Using a method call instead of a property for the ngFor directive can lead to an infinite loop of loading

Question

Using a method call instead of a property for the ngFor directive can lead to an infinite loop of loading

Within my template, I have included the following code:

<a *ngFor="let item of navItems"
   [routerLink]="item.link"
   routerLinkActive="active"
   class="navigation-item"
   [ngClass]="{'enabled': item.enabled}"
>
  <span class="color-{{item.color}}">
    <mat-icon>{{item.icon}}</mat-icon>
  </span>
  <div class="label">{{item.label}}</div>
</a>

The array navItems is defined within my component as follows:

navItems: NavItem[] = [
  {
    link: 'link1', label: 'Label 1', icon: 'thumb_up', color: 'green', enabled: true
  },
  {
    link: 'link2', label: 'Label 2', icon: 'thumb_down', color: 'red', enabled: true
  },
  {
    link: 'link3', label: 'Label 3', icon: 'insert_drive_file', color: 'blue', enabled: true
  },
  {
    link: 'link4', label: 'Label 4', icon: 'note_add', color: 'blue', enabled: true
  },
];

Although this setup works properly, I now require the ability for the contents of navItems to dynamically change. I attempted converting the navItems property into a getter like so:

get navItems(): NavItem[] {
  return [
    {
      link: 'link1', label: 'Label 1', icon: 'thumb_up', color: 'green', enabled: true
    },
    {
      link: 'link2', label: 'Label 2', icon: 'thumb_down', color: 'red', enabled: true
    },
    {
      link: 'link3', label: 'Label 3', icon: 'insert_drive_file', color: 'blue', enabled: true
    },
    {
      link: 'link4', label: 'Label 4', icon: 'note_add', color: 'blue', enabled: true
    },
  ];
}

Unfortunately, when attempting this modification, the browser tab enters an infinite loading loop upon loading the component, requiring termination via the task manager - no console output is generated.

I also experimented with using a regular method call instead of a getter to supply the array, but faced the same issue.

The array being returned consists solely of plain objects with directly assigned string and boolean values, ruling out any nested calls that could trigger recursion.

Is there an error in my approach? Can method calls / getters not be used to provide data for the ngFor directive?

angular typescript angular2-directives ngfor

Answer 1

Answer №1

Upon utilizing the trackBy function, the issue was successfully resolved.

Within the HTML file:

<a *ngFor="let item of navItems; trackBy: trackByFn" [routerLink]="item.link" routerLinkActive="active"
class="navigation-item"
[ngClass]="{'enabled': item.enabled}"
>
<span class="color-{{item.color}}">
 <mat-icon>{{item.icon}}</mat-icon>
</span>
<div class="label">{{item.label}}</div>
</a>

Within the TS file:

export class AppComponent {
   trackByFn(index,item){
     console.log(index,"n:",item);
     return index;
   }
  get navItems(): NavItem[] {
    return [
      {
        link: 'link1', label: 'Label 1', icon: 'thumb_up', color: 'green', enabled: true
      },
      {
        link: 'link2', label: 'Label 2', icon: 'thumb_down', color: 'red', enabled: true
      },
      {
        link: 'link3', label: 'Label 3', icon: 'insert_drive_file', color: 'blue', enabled: true
      },
      {
        link: 'link4', label: 'Label 4', icon: 'note_add', color: 'blue', enabled: true
      },
    ];
  }
}
class NavItem {
  link;
   label;
    icon;
     color;
     enabled;
}

Note:

Case-1 : The trackBy function works fine without the routerLinkActive directive.

Case-2 : The trackBy function works fine with the routerLinkActive directive.

Case-3 : With routerLinkActive but without trackBy, it results in a loop.

The continuous loops are caused by the update() method within the RouterLinkActive directive, which executes hooks like AfterContentInit. Commenting out update() or using trackBy resolves this issue.

Each time the navItems method is called, the number of elements created corresponds to the items in the array. Directives apply hooks such as AfterContentInit and OnDestroy to manipulate elements. Without trackBy, elements are constantly recreated, resulting in a loop.

In summary, without trackBy, each method return creates new elements which are then manipulated by directives. This repetitive process leads to an infinite loop.

For further information, refer to:

Additionally, consider creating a CustomRouterLinkActive for debugging purposes using the source from https://github.com/angular/angular

Answer 2

Upon utilizing the trackBy function, the issue was successfully resolved.

