function practice_demo_form($form, &$form_state) {
  ... ...
  return $form;
}

function practice_demo_form_validate($form, &$form_state) {
  if (...) {
    form_set_error(...);
  }
}

function practice_demo_form_submit($form, &$form_state) {
  ... ...
}

function drupal_prepare_form($form_id, &$form, &$form_state) {
  ... 

  if (!isset($form[‘#validate‘])) {
    // Ensure that modules can rely on #validate being set.
    $form[‘#validate‘] = array();
    // Check for a handler specific to $form_id.
    // 默认的表单校验函数以_validate结尾
    if (function_exists($form_id . ‘_validate‘)) { 
      $form[‘#validate‘][] = $form_id . ‘_validate‘;
    }
    // Otherwise check whether this is a shared form and whether there is a
    // handler for the shared $form_id.
    elseif (isset($form_state[‘build_info‘][‘base_form_id‘]) && function_exists($form_state[‘build_info‘][‘base_form_id‘] . ‘_validate‘)) {
      $form[‘#validate‘][] = $form_state[‘build_info‘][‘base_form_id‘] . ‘_validate‘;
    }
  }

  if (!isset($form[‘#submit‘])) {
    // Ensure that modules can rely on #submit being set.
    $form[‘#submit‘] = array();
    // Check for a handler specific to $form_id.
    // 默认的表单提交函数以_submit结尾
    if (function_exists($form_id . ‘_submit‘)) {
      $form[‘#submit‘][] = $form_id . ‘_submit‘;
    }
    // Otherwise check whether this is a shared form and whether there is a
    // handler for the shared $form_id.
    elseif (isset($form_state[‘build_info‘][‘base_form_id‘]) && function_exists($form_state[‘build_info‘][‘base_form_id‘] . ‘_submit‘)) {
      $form[‘#submit‘][] = $form_state[‘build_info‘][‘base_form_id‘] . ‘_submit‘;
    }
  }

  ...
}

function practice_menu() {
  $items[‘practice/demo‘] = array(
    ‘title‘ => ‘Demo‘,
    ‘page callback‘ => ‘drupal_get_form‘,
    // 第一个参数是表单标识practice_demo_from
    // 第二个参数是ultramen，第三个参数是25
    ‘page_arguments‘ => array(‘practice_demo_from‘, ‘ultramen‘, 25),
    ‘access callback‘ => TRUE,
  );
  return $items;
}

function practice_demo_form($form, &$form_state, $name, $age) {
  ... ...
}

function drupal_get_form($form_id) {
  $form_state = array();

  $args = func_get_args();
  // Remove $form_id from the arguments.
  array_shift($args);
  $form_state[‘build_info‘][‘args‘] = $args;

  return drupal_build_form($form_id, $form_state);
}

function drupal_prepare_form($form_id, &$form, &$form_state) {
  ... 

  // Generate a new #build_id for this form, if none has been set already. The
  // form_build_id is used as key to cache a particular build of the form. For
  // multi-step forms, this allows the user to go back to an earlier build, make
  // changes, and re-submit.
  // @see drupal_build_form()
  // @see drupal_rebuild_form()
  // Drupal为每个表单定义一个唯一的form_build_id
  if (!isset($form[‘#build_id‘])) {
    $form[‘#build_id‘] = ‘form-‘ . drupal_random_key();
  }
  // form_buikd_id会在浏览器表单中以hidden的形式出现
  $form[‘form_build_id‘] = array(
    ‘#type‘ => ‘hidden‘,
    ‘#value‘ => $form[‘#build_id‘],
    ‘#id‘ => $form[‘#build_id‘],
    ‘#name‘ => ‘form_build_id‘,
    // Form processing and validation requires this value, so ensure the
    // submitted form value appears literally, regardless of custom #tree
    // and #parents being set elsewhere.
    ‘#parents‘ => array(‘form_build_id‘),
  );

  // 同form_build_id
  if (isset($form_id)) {
    $form[‘form_id‘] = array(
      ‘#type‘ => ‘hidden‘,
      ‘#value‘ => $form_id,
      ‘#id‘ => drupal_html_id("edit-$form_id"),
      // Form processing and validation requires this value, so ensure the
      // submitted form value appears literally, regardless of custom #tree
      // and #parents being set elsewhere.
      ‘#parents‘ => array(‘form_id‘),
    );
  }
  if (!isset($form[‘#id‘])) {
    $form[‘#id‘] = drupal_html_id($form_id);
  }

  ...
}

function drupal_build_form($form_id, &$form_state) {
  ...

