yii\elasticsearch\Query::fields PHP Method

    public function fields($fields)
    {
        if (is_array($fields) || $fields === null) {
            $this->fields = $fields;
        } else {
            $this->fields = func_get_args();
        }
        return $this;
    }

 public function testFields()
 {
     $query = new Query();
     $query->from('yiitest', 'user');
     $query->fields(['name', 'status']);
     $this->assertEquals(['name', 'status'], $query->fields);
     $query->fields('name', 'status');
     $this->assertEquals(['name', 'status'], $query->fields);
     $result = $query->one($this->getConnection());
     $this->assertEquals(2, count($result['_source']));
     $this->assertArrayHasKey('status', $result['_source']);
     $this->assertArrayHasKey('name', $result['_source']);
     $this->assertArrayHasKey('_id', $result);
     $query->fields([]);
     $this->assertEquals([], $query->fields);
     $result = $query->one($this->getConnection());
     $this->assertEquals([], $result['_source']);
     $this->assertArrayHasKey('_id', $result);
     $query->fields(null);
     $this->assertNull($query->fields);
     $result = $query->one($this->getConnection());
     $this->assertEquals(3, count($result['_source']));
     $this->assertArrayHasKey('status', $result['_source']);
     $this->assertArrayHasKey('email', $result['_source']);
     $this->assertArrayHasKey('name', $result['_source']);
     $this->assertArrayHasKey('_id', $result);
 }