Issue with Angular 7: In a ReactiveForm, mat-select does not allow setting a default option without using ngModel

I have a Angular 7 app where I am implementing some reactive forms.

The initialization of my reactive form looks like this:

private initFormConfig() {
    return this.formBuilder.group({
      modeTransfert: [''],
      modeChiffrement: [''],
    });
  }

Within my form, there are various inputs and mat-select elements:

<div class="form-inline form-group">
          <label class="col-md-2 justify-content-start">Target URL</label>
          <input  id="urlCible"
                  type="text"
                  maxlength="200"
                  ngDefaultControl
                  formControlName="urlCible"
                  class="col-md-6 form-control"/>
        </div>

        <div class="form-inline form-group">
          <label class="col-md-2 justify-content-start">Transfer Mode</label>
          <mat-form-field class="col-md-3" color="warn">
            <mat-select placeholder="Select transfer mode"
                        id="modesTransfert"
                        [(value)]="selectedModeTransfert"
                        ngDefaultControl
                        formControlName="modeTransfert">
              <mat-option *ngFor="let modeTr of modeTransfertData"
                          [value]="modeTr.value">
                {{modeTr.viewValue}}
              </mat-option>
            </mat-select>
          </mat-form-field>
        </div>

I am facing challenges in setting the default value for the select input. The main issues are:

  • The use of ngModel with ngFormControl (reactive form) is deprecated in Angular 7

  • Trying to patch its value like this leads to errors:

    this.addPefForm.patchValue({'modeTransfert': this.modeTransfertData[0].value});

This cannot be done initially in the onInit or AfterViewChecked hooks due to resulting error:

ParametragePefAdministrationFormComponent.html:107 ERROR Error: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value: 'mat-selected: false'. Current value: 'mat-selected: true'.
    at viewDebugError (core.js:20342)
    at expressionChangedAfterItHasBeenCheckedError (core.js:20330)
    at checkBindingNoChanges (core.js:20432)
    at checkNoChangesNodeInline (core.js:23305)
    at checkNoChangesNode (core.js:23292)
    at debugCheckNoChangesNode (core.js:23896)
    at debugCheckRenderNodeFn (core.js:23850)
    at Object.eval [as updateRenderer] (ParametragePefAdministrationFormComponent.html:107)
    at Object.debugUpdateRenderer [as updateRenderer] (core.js:23839)
    at checkNoChangesView (core.js:23193)
  • I also attempted to bind it using 
    [(value)]="selectedModeTransfert"

However, none of these solutions worked.

Any suggestions?

javascriptangulartypescriptangular-materialangular-reactive-forms

Answer №1

When setting a default value for mat-select, make sure to specify it when initializing the reactive form like this:

  states = [
    {name: 'Arizona', abbrev: 'AZ'},
    {name: 'California', abbrev: 'CA'},
    {name: 'Colorado', abbrev: 'CO'},
    {name: 'New York', abbrev: 'NY'},
    {name: 'Pennsylvania', abbrev: 'PA'},
  ];

  form = new FormGroup({
    state: new FormControl(this.states[3].abbrev),
  });

<mat-select formControlName="state">
    <mat-option *ngFor="let state of states" [value]="state.abbrev">
        {{state.name}}
    </mat-option>
</mat-select>

By doing this, the value in your *ngFor loop will align with the value specified in the FormControl.

This method can also be used to display data fetched from an API and automatically fill in the fields with that information.

I hope this explanation clarifies things for you.

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