如何从Angular Mat-Select中获取先前值和新值? [英] How to obtain previous and new value from Angular mat-select?

问题描述

兄弟.我使用的是6号角和敏捷材料，并且有一个字符串数组，这些字符串显示在垫选择形式"字段中. 如果用户选择一个元素，然后选择另一个，则需要跟踪以前的值是什么，新的值是什么.

Hellow. I'm using angular 6, and agular material, and I have an array of strings, which I display in a mat-select form field. If an user selects one element, and then another, I need to track what was the previous value and what is the new value.

到目前为止，我已经可以使用$ event.value获取当前值，但是还没有找到存储或获取先前值的方法.

So far, I've been able to obtain current value using $event.value, but I haven't found a way to store or obtain what the previous value was.

   <mat-select  placeholder="Choose..." (selectionChange) ="checkTitle($event.value);">
            <mat-option *ngFor="let option of Titles; trackBy: trackByFn" [value]="option.title">
              {{option.title}}
            </mat-option>
    </mat-select>

到目前为止，关于如何解决此问题，我还没有提出任何想法. 感谢您的帮助.

So far, I haven't come up with any ideas as to how solve this problem. Help is appreciated.

推荐答案

您可以通过将值推送到Subject中来处理先前值和当前值，并使用成对运算符观察此Subject.该运算符将发出流的先前值和当前值. ( https://www.learnrxjs.io/operators/combination/pairwise.html)

You can handle previous and current value by pushing the value into a Subject, and observe this Subject using the pairwise operator. This operator will emit the previous and the current value of the stream. (https://www.learnrxjs.io/operators/combination/pairwise.html)

示例:

export class YOU_COMPONENT{

  private data: Subject<any> = new Subject<any>();

  checkTitle(value){
    this.data.next(value);
  }

  observeDataChange():Observable<[]>{
     // this return an observable of an array that contains [previous, current] data
     return this.data.asObservable().pipe(pairwise());
  }

}

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