  // If the incoming input contains a form_build_id, we‘ll check the cache for a
  // copy of the form in question. If it‘s there, we don‘t have to rebuild the
  // form to proceed. In addition, if there is stored form_state data from a
  // previous step, we‘ll retrieve it so it can be passed on to the form
  // processing code.
  // 用$form_state[‘input‘][‘form_id‘]标识表单，例如practice_demo_form
  // 用$form_state[‘input‘][‘form_build_id‘]标识缓存
  $check_cache = isset($form_state[‘input‘][‘form_id‘]) && $form_state[‘input‘][‘form_id‘] == $form_id && !empty($form_state[‘input‘][‘form_build_id‘]);
  if ($check_cache) {
    $form = form_get_cache($form_state[‘input‘][‘form_build_id‘], $form_state);
  }

  ... 
}

function drupal_process_form($form_id, &$form, &$form_state) {
  ...

  // After processing the form, the form builder or a #process callback may
  // have set $form_state[‘cache‘] to indicate that the form and form state
  // shall be cached. But the form may only be cached if the ‘no_cache‘ property
  // is not set to TRUE. Only cache $form as it was prior to form_builder(),
  // because form_builder() must run for each request to accommodate new user
  // input. Rebuilt forms are not cached here, because drupal_rebuild_form()
  // already takes care of that.
  // 用$form_state[‘cache‘]和$form_state[‘no_cache‘]标识是否允许表单缓存
  if (!$form_state[‘rebuild‘] && $form_state[‘cache‘] && empty($form_state[‘no_cache‘])) {
    form_set_cache($form[‘#build_id‘], $unprocessed_form, $form_state);
  }

  ...
}

function drupal_prepare_form($form_id, &$form, &$form_state) {
  ... 

  // Add a token, based on either #token or form_id, to any form displayed to
  // authenticated users. This ensures that any submitted form was actually
  // requested previously by the user and protects against cross site request
  // forgeries.
  // This does not apply to programmatically submitted forms. Furthermore, since
  // tokens are session-bound and forms displayed to anonymous users are very
  // likely cached, we cannot assign a token for them.
  // During installation, there is no $user yet.
  if (!empty($user->uid) && !$form_state[‘programmed‘]) {
    // Form constructors may explicitly set #token to FALSE when cross site
    // request forgery is irrelevant to the form, such as search forms.
    if (isset($form[‘#token‘]) && $form[‘#token‘] === FALSE) {
      unset($form[‘#token‘]);
    }
    // Otherwise, generate a public token based on the form id.
    else {
      $form[‘#token‘] = $form_id;
      // 将form token发送到客户端
      $form[‘form_token‘] = array(
        ‘#id‘ => drupal_html_id(‘edit-‘ . $form_id . ‘-form-token‘),
        ‘#type‘ => ‘token‘,
        ‘#default_value‘ => drupal_get_token($form[‘#token‘]),
        // Form processing and validation requires this value, so ensure the
        // submitted form value appears literally, regardless of custom #tree
        // and #parents being set elsewhere.
        ‘#parents‘ => array(‘form_token‘),
      );
    }
  }

  ...
}

function drupal_validate_form($form_id, &$form, &$form_state) {
  ...

  // If the session token was set by drupal_prepare_form(), ensure that it
  // matches the current user‘s session.
  // $form[‘#token‘]是表单标识，例如practice_demo_form
  if (isset($form[‘#token‘])) {
    if (!drupal_valid_token($form_state[‘values‘][‘form_token‘], $form[‘#token‘])) {
      $path = current_path();
      $query = drupal_get_query_parameters();
      $url = url($path, array(‘query‘ => $query));

      // Setting this error will cause the form to fail validation.
      form_set_error(‘form_token‘, t(‘The form has become outdated. Copy any unsaved work in the form below and then <a href="http://www.mamicode.com/@link">reload this page</a>.‘, array(‘@link‘ => $url)));

      // Stop here and don‘t run any further validation handlers, because they
      // could invoke non-safe operations which opens the door for CSRF
      // vulnerabilities.
      $validated_forms[$form_id] = TRUE;
      return;
    }
  }

