Angular 2 ngModelChange旧值 [英] Angular 2 ngModelChange old value

问题描述

有人可以告诉我比较ngModel新旧值的最佳做法是什么吗?

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

在角度1:

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

我之所以这样问，是因为(ngModelChange)从来没有带给我oldVal，只有我带了newVal.

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

在我的情况下，我在<select>标记中使用ngModel，并将旧选择与新选择进行比较:

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>

推荐答案

这可能有效

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

或

(ngModelChange)="onModelChange($event)"

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

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