Executing a single method during the ngAfterViewChecked lifecycle hook in Angular

Question

Executing a single method during the ngAfterViewChecked lifecycle hook in Angular

When tracking a change of variable and wanting to run a function after the change, I utilized the ngAfterViewChecked hook. However, since this method involves data submission which should only occur once, I incorporated a boolean status variable to manage it like so:

    ngAfterViewChecked() 
    {
        //status is a boolean variable which is true
        if (this.status)
        {
           this.status=!this.status;
           this.myFunction();
        }
    }
    myFunction
    {
        //some code
    }

Although, I noticed multiple data submissions and encountered the following error:

ERROR Error: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value for '@dialogContainer': 'enter'. Current value: 'exit'.

I suspect that NgAfterViewChecked is triggered for every change, leading to the issue where if (this.status){} might be executed before the status changes to false.

Is there a solution to resolve this problem?

angular typescript lifecycle

Answer 1

Answer №1

After spending several hours searching, I successfully found a solution to overcome my issue. Let me explain how I managed to do it.

Angular operates in a browser environment where JavaScript is executed. Although JavaScript runs on a single thread, its runtime does not operate on a single thread, so we don't need to worry about it as the Runtime manages this.
A Subject in Angular is an observable that can emit values to multiple observers listening to it.
A Subscription in Angular can store an observable for later use.

To begin, I created a Subject named paymentListener, an integer variable called counter, and a Subscription named oneTimePayment.

oneTimePayment: Subscription;
private paymentListener = new Subject<number>();
counter = 0;

Within the onInit method, I established a subscription.

ngOnInit() {
    
    this.oneTimePayment = this.paymentListener.subscribe((counter) => {
          if (counter == 1) {
            // Call my function here
            this.myFunction();
          }
        });
    
    }

In the ngAfterViewChecked lifecycle hook, I utilized the paymentListener (a Subject) to emit the counter value.

ngAfterViewChecked() {
    if (this.counter == 0) {
      this.counter++;
      if (this.counter == 1) {
        this.paymentListener.next(this.counter);
      }
    }
  }

The value is only emitted by the paymentListener (Subject) when the counter equals 1. Therefore, the subscription made in onInit will receive the emitted value, triggering the execution of myFunction only when the emitted value is 1. This ensures the method runs once after a change.

Lastly, remember to call unsubscribe inside ngOnDestroy to prevent memory leaks.

ngOnDestroy() {
    this.oneTimePayment.unsubscribe();
  }

Answer 2

After spending several hours searching, I successfully found a solution to overcome my issue. Let me explain how I managed to do it.

Angular operates in a browser environment where JavaScript is executed. Although JavaScript runs on a single thread, its runtime does not operate on a single thread, so we don't need to worry about it as the Runtime manages this.
A Subject in Angular is an observable that can emit values to multiple observers listening to it.
A Subscription in Angular can store an observable for later use.

To begin, I created a Subject named paymentListener, an integer variable called counter, and a Subscription named oneTimePayment.

oneTimePayment: Subscription;
private paymentListener = new Subject<number>();
counter = 0;

Within the onInit method, I established a subscription.

ngOnInit() {
    
    this.oneTimePayment = this.paymentListener.subscribe((counter) => {
          if (counter == 1) {
            // Call my function here
            this.myFunction();
          }
        });
    
    }

In the ngAfterViewChecked lifecycle hook, I utilized the paymentListener (a Subject) to emit the counter value.

ngAfterViewChecked() {
    if (this.counter == 0) {
      this.counter++;
      if (this.counter == 1) {
        this.paymentListener.next(this.counter);
      }
    }
  }

The value is only emitted by the paymentListener (Subject) when the counter equals 1. Therefore, the subscription made in onInit will receive the emitted value, triggering the execution of myFunction only when the emitted value is 1. This ensures the method runs once after a change.

Lastly, remember to call unsubscribe inside ngOnDestroy to prevent memory leaks.

ngOnDestroy() {
    this.oneTimePayment.unsubscribe();
  }

Executing a single method during the ngAfterViewChecked lifecycle hook in Angular

Answer №1

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