如何从angular2中的下拉列表中获取新旧值

下面的代码是带有OnChange事件的下拉列表$event.target.value只给了我新的值（在选择之后）。如何使用onChanges事件获取以前的值（选择之前的值）

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您可以保留一个将保存旧值的属性，例如下面的示例：

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> 

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您可以使用单击事件来存储以前的值，并对更改进行操作。

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

<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> 

  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;
  }