APEX has the native validation after submit, but sometimes the requirement needs immediate (read-time) validation,
For example, A column(or Page Item): COL_A, and another column: COL_B is depentdent on COL_A,
So COL_A must passed validation first, then COL_B will then has the chance to be input.
What is the best practice to do the validation for COL_A which is:
showing error alert and keeping cursor in COL_A when validation falied?
Somewhat it's like making a equivalent solution for When-Validate-Item trigger in Forms.