Working with forms in Drupal 7

Question

I have nodes keeping a picture. There is a task to provide an ability to select a type and a size of the picture in every node for further downloading to the end user device. To do this, I want to attach a form under every node to allow user select a type and a size they need. After user select it and put submit-button I want to process user's request, to generate the picture they need on the server side and make somehow his browser to download it. But I have no idea how to achieve this because I don't understand a mechanism of client-server exchange in Drupal 7. I know basic Form API, but it does not help me. So, what can you advice?

EDIT 1

This is a source code of my module testmodule.module

<?php
function testmodule_custom_form($form, &$form_state) {
    $form=array();
    $form['image_types'] = array(
        '#type' => 'radios',
        '#title' => t('Select image type to download'),
        '#options' => array(0 => t('SVG'), 1 => t('PNG'), 2 => t('ICON')),
    );
    $form['image_sizes'] = array(
        '#type' => 'radios',
        '#title' => t('Select image size'),
        '#options' => array(0 => t('100x100'), 1 => t('200x200')),
    );
    $form['submit'] = array(
        '#type' => 'submit',
        '#value' => t('Download'),

    );
    return $form;
}

function testmodule_custom_form_submit($form, &$form_state) {
    drupal_set_message('IT WORKS!!!');
}

function testmodule_node_view($node, $view_mode, $langcode) {
    if($node->type === 'article') {
        $my_form = drupal_get_form('testmodule_custom_form', $node);
        $node->content['attached_form'] = array(
            '#markup' => drupal_render($my_form),
            '#weight' => 99,
        );
    }
}

function testmodule_node_presave($node) {
    $node->body[$node->language][0]['format']  = 'full_html';
    if ($node->field_image_svg[$node->language])
    {
        $file = file_load($node->field_image_svg[$node->language][0]['fid']);
        $image = new Imagick(drupal_realpath($file->uri));
        $image->setImageFormat("png");
        $destination = 'public://pngimages/'.$file->filename.'.png';
        $image->writeImage(drupal_realpath($destination)); 
        $data = file_get_contents(drupal_realpath($destination));
        $file = file_save_data($data, $destination, FILE_EXISTS_REPLACE);
        $node->field_image[$node->language][0] = (array)$file;  
    }
}
?>  

Answer 1

One way to do this is to create a custom module with a form callback function.

Code sample:

function YOUR_MODULE_custom_form($form, &$form_state) {
    // your form controls and inputs go here
}

function YOUR_MODULE_custom_form_submit($form, &$form_state) {
    // your form submission handler.
    // your image resizing logic and any other logic should go here
}

Then in order to attach this form to nodes, create a hook_node_view implementation.

function YOUR_MODULE_node_view($node, $view_mode, $langcode)
{
    if($node->type === 'YOUR_CONTENT_TYPE') {
        $my_form = drupal_get_form('YOUR_MODULE_custom_form', $node);
        $node->content['your_attached_form'] = array(
            '#markup' => drupal_render($my_form),
            '#weight' => 99,
        );
    }
}

Hope this gets you started. I highly recommend taking your time getting familiar with Drupal form API and creating custom modules, that would make your life a lot easier :).

Answer 2

An alternative approach that I generally prefer, is to use programatic blocks with defaults.

I've implements one in your code and cleaned up a few items to bring better into line with coding standards.

/**
 * Implements hook_form().
 */
function testmodule_custom_form($form, &$form_state) {
  $form = array();
  $form['image_types'] = array(
    '#type' => 'radios',
    '#title' => t('Select image type to download'),
    '#options' => array(0 => t('SVG'), 1 => t('PNG'), 2 => t('ICON')),
  );
  $form['image_sizes'] = array(
    '#type' => 'radios',
    '#title' => t('Select image size'),
    '#options' => array(0 => t('100x100'), 1 => t('200x200')),
  );
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Download'),

  );
  return $form;
}

/**
 * Implements hook_form_submit().
 */
function testmodule_custom_form_submit($form, &$form_state) {
  drupal_set_message('IT WORKS!!!');
}

/**
 * Implements hook_block_info().
 */
function testmodule_block_info() {
  $blocks = array();

  $blocks['image_download'] = array(
    'info' => t('Node - Image download'),
    'status' => 1,
    'region' => 'content',
    'visibility' => BLOCK_VISIBILITY_LISTED,
    'pages' => 'node/*',
    'cache' => DRUPAL_CACHE_PER_PAGE,
    'weight' => 1,
  );

  return $blocks;
}

/**
 * Implements hook_block_view().
 */
function testmodule_block_view($delta = '') {
  $n = menu_get_object();
  if (!$n || $n->type != 'only_particular_content_type') {
    return FALSE;
  }

  if ($delta == 'image_download') {
    return array(
      'content' => drupal_get_form('testmodule_custom_form'),
    );
  }

  return FALSE;
}

/**
 * Implements hook_node_presave().
 */
function testmodule_node_presave($node) {
  $node->body[$node->language][0]['format']  = 'full_html';
  $field_image_svg = field_get_items('node', $node, 'field_image_svg');
  if (!$field_image_svg) {
    return;
  }

  $file = file_load($field_image_svg[0]['fid']);
  $image = new Imagick(drupal_realpath($file->uri));
  $image->setImageFormat("png");
  $destination = 'public://pngimages/'.$file->filename.'.png';
  $image->writeImage(drupal_realpath($destination));
  $data = file_get_contents(drupal_realpath($destination));
  $file = file_save_data($data, $destination, FILE_EXISTS_REPLACE);
  $node->field_image[$node->language][0] = (array)$file;

}