Drupal 8 block annotation with context example

block-annotation-with-context.php

/**
 * @Block(
 *   id = "my_block",
 *   context = {
 *     "node" = @ContextDefinition("entity:node")
 *   }
 * )
 */
class MyBlock extends BlockBase implements ContextAwarePluginInterface {

  /**
   * {@inheritdoc}
   */
  public function build() {
    // The render array.
    $build = [];
    
    $node = $this->getContextValue('node');
    
    // @todo Do something with the $node to affect the $build array
    
    return $build;
  }

}