如何在Dropdownlist中设置默认值2到3

Question

在此代码中如何在下拉列表中将2个值设置为默认值，当我刷新页面时，应将最后2个选定值设置为默认值

https://stackblitz.com/edit/angular-ku2by4-mgaqtp

Answer 1

您可以使用localStorage存储一个人选择的多个值...当我们刷新页面时，这些值将从构造函数中的localStorage中获取。

您可以使用以下2个更改来更新stackblitz以查看效果...

选择-多example.ts

import {Component} from '@angular/core';
import {FormControl} from '@angular/forms';

/** @title Select with multiple selection */
@Component({
  selector: 'select-multiple-example',
  templateUrl: 'select-multiple-example.html',
  styleUrls: ['select-multiple-example.css'],
})
export class SelectMultipleExample {
  toppings = new FormControl();
  toppingList: string[] = ['Extra cheese', 'Mushroom', 'Onion', 'Pepperoni', 'Sausage', 'Tomato'];
  selected:string[]; //= ['Mushroom', 'Onion'];
  previousSelection:string[];

  constructor(){
    this.selected = [];
    this.previousSelection = [];
    var readStore = localStorage.getItem('selected');

    if (readStore.length>0){
      this.previousSelection = readStore.split(",");
    }

    if (this.previousSelection.length>0){
      for(var i=0; i<this.previousSelection.length; i++){
        this.selected.push(this.previousSelection[i]) ; 
      }
    }
  }

  selectionChanged(passedVal){
    while(this.selected.length>2){
      this.selected.shift();
    }

    var combinedString = this.selected.join();
    localStorage.setItem('selected', combinedString);
  }

}

选择-多example.html的

   Previous selection: <b>{{previousSelection}}</b> <br/> 
2 Valid selections: <b>{{selected}}</b><br/>
<br/>
<mat-form-field>
  <mat-select placeholder="Toppings" [formControl]="toppings" multiple [(ngModel)]="selected" (selectionChange)="selectionChanged($event.value)">
<mat-option *ngFor="let topping of toppingList" [value]="topping">{{topping}}</mat-option>
  </mat-select>
</mat-form-field>

更新：用户要求只选择2个选项，尽管用户试图选择2个以上 - 更新代码以反映这一点;但是，有一个open issue，在更新ngModel时，mat-select选项不会更新;这就是为什么用户可以选择2个以上的多个选项，但只有2个选项（我们也在屏幕上显示）将继续。

更新＃2（对于材料表）：将现有的table-basic-example.html替换为：

<div class="example-container mat-elevation-z8">
  <mat-table #table [dataSource]="dataSource">

    <!--- Note that these columns can be defined in any order.
          The actual rendered columns are set as a property on the row definition" -->

    <!-- Position Column -->
    <ng-container matColumnDef="checked">
      <mat-header-cell *matHeaderCellDef>Check</mat-header-cell>
      <mat-cell *matCellDef="let element">
        <mat-checkbox [(ngModel)]="element.checked" (click)="selectionChanged(element)"></mat-checkbox>
      </mat-cell>
    </ng-container>

    <!-- Position Column -->
    <ng-container matColumnDef="position">
      <mat-header-cell *matHeaderCellDef> No. </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.position}} </mat-cell>
    </ng-container>

    <!-- Name Column -->
    <ng-container matColumnDef="name">
      <mat-header-cell *matHeaderCellDef> Name </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.name}} </mat-cell>
    </ng-container>

    <!-- Weight Column -->
    <ng-container matColumnDef="weight">
      <mat-header-cell *matHeaderCellDef> Weight </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.weight}} </mat-cell>
    </ng-container>

    <!-- Symbol Column -->
    <ng-container matColumnDef="symbol">
      <mat-header-cell *matHeaderCellDef> Symbol </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.symbol}} </mat-cell>
    </ng-container>

    <mat-header-row *matHeaderRowDef="displayedColumns"></mat-header-row>
    <mat-row *matRowDef="let row; columns: displayedColumns;" [ngClass]="{hovered: row.hovered, highlighted: row.highlighted}" (click)="row.highlighted = !row.highlighted" (mouseover)="row.hovered = true" (mouseout)="row.hovered = false"></mat-row>
  </mat-table>
</div>

