refactors (#539)

lib/data/provider/base.dart

@@ -0,0 +1,20 @@
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
+
+abstract class Provider<T> {
+  const Provider();
+
+  /// (Re)Load data from store / network / etc.
+  @mustCallSuper
+  FutureOr<void> load() {
+    all.add(this);
+    debugPrint('$runtimeType added');
+  }
+
+  static final all = <Provider>[];
+
+  static Future<void> reload() {
+    return Future.wait(all.map((e) async => await e.load()));
+  }
+}