What is change detection in Angular?
Change detection is a mechanism in Angular that ensures the view is updated with the latest data changes made to a component’s properties. Whenever a component’s property changes, Angular runs a change detection cycle to check for any changes in the component’s data and update the view accordingly.
Here is an example of change detection in Angular:
In this example, we have a simple Angular component that displays a title, a count, and a button to increment the count. Whenever the button is clicked, the ‘increment()’ method is called, which increments the ‘count’ property of the component.
Angular’s change detection mechanism will automatically detect this change to the ‘count’ property and update the view to display the new count value. This means that every time the button is clicked, the view will be updated with the latest count value without the need for any additional code to be written.
There are two types of change detection strategies in Angular:
1. Default Change Detection Strategy:
Angular uses the default change detection strategy, which is also called the “CheckOnce” strategy. In this strategy, Angular checks the component tree starting from the root component and traverses down to the leaf components. During this traversal, Angular checks the data bindings and updates the view if there are any changes. After the initial change detection cycle, the view is not updated unless there are changes to the component’s data. This strategy is used in most cases.
2. OnPush Change Detection Strategy:
The OnPush change detection strategy is a performance optimization strategy. In this strategy, Angular checks the data bindings and updates the view only if the component’s input properties or any of its child components’ input properties have changed. This strategy can be enabled by setting the ‘changeDetection’ property of a component to ‘ChangeDetectionStrategy.OnPush’. The OnPush strategy can significantly improve performance in applications with a large number of components or complex data structures. However, it requires careful use, as it can cause unexpected behavior if not used correctly.
To summarize, the default change detection strategy is used by default in Angular and checks for changes in all the components in the component tree. The OnPush change detection strategy, on the other hand, is used for performance optimization and updates the view only if the component’s input properties or any of its child components’ input properties have changed.
How to run change detection in Angular?
In Angular, change detection is automatically triggered when an event is fired or when an asynchronous operation completes. However, there may be times when you want to manually trigger change detection. You can do this by using the ‘ChangeDetectorRef’ service provided by Angular.
The ‘ChangeDetectorRef’ service provides a ‘detectChanges()’ method that triggers the change detection for the component and its child components. Here’s an example of how to use it:
In this example, we inject the ‘ChangeDetectorRef’ service into the component’s constructor. When the ‘increment()’ method is called, we increment the ‘count’ property and then call the ‘detectChanges()’ method on the ‘ChangeDetectorRef’ service. This triggers a change detection cycle for the component and its child components, which updates the view to display the new count value.
Note that manually triggering change detection should be used sparingly as it can impact the performance of your application. It’s usually better to rely on Angular’s automatic change detection mechanism whenever possible.
How to detect value change in Angular?
In Angular, you can detect changes in a component’s property values by implementing the ‘OnChanges’ interface and its ‘ngOnChanges()’ method. The ‘ngOnChanges()’ method is called whenever a data-bound input property of a component changes.
Here’s an example of how to detect changes in a component’s input property:
In this example, we have a child component that has an input property called value. We implement the ‘OnChanges’ interface and its ‘ngOnChanges()’ method to detect changes in the ‘value’ property.
In the ‘ngOnChanges()’ method, we check if the ‘value’ property has changed by looking for it in the ‘changes’ object. If it has changed, we update the ‘prevValue’ and ‘currentValue’ properties to display the previous and current values of the ‘value’ property in the view.
Note that the ‘ngOnChanges()’ method is called only when an input property of a component changes. If you want to detect changes in a component’s internal properties, you can use the ‘ngDoCheck()’ method or a change detection strategy.