<hr/>
<div *ngFor="let item of selectedCells">
  {{item.position}} - {{item.name}}
</div>

<!-- Copyright 2018 Google Inc. All Rights Reserved.
    Use of this source code is governed by an MIT-style license that
    can be found in the LICENSE file at http://angular.io/license -->

将现有的table-basic-example.ts替换为：

import { Component } from '@angular/core';

/**
 * @title Basic table
 */
@Component({
  selector: 'table-basic-example',
  styleUrls: ['table-basic-example.css'],
  templateUrl: 'table-basic-example.html',
})
export class TableBasicExample {
  displayedColumns = ['checked', 'position', 'name', 'weight', 'symbol'];
  dataSource = ELEMENT_DATA;
  selectedCells:Element[] = [];

  // highlight(element: Element) {    element.highlighted = !element.highlighted;  }


  selectionChanged(val: Element){
    console.log(val);
    if(!val.checked){
      if (this.selectedCells.length <2){
        /* just insert new entry */
        var duplicateCheck = false;
        for(var j=0; j<this.selectedCells.length;j++){
          if (this.selectedCells[j].position == val.position ){
            duplicateCheck = true;
          }
        }
        if(!duplicateCheck){
          this.selectedCells.push(val);
        }
      } else {
        for(var j=0; j<this.selectedCells.length;j++){
          if (this.selectedCells[j].position == val.position ){
            duplicateCheck = true;
          }
        }
        if(!duplicateCheck){
          this.selectedCells.push(val);
          /* remove the oldest entry  - and insert new entry */
          for(var i=0; i<this.dataSource.length; i++){
            if(this.dataSource[i].position == this.selectedCells[0].position){
              //console.log("about to remove",this.selectedCells[0].position);
              this.dataSource[i].checked = false;
            }
          }
          this.selectedCells.shift();
          //console.log(this.dataSource);
        }

      }
    }
  }
}

export interface Element {
  checked: boolean;
  name: string;
  position: number;
  weight: number;
  symbol: string;
  highlighted?: boolean;
  hovered?: boolean;
}

const ELEMENT_DATA: Element[] = [
  { checked: false, position: 1, name: 'Hydrogen', weight: 1.0079, symbol: 'H' },
  { checked: false, position: 2, name: 'Helium', weight: 4.0026, symbol: 'He' },
  { checked: false, position: 3, name: 'Lithium', weight: 6.941, symbol: 'Li' },
  { checked: false, position: 4, name: 'Beryllium', weight: 9.0122, symbol: 'Be' },
  { checked: false, position: 5, name: 'Boron', weight: 10.811, symbol: 'B' },
  { checked: false, position: 6, name: 'Carbon', weight: 12.0107, symbol: 'C' },
  { checked: false, position: 7, name: 'Nitrogen', weight: 14.0067, symbol: 'N' },
  { checked: false, position: 8, name: 'Oxygen', weight: 15.9994, symbol: 'O' },
  { checked: false, position: 9, name: 'Fluorine', weight: 18.9984, symbol: 'F' },
  { checked: false, position: 10, name: 'Neon', weight: 20.1797, symbol: 'Ne' },
  { checked: false, position: 11, name: 'Sodium', weight: 22.9897, symbol: 'Na' },
  { checked: false, position: 12, name: 'Magnesium', weight: 24.305, symbol: 'Mg' },
  { checked: false, position: 13, name: 'Aluminum', weight: 26.9815, symbol: 'Al' },
  { checked: false, position: 14, name: 'Silicon', weight: 28.0855, symbol: 'Si' },
  { checked: false, position: 15, name: 'Phosphorus', weight: 30.9738, symbol: 'P' },
  { checked: false, position: 16, name: 'Sulfur', weight: 32.065, symbol: 'S' },
  { checked: false, position: 17, name: 'Chlorine', weight: 35.453, symbol: 'Cl' },
  { checked: false, position: 18, name: 'Argon', weight: 39.948, symbol: 'Ar' },
  { checked: false, position: 19, name: 'Potassium', weight: 39.0983, symbol: 'K' },
  { checked: false, position: 20, name: 'Calcium', weight: 40.078, symbol: 'Ca' },
];


/**  Copyright 2018 Google Inc. All Rights Reserved.
    Use of this source code is governed by an MIT-style license that
    can be found in the LICENSE file at http://angular.io/license */