Angular change detection is one of the core mechanisms that ensure the application stays reactive and updates the user interface whenever there’s a change in the underlying data model. Angular uses a combination of data binding and directives to implement change detection effectively, allowing developers to build responsive applications.
At a high level, the change detection mechanism goes through two phases - the first one is where Angular checks if there are any changes in the binding sources (i.e., component inputs and properties), and the second one is where it updates the DOM with the changed values.
Let’s dive deeper into these two phases and see how Angular’s change detection impacts application performance.
1. **Checking for changes**: Angular uses a concept called *zones* to track asynchronous events in the application (such as clicks, XHRs, and timeouts). Whenever an asynchronous event occurs, Angular performs change detection on the component tree from the root component down (like a depth-first search). This process is called **digest**. The digest cycle detects changes in the application state and triggers the appropriate event handlers.
Angular tracks changes in properties using a mechanism called **dirty checking**. It compares the previous and the current values of the property to determine if a change has occurred. When Angular detects a change in a property, it triggers the change detection process, updating the component’s view and propagating changes down the component tree.
2. **Updating the DOM**: After Angular detects that a change has occurred in the model, it updates the DOM to reflect the new values. This process can involve updating text nodes, classes, styles, or attributes, and it utilizes the power of Angular’s built-in directives (e.g., *ngIf, *ngFor, and ngClass) to efficiently update the DOM.
**Impact on Performance**
Change detection is crucial for Angular applications to work, but it can also inadvertently cause performance bottlenecks if not optimized. Here are a few ways change detection can impact performance:
1. **Frequent updates**: If asynchronous events are happening very frequently, change detection is triggered repeatedly, potentially causing a slower user interface as Angular tries to detect and propagate changes.
2. **Large component trees**: If your application has many components or a deep nesting of components, the change detection process can take longer, as Angular traverses the component tree and performs dirty checking.
3. **Complex expressions**: Angular evaluates expressions in templates during each digest cycle. Complex expressions can result in significant performance overhead, especially when they involve intensive calculations or function calls.
**Optimizations**
To alleviate performance issues associated with Angular’s change detection mechanism, consider these optimizational strategies:
1. **Use OnPush change detection**: Angular provides two change detection strategies: ‘Default‘ and ‘OnPush‘. The latter strategy only updates a component when its ‘@Input()‘ properties change, significantly reducing the number of times change detection is run. To use the OnPush strategy, update the changeDetection property in your component metadata:
import { Component, ChangeDetectionStrategy } from '@angular/core';
@Component({
selector: 'my-component',
templateUrl: './my-component.component.html',
changeDetection: ChangeDetectionStrategy.OnPush
})
export class MyComponent { }2. **Minimize complex expressions in templates**: Simplify complex expressions or expensive computations in your template by moving them to a separate method or, better yet, to a pure pipe.
3. **Use trackBy with *ngFor**: When using *ngFor to loop over a list of items, provide a ‘trackBy‘ function to tell Angular how to uniquely identify each item in the list. This optimizes DOM updates and only updates those items that have actually changed.
<li *ngFor="let item of items; trackBy: trackById">
{{ item.name }}
</li>trackById(index: number, item: any): number {
return item.id;
}By understanding the inner workings of Angular’s change detection mechanism and following best practices for optimizing change detection, you can ensure your application maintains excellent performance and provides the best possible user experience.