"Losing focus: The challenge of maintaining focus on dynamic input fields in Angular 2

Question

"Losing focus: The challenge of maintaining focus on dynamic input fields in Angular 2

I am currently designing a dynamic form where each Field contains a list of values, with each value represented as a string.

export class Field {
    name: string;
    values: string[] = [];
    fieldType: string;
    constructor(fieldType: string) { this.fieldType = fieldType; }
}

Within my component, I have implemented a function that allows users to add new values to the field.

addValue(field){
    field.values.push("");
}

In the HTML template, the values and corresponding button are displayed in the following format:

<div id="dropdown-values" *ngFor="let value of field.values; let j=index">
    <input type="text" class="form-control" [(ngModel)]="field.values[j]" [name]="'value' + j + '.' + i"/><br/>
</div>
<div class="text-center">
    <a href="javascript:void(0);" (click)="addValue(field)"><i class="fa fa-plus-circle" aria-hidden="true"></i></a>
</div>

However, I encountered an issue where entering text into one input field within a value causes it to lose focus. Additionally, if multiple values are added to a field and a character is typed into one input, the focus is lost and the character appears in every input field.

javascript angular angular2-forms ngfor angular2-ngmodel

Answer 1

Answer №1

If you encounter this issue, it may be because the array is a primitive type, specifically a String array in your case. One way to resolve this is by implementing TrackBy. Adjust your template to match the following:

<div *ngFor="let value of field.values; let i=index; trackBy:trackByFn">
    <input type="text" [(ngModel)]="field.values[i]"  /><br/>
</div>
<div>
    <button (click)="addValue(field)">Click</button>
</div>

Add the function trackByFn to your TypeScript file, which should return the unique index of the value:

trackByFn(index: any, item: any) {
   return index;
}

You can find more information about a similar issue in AngularJS at this link, although the problem discussed there aligns with yours. A key point mentioned on that page is:

When you iterate over an array and modify its items (which are strings, primitives in JS compared "by value"), changes will trigger DOM updates where old elements are removed and new ones are created, causing focus loss in input fields.

By using TrackBy, Angular can monitor added or removed items based on a unique identifier, updating only what has changed and maintaining focus on input fields :)

In addition to utilizing TrackBy, you could also consider modifying your array to store objects with unique identifiers and use something like [(ngModel)]="value.id", but that may not suit your specific requirements.

Answer 2

If you encounter this issue, it may be because the array is a primitive type, specifically a String array in your case. One way to resolve this is by implementing TrackBy. Adjust your template to match the following:

<div *ngFor="let value of field.values; let i=index; trackBy:trackByFn">
    <input type="text" [(ngModel)]="field.values[i]"  /><br/>
</div>
<div>
    <button (click)="addValue(field)">Click</button>
</div>

Add the function trackByFn to your TypeScript file, which should return the unique index of the value:

trackByFn(index: any, item: any) {
   return index;
}

You can find more information about a similar issue in AngularJS at this link, although the problem discussed there aligns with yours. A key point mentioned on that page is:

When you iterate over an array and modify its items (which are strings, primitives in JS compared "by value"), changes will trigger DOM updates where old elements are removed and new ones are created, causing focus loss in input fields.

By using TrackBy, Angular can monitor added or removed items based on a unique identifier, updating only what has changed and maintaining focus on input fields :)

In addition to utilizing TrackBy, you could also consider modifying your array to store objects with unique identifiers and use something like [(ngModel)]="value.id", but that may not suit your specific requirements.

Answer 3

Answer №2

While iterating over the keys and values of an object using a helper function, I encountered a specific issue:

<div *ngFor="let item of getItems()" [attr.itemname]="item.key">
  ... {{ applyItem(item.value) }}
</div>

In my component, I was returning an array of objects with key/value pairs:

export ItemComponent {
  ...

