How to change a label value automatically when we change other text fields in flutter?

[Thunder Birds]

I have used TextEditingController to fetch the changes in the text fields. Using the same I created a submit button and when the user clicks the submit button, it will calculate the values and update the label at the end.

Below is the code.

Stackoverflow link for image and code: https://stackoverflow.com/questions/67353600/how-to-change-a-label-value-automatically-when-we-change-other-text-fields-in-fl

I want to remove this submit button and update the value automatically when the user changes the Price or Amount text fields or both.

Please guide me how to set that.