CaritasPWA/Shared/Services/OnlineStatusProvider.cs

using Microsoft.JSInterop;
using System.Collections.Generic;

namespace cwebplusApp.Shared.Services {

    public class OnlineStatusProvider {

        public bool Online { get => online; }

        public delegate void OnlineStatusChangeCallBack(bool online);
        public IJSRuntime JSRuntime { get; set; }

        private bool online;
        private HashSet<OnlineStatusChangeCallBack> OnlineStatusChangeCallBacks;

        public OnlineStatusProvider(IJSRuntime jSRuntime) {
            this.JSRuntime = jSRuntime;
            OnlineStatusChangeCallBacks = new();
            var dotNetObjRef = DotNetObjectReference.Create(this);
            JSRuntime.InvokeVoidAsync("registerOnlineStatusHandler", dotNetObjRef);
        }

        [JSInvokable]
        public void SetOnlineStatus(bool isOnline) {
            this.online = isOnline;
            foreach (OnlineStatusChangeCallBack OnlineStatusCallBack in OnlineStatusChangeCallBacks) {
                OnlineStatusCallBack?.Invoke(isOnline);
            }
        }

        public void AddOnlineStatusChangeCallBack(OnlineStatusChangeCallBack listener) {
            OnlineStatusChangeCallBacks.Add(listener);
        }

        public void RemoveOnlineStatusChangeCallBack(OnlineStatusChangeCallBack listener) {
            OnlineStatusChangeCallBacks.Remove(listener);
        }
    }
}