/collapsing correctly.

Check that you’reHere are the 5 Questions and Answers about “Angular MatTree with FlatTreeControl and CdkVirtualScrollViewport, preserve expanded node”: Frequently Asked Questions Get answers to your burning questions about Angular MatTree with FlatTreeControl and CdkVirtualScrollViewport, and learn how to preserve expanded nodes like a pro! Q: How do I implement Angular MatTree with FlatTreeControl and CdkVirtualScrollViewport? To implement Angular MatTree with FlatTreeControl and CdkVirtualScrollViewport, you need to first import the necessary modules in your Angular module. Then, create a component that uses the MatTree component and bind it to a data source. Finally, use the FlatTreeControl to control the tree’s expansion state and the CdkVirtualScrollViewport to enable virtual scrolling. Don’t forget to configure the tree’s node data and template! Q: Why do I need to use FlatTreeControl with MatTree? You need to use FlatTreeControl with MatTree because it provides a flat data structure that mirrors the tree’s hierarchical structure. This allows for efficient data manipulation and caching, making it ideal for large datasets. Plus, it enables features like node expansion and collapse, which are essential for a great user experience! Q: How do I preserve the expanded state of nodes in MatTree? To preserve the expanded state of nodes in MatTree, you can use the `expanded` property on each node data object. When the node is expanded, set `expanded` to `true`, and when it’s collapsed, set it to `false`. Then, in your component, use a service or a state management system to store the expanded state of nodes. Finally, when the tree is reinitialized, use the stored state to restore the expanded nodes! Q: Can I use CdkVirtualScrollViewport with MatTree? Absolutely! CdkVirtualScrollViewport is designed to work seamlessly with MatTree. By combining the two, you can create a performant and efficient tree component that handles large datasets with ease. Just remember to configure the viewport’s settings to match your tree’s requirements, and you’re good to go! Q: Are there any performance considerations when using MatTree with FlatTreeControl and CdkVirtualScrollViewport? Yes! When using MatTree with FlatTreeControl and CdkVirtualScrollViewport, performance is crucial. Make sure to optimize your tree’s data structure, use efficient data manipulation, and implement lazy loading or pagination to reduce the amount of data being processed. Additionally, consider using caching or memoization to reduce computation overhead. By following these best practices, you can ensure a blazing-fast tree component that delights your users! Share this: Add Comment Exit mobile version