<?php

final class PhabricatorExampleEdgeType extends PhabricatorEdgeType {

  const EDGECONST = 9999;

}

<?php

final class PhabricatorProjectExampleCustomField
  extends PhabricatorProjectCustomField {

  private $value = [];

  protected function getDatasource(): PhabricatorTypeaheadDatasource {
    return new PhabricatorProjectDatasource();
  }

  protected function getApplicationTransactionEdgeType(): PhabricatorEdgeType {
    return new PhabricatorExampleEdgeType();
  }

  public function getValue(): array {
    return $this->value;
  }

  public function setValue(array $value): void {
    $this->value = $value;
  }

/* -(  PhabricatorCustomField  )--------------------------------------------- */

  public function getFieldKey(): string {
    return 'example';
  }

  public function getFieldName(): string {
    return pht('Example');
  }

  public function readValueFromObject(PhabricatorCustomFieldInterface $object): void {
    $edges = PhabricatorEdgeQuery::loadDestinationPHIDs(
      $this->getObject()->getPHID(),
      $this->getApplicationTransactionEdgeConstant());

    $this->setValue($edges);
  }

  public function getValueForStorage(): array {
    return ['=' => array_fuse($this->getValue())];
  }

  public function setValueFromStorage($value): void {
    $this->setValue(array_values($value['=']));
  }

  public function shouldAppearInApplicationTransactions(): bool {
    return true;
  }

  public function getApplicationTransactionType(): string {
    return PhabricatorTransactions::TYPE_EDGE;
  }

  public function getApplicationTransactionMetadata(): array {
    return [
      'edge:type' => $this->getApplicationTransactionEdgeConstant(),
    ];
  }

  public function validateApplicationTransactions(
    PhabricatorApplicationTransactionEditor $editor,
    $type,
    array $xactions): array {

    $errors = parent::validateApplicationTransactions(
      $editor,
      $type,
      $xactions);

    foreach ($xactions as $xaction) {
      $errors[] = new PhabricatorApplicationTransactionValidationError(
        $type,
        pht('Invalid value'),
        pht('This value is not allowed.'),
        $xaction);
    }

    return $errors;
  }

  public function shouldAppearInEditView(): bool {
    return true;
  }

  public function readValueFromRequest(AphrontRequest $request): void {
    $this->setValue($request->getArr($this->getFieldKey()));
  }

  public function getRequiredHandlePHIDsForEdit(): array {
    return $this->getValue();
  }

  public function renderEditControl(array $handles): AphrontFormControl {
    return (new AphrontFormTokenizerControl())
      ->setViewer($this->getViewer())
      ->setLabel($this->getFieldName())
      ->setName($this->getFieldKey())
      ->setValue($this->getValue())
      ->setDatasource($this->getDatasource());
  }

  public function shouldAppearInPropertyView(): bool {
    return true;
  }

  public function renderPropertyViewValue(array $handles): ?PhutilSafeHTML {
    return $this->renderHandleList($handles);
  }

  public function getRequiredHandlePHIDsForPropertyView(): array {
    return $this->getValue();
  }

}