/**
* Returns the list of fields that should be returned by default by {@see \rock\components\ArrayableTrait::toArray()} when no specific fields are specified.
*
* A field is a named element in the returned array by {@see \rock\components\ArrayableTrait::toArray()}.
*
* This method should return an array of field names or field definitions.
* If the former, the field name will be treated as an object property name whose value will be used
* as the field value. If the latter, the array key should be the field name while the array value should be
* the corresponding field definition which can be either an object property name or a PHP callable
* returning the corresponding field value. The signature of the callable should be:
*
* ```php
* function ($field, $model) {
* // return field value
* }
* ```
*
* For example, the following code declares four fields:
*
* - `email`: the field name is the same as the property name `email`;
* - `firstName` and `lastName`: the field names are `firstName` and `lastName`, and their
* values are obtained from the `first_name` and `last_name` properties;
* - `fullName`: the field name is `fullName`. Its value is obtained by concatenating `first_name`
* and `last_name`.
*
* ```php
* return [
* 'email',
* 'firstName' => 'first_name',
* 'lastName' => 'last_name',
* 'fullName' => function () {
* return $this->first_name . ' ' . $this->last_name;
* },
* ];
* ```
*
* In this method, you may also want to return different lists of fields based on some context
* information. For example, depending on the privilege of the current application user,
* you may return different sets of visible fields or filter out some fields.
*
* The default implementation of this method returns the public object member variables.
*
* @return array the list of field names or field definitions.
* @see toArray()
*/
public function fields()
{
$fields = array_keys(ObjectHelper::getObjectVars($this));
return array_combine($fields, $fields);
}
