How to set default value of a managed file field in Drupal 8 & 9

in this article, I'll show you How to set default value of a managed file field in Drupal 8 & 9.

Question:

I created an edit custom form via a custom module. From this form, I have a managed file field and I set his default value to a fid; However, when my form is rendered, the managed file field doesn't show the image ratached. I don't understand why ?

My code:

$form['picture'] = array(
      '#title' => t('picture'),
      '#description' => $this->t('Chossir Image gif png jpg jpeg'),
      '#type' => 'managed_file',
      '#required' => true,
      '#default_value' = 10, //Set 10 for the test, the fid "10" exist, I checked
      '#upload_location' => 'public://images/',
      '#upload_validators' => array(
        'file_validate_extensions' => array('gif png jpg jpeg')),
    );

Solution: 

You need to give the fid in array format. Please try it and let me know whether you got the result.

$form['picture'] = array(
      '#title' => t('picture'),
      '#description' => $this->t('Chossir Image gif png jpg jpeg'),
      '#type' => 'managed_file',
      '#required' => true,
      '#default_value' => (isset($data['fid'])) ? [$data['fid']] : [],
      '#upload_location' => 'public://images/',
      '#upload_validators' => array(
        'file_validate_extensions' => array('gif png jpg jpeg')),
    );