I assume that you are using Business project template of Crosslight. To use async mode, you can modify OnNavigated in CategoryListViewModel into the following snippet code:

public override async void Navigated(NavigatedParameter parameter)
{
    base.Navigated(parameter);

    ISelectResult<Category> selectResult = await this.Repository.GetAllAsync();
    this.Items = selectResult.Items.ToObservable();
}

Hope this helps.

Thank you for your explaination.

But I mean server side async methods in Controller generated by Crosslight VS Extension. Does it support async mode?

Currently, the generated code of Web API controller doesn't use async mode. I have forwarded this to Crosslight development team as feature request. It is submitted under CROS-678.

Once implemented, there are no code changes needed in your existing Crosslight mobile project.

I will keep this thread updated with any news I heard from the team regarding CROS-678.