private void BeginLoading()
{
    IsLoading = true;
}

private void EndLoading()
{
    IsLoading = false;
}

protected IDisposable Loading()
{
    return new LoadingHelper(this);
}

private class LoadingHelper : IDisposable
{
    private readonly BaseViewModel _viewModel;
    private readonly bool _wasLoading;

    public LoadingHelper(BaseViewModel viewModel)
    {
        _viewModel = viewModel;
        _wasLoading = _viewModel.IsLoading;
        if (!_wasLoading)
        {
            _viewModel.BeginLoading();
        }
    }

    public void Dispose()
    {
        if (!_wasLoading)
        {
            _viewModel.EndLoading();
        }
    }
}