Add a content entity form which allows to set revisions

src/Entity/RevisionableEntityBundleInterface.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity\Entity\RevisionableEntityBundleInterface.
+ */
+
+namespace Drupal\entity\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityInterface;
+
+interface RevisionableEntityBundleInterface extends ConfigEntityInterface {
+
+  /**
+   * Returns whether a new revision should be created by default.
+   *
+   * @return bool
+   *   TRUE if a new revision should be created by default.
+   */
+  public function shouldCreateNewRevision();
+
+}

src/Form/ContentEntityFormWithRevisions.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity\Form\ContentEntityFormWithRevisions.
+ */
+
+namespace Drupal\entity\Form;
+
+use Drupal\Core\Entity\ContentEntityForm;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\entity\Entity\RevisionableEntityBundleInterface;
+
+class ContentEntityFormWithRevisions extends ContentEntityForm {
+
+  /**
+   * The entity being used by this form.
+   *
+   * @var \Drupal\Core\Entity\EntityInterface|\Drupal\Core\Entity\RevisionableInterface|\Drupal\entity\Revision\EntityRevisionLogInterface
+   */
+  protected $entity;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function prepareEntity() {
+    parent::prepareEntity();
+
+    $bundle_entity = $this->getBundleEntity();
+
+    // Set up default values, if required.
+    if (!$this->entity->isNew()) {
+      $this->entity->setRevisionLogMessage(NULL);
+    }
+
+    if ($bundle_entity instanceof RevisionableEntityBundleInterface) {
+      // Always use the default revision setting.
+      $this->entity->setNewRevision($bundle_entity && $bundle_entity->shouldCreateNewRevision());
+    }
+  }
+
+  protected function getBundleEntity() {
+    $bundle_entity = $this->entity->{$this->entity->getEntityType()->getKey('bundle')}->referencedEntities()[0];
+    return $bundle_entity;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function form(array $form, FormStateInterface $form_state) {
+
+    $entity_type = $this->entity->getEntityType();
+
+    $bundle_entity = $this->getBundleEntity();
+
+    $account = $this->currentUser();
+
+    if ($this->operation == 'edit') {
+      $form['#title'] = $this->t('Edit %bundle_label @label', [
+        '%bundle_label' => $bundle_entity ? $bundle_entity->label() : '',
+        '@label' => $this->entity->label(),
+      ]);
+    }
+
+    $form['advanced'] = [
+      '#type' => 'vertical_tabs',
+      '#weight' => 99,
+    ];
+
+    // Add a log field if the "Create new revision" option is checked, or if the
+    // current user has the ability to check that option.
+    // @todo Could we autogenerate this form by using some widget on the
+    //   revision info field.
+    $form['revision_information'] = [
+      '#type' => 'details',
+      '#title' => $this->t('Revision information'),
+      // Open by default when "Create new revision" is checked.
+      '#open' => $this->entity->isNewRevision(),
+      '#group' => 'advanced',
+      '#weight' => 20,
+      '#access' => $this->entity->isNewRevision() || $account->hasPermission($entity_type->get('admin_permission')),
+    ];
+
+    $form['revision_information']['revision'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Create new revision'),
+      '#default_value' => $this->entity->isNewRevision(),
+      '#access' => $account->hasPermission($entity_type->get('admin_permission')),
+    ];
+
+    // Check the revision log checkbox when the log textarea is filled in.
+    // This must not happen if "Create new revision" is enabled by default,
+    // since the state would auto-disable the checkbox otherwise.
+    if (!$this->entity->isNewRevision()) {
+      $form['revision_information']['revision']['#states'] = [
+        'checked' => [
+          'textarea[name="revision_log"]' => ['empty' => FALSE],
+        ],
+      ];
+    }
+
+    $form['revision_information']['revision_log'] = [
+      '#type' => 'textarea',
+      '#title' => $this->t('Revision log message'),
+      '#rows' => 4,
+      '#default_value' => $this->entity->getRevisionLogMessage(),
+      '#description' => $this->t('Briefly describe the changes you have made.'),
+    ];
+
+    return parent::form($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save(array $form, FormStateInterface $form_state) {
+    // Save as a new revision if requested to do so.
+    if (!$form_state->isValueEmpty('revision')) {
+      $this->entity->setNewRevision();
+    }
+
+    $insert = $this->entity->isNew();
+    $this->entity->save();
+    $context = ['@type' => $this->entity->bundle(), '%info' => $this->entity->label()];
+    $logger = $this->logger($this->entity->id());
+    $bundle_entity = $this->getBundleEntity();
+    $t_args = ['@type' => $bundle_entity ? $bundle_entity->label() : 'None', '%info' => $this->entity->label()];
+
+    if ($insert) {
+      $logger->notice('@type: added %info.', $context);
+      drupal_set_message($this->t('@type %info has been created.', $t_args));
+    }
+    else {
+      $logger->notice('@type: updated %info.', $context);
+      drupal_set_message($this->t('@type %info has been updated.', $t_args));
+    }
+
+    if ($this->entity->id()) {
+      $form_state->setValue('id', $this->entity->id());
+      $form_state->set('id', $this->entity->id());
+      $form_state->setRedirectUrl($this->entity->urlInfo('collection'));
+    }
+    else {
+      // In the unlikely case something went wrong on save, the entity will be
+      // rebuilt and entity form redisplayed.
+      drupal_set_message($this->t('The entity could not be saved.'), 'error');
+      $form_state->setRebuild();
+    }
+  }
+
+}