where to put composers umbraco code example

Example: umbraco 8 composer

public void Initialize()
{
    // subscribe to content service published event
    ContentService.Published += ContentService_Published;
}

private void ContentService_Published(Umbraco.Core.Services.IContentService sender, Umbraco.Core.Events.ContentPublishedEventArgs e)
{
    // the custom code to fire everytime content is published goes here!
}