===========================
How to validate field value
===========================

Write the
:doc:`on_field_validate </refs/client/item/on_field_validate>`
event handler to validate field value.

For example, The event will triggered when the 
:doc:`post </refs/client/item/m_post>`
method is called, that saves the record in memory or 
when the user leaves the input used to edit the unitprice field value.

.. code-block:: js

  function on_field_validate(field) {
       if (field.field_name === 'unitprice' && field.value <= 0) {
          return 'Unit price must be greater that 0';
      }
  }

As an example, below is the code that doesn't use the
:doc:`on_field_validate </refs/client/item/on_field_validate>` 
method and checks the value of the unitprice field and prevents the user 
from leaving the input when the value is less than or equal to zero:

.. code-block:: js

  function on_edit_form_shown(item) {
      item.each_field( function(field) {
          var input = item.edit_form.find('input.' + field.field_name);
          input.blur( function(e) {
              var err;
              if ($(e.relatedTarget).attr('id') !== "cancel-btn") {
                  err = check_field_value(field);
                  if (err) {
                      item.alert_error(err);
                      input.focus();             
                  }
              }
          });
      });
  }
  
  function check_field_value(field) {
      if (field.field_name === 'album' && !field.value) {
          return 'Album must be specified';
      }
      if (field.field_name === 'unitprice' && field.value <= 0) {
          return 'Unit price must be greater that 0';
      }
  }

In the on_edit_form_shown event handler, we iterate through all the fields using the each_field method and find the input data for each field, if it exists.

In the 
:doc:`on_edit_form_shown </refs/client/item/on_edit_form_shown>` 
event handler we iterate through all the fields using the
:doc:`each_field </refs/client/item/m_each_field>` 
method and find the input for each field, if it exists. Each input has a class 
with the name of the field (:doc:`field_name </refs/client/field/at_field_name>`).

Then we assign a jQuery blur event to it, in which we call the ``check_field_value`` 
function, and, if it returns text string, we warn the user and focus the input. 
Before calling the function, we check whether the "Cancel" button was pressed.

We declared the 
:doc:`on_edit_form_shown </refs/client/item/on_edit_form_shown>` 
event handler in the item's module, so it will work in this module only.

We can declare the following event handler in the task client module so we can 
write ``check_field_value`` function in any module we need to enable this field 
validation. The 
:doc:`on_edit_form_shown </refs/client/task/on_edit_form_shown>` of the task
is called first for every item when edit form is shown. See 
:doc:`Form events </programming/interface/form_events>`.

.. code-block:: js

  function on_edit_form_shown(item) {
      if (item.check_field_value) {
          item.each_field( function(field) {
              var input = item.edit_form.find('input.' + field.field_name);
              input.blur( function(e) {
                  var err;
                  if ($(e.relatedTarget).attr('id') !== "cancel-btn") {
                      err = item.check_field_value(field);
                      if (err) {
                          item.alert_error(err);
                          input.focus();             
                      }
                  }
              });
          });
      }
  }

In this event handler we check if the item has the ``check_field_value`` attribute. 
Each function declared in a module becomes an attribute of the item.