Unable to transfer information from the Parent component to the Child component

Question

Unable to transfer information from the Parent component to the Child component

Can you help me solve this strange issue?

I am experiencing a problem where I am passing data from a parent component to a child component using a service method that returns data as Observable<DemoModel>. The issue is that when the child component is loading, the data is undefined and only fills up after ngAfterViewInit. I have also attempted to retrieve the data within this method, but it remains undefined. I tried implementing an approach using ngOnChanges, but it seems that the data received from the Parent Component is not ready while the Child Component is loading. I also attempted to use async instead of subscribe. How can I ensure that the data is available when the child component is loading?

The code for the Parent and Child Components is provided below:

Parent Component

<child-component
    [courses]="courses|async" 
>
</child-component>

courses: any;

this.service.listCourses().subscribe((course: Course) => {
  this.courses = course;
});

Child Component

private courses: any;

@Input()
set data(data: any) {
    this.courses.next(data);
}

myControl = new FormControl('');

ngAfterViewInit() {

  // >>> THIS THROWS ERROR AS this.courses is undefined
  this.myControl.setValidators([
    Validators.required,
    forbiddenNamesValidator(this.courses)
  ]);
}

I have also attempted to use *ngIf in HTML, but since the this.courses parameter is used in the methods, it doesn't make sense to check the data in HTML.

The issue could be related to the subscribe method. I have also attempted to use promise, but I'm uncertain if I implemented it correctly.

javascript angular typescript

Answer 1

Answer №1

I have identified a couple of issues with your current setup:

In the Parent component, it seems that `courses` is being treated as an array rather than an observable, so there is no need to use the `async` pipe.
Within the Child component, you have named the input field `data`, and used a setter to call `.next` on a variable that should be an array - this will not work as intended.

The following modifications should rectify the issues in your implementation:

Parent Component

<child-component
    [courses]="courses" 
>
</child-component>

courses: any;

this.service.listCourses().subscribe((course: Course) => {
  this.courses = course;
});

Child Component

@Input() courses: any;

Note that `listCourses` is asynchronous

This means that the value of `courses` may not be available when `ngAfterViewInit` is called, potentially resulting in an error. To address this, consider the following approach:

<child-component
    *ngIf="courses?.length"
    [courses]="courses" 
>
</child-component>

By implementing this, you can avoid waiting for `ngAfterViewInit` and instead rely on `ngOnInit`.

ngOnInit(): void {
    this.myControl.setValidators([
        Validators.required,
        forbiddenNamesValidator(this.courses)
    ]);
}

Additional Comments

When passing a list from parent to child, which approach should I take: observable, promise, array, etc.?

The choice between observable, promise, or array depends on your specific requirements. Personally, I find using the `async` pipe with observables convenient as it simplifies subscription management.

<child-component
    [courses]="courses | async" 
>
</child-component>

courses = this.service.listCourses()

Given that the list in the Child Component is static, do I still need to utilize get/set methods for `courses`?

No, you do not necessarily require get/set methods if the data remains constant. Angular automatically updates the @Input data, eliminating the need for explicit getter and setter functions unless strictly necessary.

Should I set the validators and filtering method in `onInit` or `afterViewInit`?

You can define the validator without waiting for View initialization in `afterViewInit`. There is no requirement to delay setting the validator until after the Component's View has initialized.

Answer 2

I have identified a couple of issues with your current setup:

In the Parent component, it seems that `courses` is being treated as an array rather than an observable, so there is no need to use the `async` pipe.
Within the Child component, you have named the input field `data`, and used a setter to call `.next` on a variable that should be an array - this will not work as intended.

The following modifications should rectify the issues in your implementation:

Parent Component

<child-component
    [courses]="courses" 
>
</child-component>

courses: any;

this.service.listCourses().subscribe((course: Course) => {
  this.courses = course;
});

Child Component

@Input() courses: any;

Note that `listCourses` is asynchronous

This means that the value of `courses` may not be available when `ngAfterViewInit` is called, potentially resulting in an error. To address this, consider the following approach:

<child-component
    *ngIf="courses?.length"
    [courses]="courses" 
>
</child-component>

By implementing this, you can avoid waiting for `ngAfterViewInit` and instead rely on `ngOnInit`.

ngOnInit(): void {
    this.myControl.setValidators([
        Validators.required,
        forbiddenNamesValidator(this.courses)
    ]);
}

Additional Comments

When passing a list from parent to child, which approach should I take: observable, promise, array, etc.?

The choice between observable, promise, or array depends on your specific requirements. Personally, I find using the `async` pipe with observables convenient as it simplifies subscription management.

<child-component
    [courses]="courses | async" 
>
</child-component>

courses = this.service.listCourses()

Given that the list in the Child Component is static, do I still need to utilize get/set methods for `courses`?

No, you do not necessarily require get/set methods if the data remains constant. Angular automatically updates the @Input data, eliminating the need for explicit getter and setter functions unless strictly necessary.

Should I set the validators and filtering method in `onInit` or `afterViewInit`?

You can define the validator without waiting for View initialization in `afterViewInit`. There is no requirement to delay setting the validator until after the Component's View has initialized.

Answer 3

Answer №2

One way to handle data: Use the ngIf directive to check for the presence of data.

<child-component [courses]="courses" *ngIf="courses.length > 0"> </child-component>

A second approach: If you prefer using async, avoid subscribing in your component.

<child-component [courses]="courses$ | async" *ngIf="(courses$| async)?.length > 0"> </child-component>

Component setup:

courses$: Observable<any>;

this.courses$ = this.service.listCourses().pipe(shareReplay());

Answer 4

One way to handle data: Use the ngIf directive to check for the presence of data.

<child-component [courses]="courses" *ngIf="courses.length > 0"> </child-component>

A second approach: If you prefer using async, avoid subscribing in your component.

<child-component [courses]="courses$ | async" *ngIf="(courses$| async)?.length > 0"> </child-component>

Component setup:

courses$: Observable<any>;

this.courses$ = this.service.listCourses().pipe(shareReplay());

Unable to transfer information from the Parent component to the Child component

Answer №1

I have identified a couple of issues with your current setup:

The following modifications should rectify the issues in your implementation:

Note that `listCourses` is asynchronous

Additional Comments

Answer №2

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