Introduced auto refresh for API Data

ac_web/lib/Widgets/AgentAvailabilityColumn.dart

@@ -3,6 +3,7 @@ import 'package:ac_web/Api/Dtos/AgentDto.dart';
 import 'package:ac_web/Pages/AgentDetails.dart';
 import 'package:ac_web/Widgets/AgentStatusIndicator.dart';
 import 'package:ac_web/Widgets/AgentTypeIcons.dart';
+import 'package:ac_web/Widgets/AutoRefreshState.dart';
 import 'package:flutter/material.dart';
 
 /// Fetches all agents from the AgentsController and lists them with a colored
@@ -14,7 +15,8 @@ class AgentAvailabilityColumn extends StatefulWidget{
   State<AgentAvailabilityColumn> createState() => _AgentAvailabilityColumnState();
 }
 
-class _AgentAvailabilityColumnState extends State<AgentAvailabilityColumn>{
+class _AgentAvailabilityColumnState extends State<AgentAvailabilityColumn>
+    with AutoRefreshState<AgentAvailabilityColumn> {
 
   late Future<List<AgentDto>> mAgentsFuture;
 
@@ -24,23 +26,44 @@ class _AgentAvailabilityColumnState extends State<AgentAvailabilityColumn>{
     mAgentsFuture = AcApi.instance.agents.getAgents();
   }
 
+  /// Reload triggered by the user. Shows the loading indicator again because
+  /// the user asked for the request and should see that it is running.
   void ReloadAgents(){
     setState(() {
       mAgentsFuture = AcApi.instance.agents.getAgents();
     });
   }
 
+  /// Automatic reload. The agents that are already on screen stay visible
+  /// while the request runs, the future is only replaced once it succeeded.
+  @override
+  Future<void> RefreshData() async {
+
+    final List<AgentDto> agents = await AcApi.instance.agents.getAgents();
+
+    if (!mounted) {
+      return;
+    }
+
+    setState(() {
+      mAgentsFuture = Future<List<AgentDto>>.value(agents);
+    });
+  }
+
   @override
   Widget build(BuildContext context) {
     return FutureBuilder<List<AgentDto>>(
       future: mAgentsFuture,
       builder: (context, snapshot) {
 
-        if (snapshot.connectionState != ConnectionState.done) {
+        // Not `connectionState`: a refresh puts the builder back into the
+        // waiting state although the previous agents are still available, and
+        // those should stay on screen instead of flashing a spinner.
+        if (!snapshot.hasData && !snapshot.hasError) {
           return const Center(child: CircularProgressIndicator());
         }
 
-        if (snapshot.hasError) {
+        if (snapshot.hasError && !snapshot.hasData) {
           return Center(child: Text('Could not load agents: ${snapshot.error}'));
         }