InstallHandlerBase.cls

public abstract class InstallHandlerBase implements InstallHandler
{
    public abstract Version GetLastPackageVersionWithoutFeature ();

    public void onInstall(InstallContext context)
    {
        if (isFreshInstall (context))
        {
            OnFreshInstall (context);
        }
        else if (previousVersionContainsFeature(context))
        {
            OnUpgradeFromRecentPackage (context);
        }
        else
        {
            OnUpgradeFromOldPackage (context);
        }
    }

    protected Boolean isFreshInstall (InstallContext context)
    {
        return context.previousVersion() == null;
    }

    protected boolean previousVersionContainsFeature(InstallContext context)
    {
        Version previousVersion = context.PreviousVersion();
        return previousVersion.compareTo (GetLastPackageVersionWithoutFeature()) > 0;
    }

    public virtual void OnUpgradeFromOldPackage (InstallContext context) {}
    public virtual void OnUpgradeFromRecentPackage (InstallContext context) {}
    public virtual void OnFreshInstall (InstallContext context) {}
}