Updating a component's value in Angular 6 when there is a change in the corresponding service

Question

Updating a component's value in Angular 6 when there is a change in the corresponding service

My objective sounds straightforward, but I am struggling to implement it: I want one of my components to automatically update when a variable in a service changes.

To illustrate my issue, consider the following example:

Imagine having a service that increases or decreases a points counter. The counter value is modified by calling specific functions within the service. Additionally, the service determines whether the counter value is even or odd.

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

@Injectable({
  providedIn: 'root'
})
export class TestService {
  Number: number = 0;
  IsOdd: boolean = false;

  constructor() {}

  IncreaseNumber() {
    this.Number = this.Number + 1;
    this.IsOdd = !this.IsOdd;
  }

  DecreaseNumber() {
    this.Number = this.Number - 1;
    this.IsOdd = !this.IsOdd;
  }
}

*Now, let's consider the component which requires knowledge about whether the figure is even or odd.

Initially, everything works fine! It correctly determines the parity!

But how can I ensure that every time the number in my service (test.service.ts) changes, the even/odd value also updates in my component (test.component.ts)?*

import { Component, OnInit } from '@angular/core';
import { TestService } from '../test.service'

@Component({
  selector: 'app-test',
  templateUrl: './test.component.html',
  styleUrls: ['./test.component.scss']
})

export class TestComponent implements OnInit {

  IsOdd: boolean = false;

  constructor(MyService: TestService) {}

  ngOnInit() {
    this.IsOdd = MyService.IsOdd;
  }

}

What approach should I take?

Should my component subscribe to the service somehow? Or is there a function similar to ngOnInit that I should use for updates?

Thank you in advance

angular typescript service components

Answer 1

Answer №1

To ensure automatic updates for service variables, they should be of a complex type like an Object or an Array which are reference types. Primitive types like number and boolean will not be updated automatically as they are passed by value.

Using BehaviorSubjects and exposing them as observables is the way to handle this. By calling the next method on the BehaviorSubjects, you can update their values effectively.

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

@Injectable({
 providedIn: 'root'
})
export class TestService {
 private myNumberValue = 0;
 private isOddValue = false;
 private myNumber: BehaviorSubject<number> = new BehaviorSubject<number>(this.myNumberValue);
 private isOdd: BehaviorSubject<boolean> = new BehaviorSubject<boolean>(false);

 myNumber$: Observable<number> = this.myNumber.asObservable();
 isOdd$:  Observable<boolean> = this.isOdd.asObservable();

 constructor() {}

 increaseNumber() {
 this.myNumberValue = this.myNumberValue + 1;
 this.myNumber.next(this.myNumberValue);
 this.isOddValue = !this.isOddValue;
 this.isOdd.next(this.isOddValue);
 }

 decreaseNumber() {
 this.myNumberValue = this.myNumberValue - 1;
 this.myNumber.next(this.myNumberValue);
 this.isOddValue = !this.isOddValue;
 this.isOdd.next(this.isOddValue);
 }
}

In your Component, simply subscribe to the publicly exposed Observables from the Service:

import { Component, OnInit, OnDestroy } from '@angular/core';
import { TestService } from '../test.service'
import { Subscription } from 'rxjs';

@Component({
 selector: 'app-test',
 templateUrl: './test.component.html',
 styleUrls: ['./test.component.scss']
})

export class TestComponent implements OnInit, OnDestroy {

 isOdd: boolean;
 subscription: Subscription;

 constructor(private testService: TestService) {}

 ngOnInit() {
 this.subscription = this.testService.isOdd$.subscribe(isOdd => this.isOdd = isOdd);
}

 ngOnDestroy() {
 this.subscription && this.subscription.unsubscribe();
}

}

By subscribing to isOdd$ in ngOnInit, any changes in isOddValue will reflect in the component's isOdd.

Remember to use lower camel case when naming properties and methods in Angular classes according to Angular's Styleguide.

Do use lower camel case to name properties and methods.

Answer 2

To ensure automatic updates for service variables, they should be of a complex type like an Object or an Array which are reference types. Primitive types like number and boolean will not be updated automatically as they are passed by value.

