blazor-lifetime/Shared/AsyncContentSupport.razor

@using blazor_lifetime.Models

@if (_isContentVisible)
{
    @ChildContent
}
else
{
    @if (LoadingContent != null)
    {
        @LoadingContent
    }
}

@code{
    private bool _isContentVisible = false;

    [Parameter] public RenderFragment ChildContent { get; set; }
    [Parameter] public RenderFragment LoadingContent { get; set; }
    [Parameter] public IAsyncComponentBase Component { get; set; }

    protected override void OnInitialized()
    {
        if (Component == null)
        {
            throw new Exception(nameof(Component));
        }
        Component.RenderChildContentAsync += OnRenderChildContentAsync;
    }

    protected async Task OnRenderChildContentAsync()
    {
        _isContentVisible = true;
        await InvokeAsync(StateHasChanged);
    }
}