Fix NgClass & Dialog Sync: 3 Simple Steps for 2025

Step 2: A Modern RxJS Approach with the async Pipe

A more declarative and "Angular-way" solution is to leverage RxJS and the async pipe. The async pipe is brilliant because it not only subscribes to an Observable or Promise for you, but it also automatically triggers change detection whenever a new value is emitted.

By converting your state into a stream, you let Angular's built-in tools handle the change detection lifecycle for you.

How to do it:

Instead of a simple property for your class, create an Observable. A BehaviorSubject is perfect for this, as it holds the current value for you. Then, in your template, unwrap the value using the async pipe.

 import { Component, Inject } from '@angular/core'; import { MAT_DIALOG_DATA } from '@angular/material/dialog'; import { BehaviorSubject, map } from 'rxjs';  @Component({   selector: 'app-status-dialog-rxjs',   template: `     <h1 mat-dialog-title [ngClass]="statusClass$ | async">Update Status</h1>     <!-- ... rest of the template ... -->   `, }) export class StatusDialogRxjsComponent {   private status$ = new BehaviorSubject<string>(this.data.status);    // We derive the class object from the status stream   public statusClass$ = this.status$.pipe(     map(status => {       if (status === 'approved') return 'text-success';       if (status === 'rejected') return 'text-danger';       return '';     })   );    constructor(@Inject(MAT_DIALOG_DATA) public data: { status: string }) {}    updateStatus(newStatus: string): void {     // Just emit the new value. The async pipe handles the rest!     this.status$.next(newStatus);   } } 

Look how clean that updateStatus method is! No manual change detection needed. You simply push the new state into the stream, and the template reacts accordingly.

Pros:

• Declarative and aligns with reactive programming principles.
• Reduces boilerplate by letting the async pipe manage subscriptions and change detection.
• Integrates beautifully if your application already uses RxJS for state management.

Cons:

• Requires a foundational understanding of RxJS.
• Can feel like overkill for a very simple, isolated component.

Step 3: The Future-Proof Solution for 2025 - Embracing Angular Signals

Welcome to the future. Angular Signals, introduced in v16 and stabilized in v17, are the new paradigm for reactivity in Angular. They provide an extremely efficient, fine-grained change detection mechanism that sidesteps many of the complexities of Zone.js.

For our dialog problem, Signals are almost a magic bullet. Because they track dependencies and updates at a very granular level, they work flawlessly even in dynamically created components without any manual intervention or stream-wrapping.

How to do it:

Use the signal primitive to hold your state and a computed signal to derive the class name. The syntax is incredibly clean and intuitive.

 import { Component, Inject, signal, computed } from '@angular/core'; import { MAT_DIALOG_DATA } from '@angular/material/dialog';  @Component({   selector: 'app-status-dialog-signals',   template: `     <h1 mat-dialog-title [ngClass]="statusClass()">Update Status</h1>     <!-- ... rest of the template ... -->   `, }) export class StatusDialogSignalsComponent {   // Create a writable signal for the status   status = signal(this.data.status);    // Create a computed signal for the class   // This will automatically re-evaluate when `status` changes   statusClass = computed(() => {     const currentStatus = this.status();     if (currentStatus === 'approved') return 'text-success';     if (currentStatus === 'rejected') return 'text-danger';     return '';   });    constructor(@Inject(MAT_DIALOG_DATA) public data: { status: string }) {}    updateStatus(newStatus: string): void {     // Simply update the signal's value. That's it!     this.status.set(newStatus);   } } 

Notice the template binding: [ngClass]="statusClass()". You call the computed signal like a function to get its value. When you call this.status.set(), the signal notifies its dependencies—in this case, the statusClass computed signal—which in turn notifies the template to update. It's fast, efficient, and requires zero manual change detection.

Pros:

• The most modern and future-proof approach.
• Extremely performant due to fine-grained reactivity.
• Simplest and cleanest syntax of all three methods.
• Glitch-free by design, avoiding intermediate states.

Cons:

• Requires your project to be on Angular v16+ (and ideally v17+ for stable APIs).

Comparison: Which Method Should You Choose?

Let's put all three side-by-side to help you decide.

Conclusion: Syncing Up for Good

The mystery of the unsynced [ngClass] in dialogs has haunted Angular developers for years, but the path forward has never been clearer. While manually triggering change detection with ChangeDetectorRef still works in a pinch, it's a relic of an older, more imperative style of coding.

The RxJS and async pipe approach offers a robust, declarative solution that fits perfectly into a reactive architecture. However, for any new project started today or updated for 2025, Angular Signals are the definitive answer. They offer the best performance, the cleanest syntax, and the most intuitive developer experience, solving this classic problem with effortless elegance.

So next time your dynamic component refuses to update, don't reach for detectChanges(). Instead, embrace the reactive power of Signals and write code that is not only bug-free but also a joy to maintain.

Happy coding, and may your classes always sync!