Using BehaviorSubjects and exposing them as observables is the way to handle this. By calling the next method on the BehaviorSubjects, you can update their values effectively.

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

@Injectable({
 providedIn: 'root'
})
export class TestService {
 private myNumberValue = 0;
 private isOddValue = false;
 private myNumber: BehaviorSubject<number> = new BehaviorSubject<number>(this.myNumberValue);
 private isOdd: BehaviorSubject<boolean> = new BehaviorSubject<boolean>(false);

 myNumber$: Observable<number> = this.myNumber.asObservable();
 isOdd$:  Observable<boolean> = this.isOdd.asObservable();

 constructor() {}

 increaseNumber() {
 this.myNumberValue = this.myNumberValue + 1;
 this.myNumber.next(this.myNumberValue);
 this.isOddValue = !this.isOddValue;
 this.isOdd.next(this.isOddValue);
 }

 decreaseNumber() {
 this.myNumberValue = this.myNumberValue - 1;
 this.myNumber.next(this.myNumberValue);
 this.isOddValue = !this.isOddValue;
 this.isOdd.next(this.isOddValue);
 }
}

In your Component, simply subscribe to the publicly exposed Observables from the Service:

import { Component, OnInit, OnDestroy } from '@angular/core';
import { TestService } from '../test.service'
import { Subscription } from 'rxjs';

@Component({
 selector: 'app-test',
 templateUrl: './test.component.html',
 styleUrls: ['./test.component.scss']
})

export class TestComponent implements OnInit, OnDestroy {

 isOdd: boolean;
 subscription: Subscription;

 constructor(private testService: TestService) {}

 ngOnInit() {
 this.subscription = this.testService.isOdd$.subscribe(isOdd => this.isOdd = isOdd);
}

 ngOnDestroy() {
 this.subscription && this.subscription.unsubscribe();
}

}

By subscribing to isOdd$ in ngOnInit, any changes in isOddValue will reflect in the component's isOdd.

Remember to use lower camel case when naming properties and methods in Angular classes according to Angular's Styleguide.

Do use lower camel case to name properties and methods.

Answer 3

Answer №2

In order for the suggested solution to be effective, it is crucial that you adjust the number within the TestComponent's service specifically instead of altering it in other components.

import { Component, OnInit } from '@angular/core';
import { TestService } from '../test.service'

@Component({
  selector: 'app-test',
  templateUrl: './test.component.html',
  styleUrls: ['./test.component.scss']
})

export class TestComponent implements OnInit {

  IsOdd: boolean = false;

  constructor(MyService: TestService) {}

  ngOnInit() {
    this.IsOdd = MyService.IsOdd;
  }

  increase() {
    MyService.IncreaseNumber();
  }

  decrease() {
    MyService.DecreaseNumber();
  }

  getIsOdd() {
    return MyService.IsOdd;
  }

}

Answer 4

In order for the suggested solution to be effective, it is crucial that you adjust the number within the TestComponent's service specifically instead of altering it in other components.

import { Component, OnInit } from '@angular/core';
import { TestService } from '../test.service'

@Component({
  selector: 'app-test',
  templateUrl: './test.component.html',
  styleUrls: ['./test.component.scss']
})

export class TestComponent implements OnInit {

  IsOdd: boolean = false;

  constructor(MyService: TestService) {}

  ngOnInit() {
    this.IsOdd = MyService.IsOdd;
  }

  increase() {
    MyService.IncreaseNumber();
  }

  decrease() {
    MyService.DecreaseNumber();
  }

  getIsOdd() {
    return MyService.IsOdd;
  }

}

Answer 5

Answer №3

Exploring various solutions is key when working with Observables, such as subscribing to changes. It's important to consider all valid options available.

A straightforward approach is to bind the service in the template instead of the value:

<div> {{ MyService.IsOdd }} </div>

When using ngOnInit, you assign a boolean value to a component property that remains constant. To establish data binding and react to changes, bind a reference to a property using an object requiring a .. Therefore, utilizing MyService.IsOdd will function correctly in the template.

https://stackblitz.com/edit/angular-rktij7

Answer 6

Exploring various solutions is key when working with Observables, such as subscribing to changes. It's important to consider all valid options available.