  ...
}

function drupal_build_form($form_id, &$form_state) {
  ...

  if (!isset($form_state[‘input‘])) {
    $form_state[‘input‘] = $form_state[‘method‘] == ‘get‘ ? $_GET : $_POST;
  }

  ... 
}

function practice_demo_form_submit($form, &$form_state) {
  $form_state[‘redirect‘] = ‘admin/people/expiration‘;
  // 等同 drupal_goto(‘admin/people/expiration‘);
}

function drupal_redirect_form($form_state) {
  // Skip redirection for form submissions invoked via drupal_form_submit().
  if (!empty($form_state[‘programmed‘])) {
    return;
  }
  // Skip redirection if rebuild is activated.
  if (!empty($form_state[‘rebuild‘])) {
    return;
  }
  // Skip redirection if it was explicitly disallowed.
  if (!empty($form_state[‘no_redirect‘])) {
    return;
  }
  // Only invoke drupal_goto() if redirect value was not set to FALSE.
  if (!isset($form_state[‘redirect‘]) || $form_state[‘redirect‘] !== FALSE) {
    if (isset($form_state[‘redirect‘])) {
      if (is_array($form_state[‘redirect‘])) {
        call_user_func_array(‘drupal_goto‘, $form_state[‘redirect‘]);
      }
      else {
        // This function can be called from the installer, which guarantees
        // that $redirect will always be a string, so catch that case here
        // and use the appropriate redirect function.
        $function = drupal_installation_attempted() ? ‘install_goto‘ : ‘drupal_goto‘;
        $function($form_state[‘redirect‘]);
      }
    }
    drupal_goto(current_path(), array(‘query‘ => drupal_get_query_parameters()));
  }
}

// If the form was submitted by the browser rather than via Ajax, then it
// can only have been triggered by a button, and we need to determine which
// button within the constraints of how browsers provide this information.
if (isset($element[‘#button_type‘])) {
  // All buttons in the form need to be tracked for
  // form_state_values_clean() and for the form_builder() code that handles
  // a form submission containing no button information in $_POST.
  $form_state[‘buttons‘][] = $element;
  if (_form_button_was_clicked($element, $form_state)) {
    $form_state[‘triggering_element‘] = $element;
  }
}

function system_element_info() {
  $types[‘submit‘] = array(
    ‘#input‘ => TRUE,
    ‘#name‘ => ‘op‘,
    ‘#button_type‘ => ‘submit‘,
    ‘#executes_submit_callback‘ => TRUE,
    ‘#limit_validation_errors‘ => FALSE,
    ‘#process‘ => array(‘ajax_process_form‘),
    ‘#theme_wrappers‘ => array(‘button‘),
  );
  $types[‘button‘] = array(
    ‘#input‘ => TRUE,
    ‘#name‘ => ‘op‘,
    ‘#button_type‘ => ‘submit‘,
    ‘#executes_submit_callback‘ => FALSE,
    ‘#limit_validation_errors‘ => FALSE,
    ‘#process‘ => array(‘ajax_process_form‘),
    ‘#theme_wrappers‘ => array(‘button‘),
  );
}

function drupal_prepare_form($form_id, &$form, &$form_state) {
  ... 

  // If no #theme has been set, automatically apply theme suggestions.
  // theme_form() itself is in #theme_wrappers and not #theme. Therefore, the
  // #theme function only has to care for rendering the inner form elements,
  // not the form itself.
  if (!isset($form[‘#theme‘])) {
    $form[‘#theme‘] = array($form_id); // 表单默认主题
    if (isset($form_state[‘build_info‘][‘base_form_id‘])) {
      $form[‘#theme‘][] = $form_state[‘build_info‘][‘base_form_id‘];
    }
  }

  ...
}

function drupal_prepare_form($form_id, &$form, &$form_state) {
  ... 

  // Invoke hook_form_alter(), hook_form_BASE_FORM_ID_alter(), and
  // hook_form_FORM_ID_alter() implementations.
  $hooks = array(‘form‘); // 表单通用钩子
  if (isset($form_state[‘build_info‘][‘base_form_id‘])) {
    $hooks[] = ‘form_‘ . $form_state[‘build_info‘][‘base_form_id‘];
  }
  $hooks[] = ‘form_‘ . $form_id; // 表单专用钩子
  drupal_alter($hooks, $form, $form_state, $form_id);

  ...
}