A Drupal 8 block condition to set visibility based on a node's field value.

SidebarCondition.php

<?php

namespace Drupal\my_block\Plugin\Condition;

use Drupal\Core\Condition\ConditionPluginBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides the 'Sidebar condition' condition.
 *
 * @Condition(
 *   id = "sidebar_condition",
 *   label = @Translation("Sidebar block condition"),
 *   context_definitions = {
 *     "node" = @ContextDefinition(
 *        "entity:node",
 *        required = TRUE ,
 *        label = @Translation("node")
 *     )
 *   }
 * )
 */
class SidebarCondition extends ConditionPluginBase implements ContainerFactoryPluginInterface {

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition
    );
  }

  /**
   * Creates a new SidebarCondition object.
   *
   * @param array $configuration
   *   The plugin configuration, i.e. an array with configuration values keyed
   *   by configuration option name. The special key 'context' may be used to
   *   initialize the defined contexts by setting it to an array of context
   *   values keyed by context names.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
  }

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {

    $form['sidebarActive'] = [
      '#type'          => 'checkbox',
      '#title'         => $this->t('When Sidebar Field Active'),
      '#default_value' => $this->configuration['sidebarActive'],
      '#description'   => $this->t('Enable this block when the sidebar field on the node is active.'),
    ];

    return parent::buildConfigurationForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $this->configuration['sidebarActive'] = $form_state->getValue('sidebarActive');
    parent::submitConfigurationForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return ['sidebarActive' => 0] + parent::defaultConfiguration();
  }

  /**
   * Provides a human readable summary of the condition's configuration.
   */
  public function summary() {
    $status = $this->getContextValue('sidebarActive') ? t('enabled') : t('disabled');

    return t(
      'The node has sidebar block @status.',
      ['@status' => $status]);
  }
  
  /**
   * Evaluates the condition and returns TRUE or FALSE accordingly.
   *
   * @return bool
   *   TRUE if the condition has been met, FALSE otherwise.
   */
  public function evaluate() {
    if (empty($this->configuration['sidebarActive']) && !$this->isNegated()) {
      return TRUE;
    }

    $node = $this->getContextValue('node');
    if ($node->hasField('field_enable_sidebar') && $node->field_enable_sidebar->value) {
      return TRUE;
    }

    return FALSE;

  }

}

Thanks for the article and gist. I think https://gist.github.com/acrosman/ab7e9ffce3996f6bc1798f9f1ca34c06#file-sidebarcondition-php-L89 should be:

$status = $this->configuration['sidebarActive'] ? t('enabled') : t('disabled');

Adding a link back to your article https://spinningcode.org/2017/06/controlling-block-visibility-with-a-custom-field-in-drupal-8/ since I found the gist first

Context is now context_definitions:

 *   context_definitions = {
 *     "node" = @ContextDefinition("entity:node", label = @Translation("Node"))
 *   }

Oh thanks for catching that. For anyone interested the change record is here: https://www.drupal.org/node/3016699