Remove validation in angular 4 when it's not render in NgIf

This Angular source GitHub Issue comment by Kara seems extremely relevant, and illustrates how you might solve the problem by treating the reactive model as "source of truth" and create your ngIf expression off of that source of truth, instead of the reverse. This shows it's by design and you have to make some effort not to mix up template-driven and reactive form ideas.

https://github.com/angular/angular/issues/7970#issuecomment-228624899

Thanks for taking the time to describe the problem. I took a look at your example code, and it seems that you are using the reactive form directives (a.k.a "model-driven" directives: ngFormModel, etc), but a template-driven strategy. The fact that the ngIf does not remove the control from the form's serialization and validation is actually by design, and here's why.

In each form paradigm - template-driven and reactive - there can only be one source of truth for the list of active controls. In the template-driven paradigm, the source of truth is the template. In the reactive equivalent, the source of truth is the form model created in the parent component. The DOM does not dictate the state of your form. For this reason, if you remove form control elements from the DOM while using a reactive approach, the form controls are not necessarily changed in the source of truth unless you want them to be. You can choose to update the controls imperatively by calling this.form.removeControl('controlName'), or you can choose to keep the controls in your form. This flexibility allows you to add or remove inputs from the DOM temporarily while keeping their form values serialized (e.g. if you have a number of collapsible sections to your form, you can remove sections on collapse without impacting the value of your form). We don't want to restrict this flexibility and complicate ownership by forcing the model to always match the DOM.

So in your case, if you choose a reactive strategy, you'll want to invert your logic to rely on the source of truth - the model. Specifically, this means removing the control imperatively in the model by calling this.form.removeControl('name') when the button is clicked. Then, the ngIf should depend on the control's presence in the model with *ngIf="form.contains('name')", rather than the other way around. See example plunker here: http://plnkr.co/edit/V7bCFLSIEKTuxU9jcp6v?p=preview

It's worth noting that if you're still using beta.14 (as in your plunker), you'll need to call this.form.updateValueAndValidity() manually. This requirement was removed in #9097, so versions after RC.2 don't require the call.

Another option is to convert to a template-driven strategy (no ngFormModel), which will remove the control from the form when it's destroyed from the template. Example: http://plnkr.co/edit/s9QWy9T8azQoTZKdm7uI?p=preview

I'm going to close this issue as it works as intended, but I think we could make the experience a lot friendlier. A good start would be some more cookbooks and guides in the documentation.

When the condition property is changed then call the method dynamically to set and remove the validation. for example,

whenConditionChanges(condition:boolean){
  if(!condition){
    this.form.controls["text2"].setValidators([Validators.required]);
    this.form.controls["text2"].updateValueAndValidity();
  } else {
    this.form.controls["text2"].setValidators(null);
    this.form.controls["text2"].updateValueAndValidity();
  }
}