A straightforward approach is to bind the service in the template instead of the value:

<div> {{ MyService.IsOdd }} </div>

When using ngOnInit, you assign a boolean value to a component property that remains constant. To establish data binding and react to changes, bind a reference to a property using an object requiring a .. Therefore, utilizing MyService.IsOdd will function correctly in the template.

https://stackblitz.com/edit/angular-rktij7

Answer 7

Answer №4

Using Observable is a fantastic method.

Alternatively, a straightforward approach, in my view, is to trigger an event every time the IncreaseNumber or DecreaseNumber functions are invoked within the TestService.

Subsequently, within the constructor of your TestComponent, you can subscribe to the custom event created in the TestService and update the IsOdd field (or any other desired field) in your component. This subscription is feasible because an event serves as a container for an observable.

Here's how: Firstly, establish an event emitter in your service

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

    @Injectable({
      providedIn: 'root'
    })
    export class TestService {
      Number: number = 0;
      IsOdd: boolean = false;
      EmitIsOdd = new EventEmitter<any>();

      constructor() {}

      IncreaseNumber() {
        this.Number = this.Number + 1;
        this.IsOdd = !this.IsOdd;
        this.EmitIsOdd.emit();
      }

      DecreaseNumber() {
        this.Number = this.Number - 1;
        this.IsOdd = !this.IsOdd;
        this.EmitIsOdd.emit();
      }
    }

Then, within the Component constructor, subscribe to the event

    import { Component, OnInit } from '@angular/core';
    import { TestService } from '../test.service'

    @Component({
      selector: 'app-test',
      templateUrl: './test.component.html',
      styleUrls: ['./test.component.scss']
    })

    export class TestComponent implements OnInit {

      IsOdd: boolean = false;

      constructor(MyService: TestService) {
        this.MyService.EmitIsOdd.subscribe(
          () => {
            this.IsOdd = this.MyService.IsOdd;
          }    
        );
      }

      ngOnInit() {
        this.IsOdd = MyService.IsOdd;
      }
    }

That's it – straightforward and efficient.

Answer 8

Using Observable is a fantastic method.

Alternatively, a straightforward approach, in my view, is to trigger an event every time the IncreaseNumber or DecreaseNumber functions are invoked within the TestService.

Subsequently, within the constructor of your TestComponent, you can subscribe to the custom event created in the TestService and update the IsOdd field (or any other desired field) in your component. This subscription is feasible because an event serves as a container for an observable.

Here's how: Firstly, establish an event emitter in your service

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

    @Injectable({
      providedIn: 'root'
    })
    export class TestService {
      Number: number = 0;
      IsOdd: boolean = false;
      EmitIsOdd = new EventEmitter<any>();

      constructor() {}

      IncreaseNumber() {
        this.Number = this.Number + 1;
        this.IsOdd = !this.IsOdd;
        this.EmitIsOdd.emit();
      }

      DecreaseNumber() {
        this.Number = this.Number - 1;
        this.IsOdd = !this.IsOdd;
        this.EmitIsOdd.emit();
      }
    }

Then, within the Component constructor, subscribe to the event

    import { Component, OnInit } from '@angular/core';
    import { TestService } from '../test.service'

    @Component({
      selector: 'app-test',
      templateUrl: './test.component.html',
      styleUrls: ['./test.component.scss']
    })

    export class TestComponent implements OnInit {

      IsOdd: boolean = false;

      constructor(MyService: TestService) {
        this.MyService.EmitIsOdd.subscribe(
          () => {
            this.IsOdd = this.MyService.IsOdd;
          }    
        );
      }

      ngOnInit() {
        this.IsOdd = MyService.IsOdd;
      }
    }

That's it – straightforward and efficient.

Updating a component's value in Angular 6 when there is a change in the corresponding service

Answer №1

Answer №2

Answer №3

Answer №4

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