Drupal updating form fields on file upload

Question

I am trying to update form fields based on an uploaded file. Specifically, the text that is displayed on the webpage. If the user then presses a submit button, these values are then passed to hook_node_submit() and saved in the database. I'm using hook_form_alter() to perform these updates, and I have found that I can change the default_value of a field when the webpage is first loaded. However, when the upload button is pressed by a user, and hook_form_alter() is called again, the same operation does not work. For instance,

function mymodule_form_alter(&$form, &$form_state, $form_id){
   if ($form_id == 'mymodule_node_form'){

       // I can change the default value here
       $form['field_myfield']['und'][0]['value']['#default_value'] = "Some Value";

       // Check to see if the upload button has been pressed
       if (array_key_exists('clicked_button', $form_state))){
           $trigger = $form_state['clicked_button']['#value'];

           if($trigger == 'Upload'){

               // Try to change the default value here.  The value is changed in $form, 
               // but the field is not updated on the website.  
               $form['field_myfield']['und'][0]['value']['#default_value'] = "New Value";

               // try changing the value after_build
               $form['#after_build'][] = 'mymodule_after_build';
           }
       }
    }
}

function mymodule_after_build($form, &$form_state){
    $form['field_type']['und'][0]['value']['#value'] = 'New Value';
    $form_state['values']['field_type']['und'][0]['value'] = 'New Value';
    return $form;
}

Do I need to perform some sort of refresh on the form? I briefly looked at AHAH, is this something that I need to use?

I am new to Drupal so I apologize in advance if there is something fundamentally wrong with this approach. Any help is greatly appreciated.

Edit:

I added some code trying to use an after_build function to change the cck values. This still doesn't change the value that is rendered on the website.

Answer 1

After many hours, I've found a solution! Thanks goes to Matthew Davidson, who posted this workaround on drupal.org, full post The working code looks like this:

function mymodule_form_alter(&$form, &$form_state, $form_id){
   if ($form_id == 'mymodule_node_form'){

       // create a button to populate the form
       $form['mymodule_pop_btn'] = array(
           '#type' => 'submit'.
           '#value'=>t('Populate Fields'),
           '#submit'=>array('mymodule_populate_fields'),
       );
    }
}

//function to populate the form fields
function mymodule_populate_fields($form, &$form_fields){
    $form_state['input']['my_field']['und'][0]['value'] = 'Some New Value';
    $form_state['rebuild'] = TRUE;
    $form_state['no-cache'] = TRUE;
}

Answer 2

1. Make sure you are altering the desired form by checking $form_id value. hook_form_alter alters all the forms.
2. You can use the submit handler of the form - function the_form_id_submit() - to add your custom functionality.