CODEWITHSUNDEEP
angularangular-ngmodelangular2-forms
Ron Avraham
Ron Avraham

Reputation: 478

Angular 2 ngModelChange old value

Can someone please tell me what is the best practice for comparing ngModel old and new value?

In angular 1:

$scope.$watch('someProperty', funciton(oldVal, newVal){
    // code goes here
})

I am asking this because (ngModelChange) never brings me the oldVal , only the newVal.

In my case, I am using ngModel in a <select> tag and compare the old selection with the new one:

<select [(ngModel)]="current" (ngModelChange)="onModelChange($event)">
     <option *ngFor="let item of myArray" [ngValue]="item">{{item.name}} </option>
</select>

Upvotes: 18

Views: 35274

Answers (3)

Disaster
Disaster

Reputation: 901

Just for the future

we need to observe that [(ngModel)]="hero.name" is just a short-cut that can be de-sugared to: [ngModel]="hero.name" (ngModelChange)="hero.name = $event".

So if we de-sugar code we would end up with:

<select (ngModelChange)="onModelChange()" [ngModel]="hero.name" (ngModelChange)="hero.name = $event">

or

<[ngModel]="hero.name" (ngModelChange)="hero.name = $event" select (ngModelChange)="onModelChange()">

If you inspect the above code you will notice that we end up with 2 ngModelChange events and those need to be executed in some order.

Summing up: If you place ngModelChange before ngModel, you get the $event as the new value, but your model object still holds previous value. If you place it after ngModel, the model will already have the new value.

SOURCE

Sample stack blitz: https://stackblitz.com/edit/angular-ivy-zlhwex

Upvotes: 17

G&#252;nter Z&#246;chbauer
G&#252;nter Z&#246;chbauer

Reputation: 658017

This might work

(ngModelChange)="onModelChange(oldVal, $event); oldVal = $event;"

or

(ngModelChange)="onModelChange($event)"

oldValue:string;
onModelChange(event) {
  if(this.oldValue != event) {
    ...
  }
  this.oldValue = event;
}

Upvotes: 19

AVJT82
AVJT82

Reputation: 73377

Example with input field... 

<div *ngFor="let value of values">{{value}}
    <input [(ngModel)]="value" (focus)="old=value" (ngModelchange)="doSomething(old, value)">
</div>

doSomething(oldVal, newVal) {
    // some code
}

Upvotes: 9

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