How can I only validate a field when field changed ?

[fallenleavesguy]

How can I only validate field when field changed, but doesn't validate when submitting.

For example, I want to check if my username had been occupied when field changed, but doesn't validate when submitting, because submit api will also do this work.

"onChange" validateTrigger will be triggered when submitting. How can I only validate a field only on change but not when submitting