In my option, it is the best idea to a create Class on Model and also define a Method.
After that, you have to call that method on your observer classes.
Model Class:
namespace {Vendor}\{ModuleName}\Model;
class {MyClass}
{
public function myFunction()
{
// do whatever you need
}
}
After that called myFunction() on MyObserver using injection.
<?php
namespace {NameSpace};
class MyObserver implements \Magento\Framework\Event\ObserverInterface
{
private $MyClass;
/**
* Constructor
*
* @param \{Vendor}\{ModuleName}\Model\{MyClass} $eavSetupFactory
*/
public function __construct(\{Vendor}\{ModuleName}\Model\{MyClass} $MYClass)
{
$this->MyClass = $MYClass;
}
public function execute(\Magento\Framework\Event\Observer $observer)
{
$this->MyClass->myFunction();
}
}