Within the HTML file:

<a *ngFor="let item of navItems; trackBy: trackByFn" [routerLink]="item.link" routerLinkActive="active"
class="navigation-item"
[ngClass]="{'enabled': item.enabled}"
>
<span class="color-{{item.color}}">
 <mat-icon>{{item.icon}}</mat-icon>
</span>
<div class="label">{{item.label}}</div>
</a>

Within the TS file:

export class AppComponent {
   trackByFn(index,item){
     console.log(index,"n:",item);
     return index;
   }
  get navItems(): NavItem[] {
    return [
      {
        link: 'link1', label: 'Label 1', icon: 'thumb_up', color: 'green', enabled: true
      },
      {
        link: 'link2', label: 'Label 2', icon: 'thumb_down', color: 'red', enabled: true
      },
      {
        link: 'link3', label: 'Label 3', icon: 'insert_drive_file', color: 'blue', enabled: true
      },
      {
        link: 'link4', label: 'Label 4', icon: 'note_add', color: 'blue', enabled: true
      },
    ];
  }
}
class NavItem {
  link;
   label;
    icon;
     color;
     enabled;
}

Note:

Case-1 : The trackBy function works fine without the routerLinkActive directive.

Case-2 : The trackBy function works fine with the routerLinkActive directive.

Case-3 : With routerLinkActive but without trackBy, it results in a loop.

The continuous loops are caused by the update() method within the RouterLinkActive directive, which executes hooks like AfterContentInit. Commenting out update() or using trackBy resolves this issue.

Each time the navItems method is called, the number of elements created corresponds to the items in the array. Directives apply hooks such as AfterContentInit and OnDestroy to manipulate elements. Without trackBy, elements are constantly recreated, resulting in a loop.

In summary, without trackBy, each method return creates new elements which are then manipulated by directives. This repetitive process leads to an infinite loop.

For further information, refer to:

Additionally, consider creating a CustomRouterLinkActive for debugging purposes using the source from https://github.com/angular/angular

Answer 3

Answer №2

It appears that the issue at hand arises from having the array stored in an instance variable, causing it to remain constant. Each time Angular checks for changes, it sees the same array and doesn't take further action.

However, by converting the instance variable into a getter method, a new array is generated each time navItems is accessed. Consequently, Angular detects a different value during change detection, even if the items themselves are conceptually identical. In essence:

// Previously:
const a = comp.navItems;
const b = comp.navItems;
console.log(a === b); // true. Both point to the same object.

But now:

const a = comp.navItems; // assigning a to the result of a function call!
const b = comp.navItems: // assigning b to a NEW function call's return
console.log(a === b); // false. Two separate arrays are created here. Two distinct objects.

To resolve this issue, consider implementing trackByFunction as recommended earlier. This approach ensures that only new or modified items trigger a re-render. Avoid returning a new array every time unless the data has genuinely changed. If you prefer dynamically constructing the array within the getter, think about utilizing ChangeDetectionStrategy.OnPush in your component. Then, utilize ChangeDetectorRef.markForCheck() to signal that component data has been updated and requires checking during change detection. Neglecting this step will prevent Angular from re-rendering the component unless any Input properties alter.

Answer 4

It appears that the issue at hand arises from having the array stored in an instance variable, causing it to remain constant. Each time Angular checks for changes, it sees the same array and doesn't take further action.

However, by converting the instance variable into a getter method, a new array is generated each time navItems is accessed. Consequently, Angular detects a different value during change detection, even if the items themselves are conceptually identical. In essence:

// Previously:
const a = comp.navItems;
const b = comp.navItems;
console.log(a === b); // true. Both point to the same object.

But now:

const a = comp.navItems; // assigning a to the result of a function call!
const b = comp.navItems: // assigning b to a NEW function call's return
console.log(a === b); // false. Two separate arrays are created here. Two distinct objects.

To resolve this issue, consider implementing trackByFunction as recommended earlier. This approach ensures that only new or modified items trigger a re-render. Avoid returning a new array every time unless the data has genuinely changed. If you prefer dynamically constructing the array within the getter, think about utilizing ChangeDetectionStrategy.OnPush in your component. Then, utilize ChangeDetectorRef.markForCheck() to signal that component data has been updated and requires checking during change detection. Neglecting this step will prevent Angular from re-rendering the component unless any Input properties alter.

Using a method call instead of a property for the ngFor directive can lead to an infinite loop of loading

Answer №1

Answer №2

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