  //this.items = { x: { ... }, y: { ... }, z: { ... } }

  public getItems() {
    return Object.keys(this.items).map((key) => {
      return {key: key, value: this.items[key] }
    })
  }
}

The solution provided by @AJT_82 does work, but in my case, the issue was that the getItems() function was generating a new list of objects each time it was called. Even though the content remained the same, the objects were being recreated on every call, causing them to have different identities in the eyes of the change detector. This resulted in the form being regenerated upon each model change.

To solve this problem, I decided to cache the result of getItems() and use that cached data for iteration:

<div *ngFor="let item of cachedItems" [attr.itemname]="item.key">
  ... {{ applyItem(item.value) }}
</div>

...

export ItemComponent {
  public cachedItems = getItems()
  ...

  private getItems() {
    return Object.keys(this.items).map((key) => {
      return {key: key, value: this.items[key] }
    })
  }
}

If there is a scenario where cachedItems needs to be updated dynamically, it must be done manually to trigger re-rendering by the change detector.

Answer 4

While iterating over the keys and values of an object using a helper function, I encountered a specific issue:

<div *ngFor="let item of getItems()" [attr.itemname]="item.key">
  ... {{ applyItem(item.value) }}
</div>

In my component, I was returning an array of objects with key/value pairs:

export ItemComponent {
  ...

  //this.items = { x: { ... }, y: { ... }, z: { ... } }

  public getItems() {
    return Object.keys(this.items).map((key) => {
      return {key: key, value: this.items[key] }
    })
  }
}

The solution provided by @AJT_82 does work, but in my case, the issue was that the getItems() function was generating a new list of objects each time it was called. Even though the content remained the same, the objects were being recreated on every call, causing them to have different identities in the eyes of the change detector. This resulted in the form being regenerated upon each model change.

To solve this problem, I decided to cache the result of getItems() and use that cached data for iteration:

<div *ngFor="let item of cachedItems" [attr.itemname]="item.key">
  ... {{ applyItem(item.value) }}
</div>

...

export ItemComponent {
  public cachedItems = getItems()
  ...

  private getItems() {
    return Object.keys(this.items).map((key) => {
      return {key: key, value: this.items[key] }
    })
  }
}

If there is a scenario where cachedItems needs to be updated dynamically, it must be done manually to trigger re-rendering by the change detector.

Answer 5

Answer №3

Whenever you enter some text, the angular ngOnChange listener will trigger, causing the ngFor indexes to be rerendered and ultimately resulting in a loss of focus. To prevent this issue, a trackFn can be included in the ngFor. For more information on this topic, refer to https://angular.io/api/core/TrackByFunction. Below is the code solution for addressing this problem:

In your HTML code, define a trackFn function for the ngFor that corresponds to the trackByFn function defined in the .ts file.

    <div *ngFor="let value of field.values; let i=index; trackBy:trackByFn">
    <input type="text" [(ngModel)]="field.values[i]"  /><br/>
</div>
<div>
    <button (click)="addValue(field)">Click</button>
</div>

In the .ts file, declare the trackByFn function mentioned above:

trackByFn(i: number, items: any) {
return index //returning the index itself for avoiding ngFor to change focus after ngModelChange
}

Answer 6

Whenever you enter some text, the angular ngOnChange listener will trigger, causing the ngFor indexes to be rerendered and ultimately resulting in a loss of focus. To prevent this issue, a trackFn can be included in the ngFor. For more information on this topic, refer to https://angular.io/api/core/TrackByFunction. Below is the code solution for addressing this problem:

In your HTML code, define a trackFn function for the ngFor that corresponds to the trackByFn function defined in the .ts file.

    <div *ngFor="let value of field.values; let i=index; trackBy:trackByFn">
    <input type="text" [(ngModel)]="field.values[i]"  /><br/>
</div>
<div>
    <button (click)="addValue(field)">Click</button>
</div>

In the .ts file, declare the trackByFn function mentioned above:

trackByFn(i: number, items: any) {
return index //returning the index itself for avoiding ngFor to change focus after ngModelChange
}

"Losing focus: The challenge of maintaining focus on dynamic input fields in Angular 2

Answer №1

Answer №2

Answer №3

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