Javascript 在模板驱动表单中，如何在表单初始化后仅在keyup或input blur上触发表单值更改订阅

在我的角度表单中，当在ngOnInit（）中设置表单值时，即使用某些默认值初始化表单时，值更改触发。但我只想在用户更改字段值时触发，而不是在页面加载时触发。如果用户在表单上更改或键入任何内容，我需要设置标志

 <form #schoolProfileForm="ngForm" (ngSubmit)="schoolProfileFormSubmit()">
          <div class="col-lg-4 col-md-6">
        <mat-form-field class="mat-form-full-width"  appearance="standard">
          <mat-label>schoolCode</mat-label>
          <input [(ngModel)]="schoolCode" name="schoolCode" matInput>
        </mat-form-field>
      </div>
          <div class="col-lg-4 col-md-6">
        <mat-form-field class="mat-form-full-width"  appearance="standard">
          <mat-label>schoolName</mat-label>
          <input [(ngModel)]="schoolName" name="schoolName" matInput>
        </mat-form-field>
      </div>
          <div class="col-lg-4 col-md-6">
        <mat-form-field class="mat-form-full-width"  appearance="standard">
          <mat-label>schoolPlace</mat-label>
          <input [(ngModel)]="schoolPlace" name="schoolPlace" matInput>
        </mat-form-field>
      </div>
</form>

 ngOnInit() {
     this.schoolProfileForm.valueChanges.subscribe(x => {
    this.valueChangedFlag = true;
    })
  }