Начал экран успешного проведения покупки

lib/base_state.dart

@@ -11,11 +11,21 @@ abstract class BaseState<T> extends State<T> {
   String error = null;
   String textFieldValue = "";
 
-  AppBar getAppBar() {
-    return new AppBar(title: new Text(getTitle(), style: new TextStyle(fontSize: 18.0)),
-        backgroundColor: primaryColor,  actions: getMenuButtons());
+  @override Widget build(BuildContext context) {
+    return new Scaffold(appBar: getAppBar(context), body: getBody(context));
   }
 
+  AppBar getAppBar(BuildContext context) {
+    return new AppBar(title: new Text(getTitle(), style: new TextStyle(fontSize: 18.0)),
+        backgroundColor: primaryColor,  actions: getMenuButtons(context));
+  }
+
+  Widget getBody(BuildContext context) {
+    return new Stack(children: <Widget>[getScreenContent(), getProgressIndicator()]);
+  }
+
+  Widget getScreenContent(BuildContext context);
+
   String getTitle();
 
   String getHint();