如何从angular2的下拉列表中获取旧值和新值

Question

下面的代码是带有 OnChange 事件的下拉列表。 $event.target.value 只给我新值（选择后）。如何使用 onChanges 事件获取以前的值（选择前的值）。

---

 <select class="form-control selectpicker selector" name="selectedQuestion1" [ngModel]="selectedQuestion1"   (Onchange)="filterSecurityQuestions($event.target.value,0)">
 <option [value]="0"  disabled selected>Select Secuirty Question1</option>
 <option *ngFor="let securityQuestion of securityQuestions[0]"  [value]="securityQuestion.SecurityQuestionID" >
      {{securityQuestion.Question}}
</option>
 </select>  <br />                       
<div>
<input type="text" class="form-control" id="first_name" name="securityAnswer1" placeholder="Your first security answer">
</div>

---

filterSecurityQuestions(questionNo: number, securityQtnDropdownNo: number) {
// I want previous value of dropdown here. 
}

Answer 1

您可以保留一个属性来保存旧值，如下例所示：

HTML

<select #select id="pageSize" [ngModel]="myValue" (ngModelChange)="select.value = onChange($event)"> 
     <option value="1">value1</option> 
     <option value="2">value2</option> 
     <option value="3">value3</option> 
</select> 

打字稿

  myValue = 1;
  oldValue=1

  onChange(event) { 
    const response = window.confirm("Are you sure you want change the page size? Your edits will be lost?"); 
    if (response) { 
      this.myValue = event;
      this.oldValue = event;
    }
    else{
      this.myValue = this.oldValue;
    }
    console.log(this.myValue)
    console.log(this.oldValue)
    return this.myValue;
  }

演示