Improve OS loading

Rather than waiting for the OS list to load, when the app starts, the
loads them in the background, and only waits if we reach the list of
available downloads without them loaded yet. In this case, we use a
FutureBuilder to display a loading spinner until the list is ready.

lib/main.dart

@@ -80,7 +80,7 @@ void main() async {
   setWindowMaxSize(const Size(692, 580));
   final foundQuickGet = await Process.run('which', ['quickget']);
   if (foundQuickGet.exitCode == 0) {
-    gOperatingSystems = await loadOperatingSystems(false);
+    gOperatingSystems = loadOperatingSystems(false);
     getIcons();
     AppVersion.packageInfo = await PackageInfo.fromPlatform();
   }

lib/src/model/operating_system.dart

@@ -8,4 +8,4 @@ class OperatingSystem {
   List<Version> versions;
 }
 
-var gOperatingSystems = <OperatingSystem>[];
+Future<List<OperatingSystem>>? gOperatingSystems;

lib/src/pages/operating_system_selection.dart

@@ -25,9 +25,6 @@ class _OperatingSystemSelectionState extends State<OperatingSystemSelection> {
 
   @override
   Widget build(BuildContext context) {
-    var list = gOperatingSystems
-        .where((os) => os.name.toLowerCase().contains(term.toLowerCase()))
-        .toList();
     return Scaffold(
       appBar: AppBar(
         title: Text(context.t('Select operating system')),
@@ -69,36 +66,61 @@ class _OperatingSystemSelectionState extends State<OperatingSystemSelection> {
       body: SingleChildScrollView(
         child: Column(
           children: [
-            ListView.builder(
-              padding: const EdgeInsets.only(top: 4),
-              shrinkWrap: true,
-              itemCount: list.length,
-              itemBuilder: (context, index) {
-                var item = list[index];
-                Widget icon;
+            FutureBuilder(
+              future: gOperatingSystems,
+              builder: (BuildContext context, AsyncSnapshot<List> snapshot) {
+                if (snapshot.hasData) {
+                  List list = snapshot.data!
+                      .where((os) => os.name.toLowerCase().contains(term.toLowerCase()))
+                      .toList();
+                  return ListView.builder(
+                    padding: const EdgeInsets.only(top: 4),
+                    shrinkWrap: true,
+                    itemCount: list.length,
+                    itemBuilder: (context, index) {
+                      var item = list[index];
+                      Widget icon;
 
-                if (osIcons.containsKey(item.code)) {
-                  icon = SvgPicture.asset(
-                    osIcons[item.code]!,
-                    width: 32,
-                    height: 32,
+                      if (osIcons.containsKey(item.code)) {
+                        icon = SvgPicture.asset(
+                          osIcons[item.code]!,
+                          width: 32,
+                          height: 32,
+                        );
+                      } else {
+                        // Replace with generic icon
+                        icon = const Icon(Icons.computer, size: 32);
+                      }
+                      return Card(
+                        child: ListTile(
+                          title: Text(item.name),
+                          leading: icon,
+                          onTap: () {
+                            Navigator.of(context).pop(item);
+                          },
+                        ),
+                      );
+                    },
                   );
                 } else {
-                  // Replace with generic icon
-                  icon = const Icon(Icons.computer, size: 32);
+                  return Row(
+                    mainAxisAlignment: MainAxisAlignment.center,
+                    children: [
+                      Column(
+                        children: [
+                          const Padding(
+                              padding: EdgeInsets.all(16.0),
+                              child: CircularProgressIndicator()
+                          ),
+                          Text(context.t('Loading available downloads')),
+                        ],
+                      )
+                    ],
+                  );
                 }
-                return Card(
-                  child: ListTile(
-                    title: Text(item.name),
-                    leading: icon,
-                    onTap: () {
-                      Navigator.of(context).pop(item);
-                    },
-                  ),
-                );
-              },
+              }
             ),
-          ],
+          ]
         ),
       ),
     );

pubspec.yaml

@@ -4,7 +4,7 @@ publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 version: 1.2.8-1
 
 environment:
-  sdk: ">=2.14.4 <3.0.0"
+  sdk: ">=2.15.0 <3.0.0"
 
 dependencies:
   flutter: