PurchaseWidget: Move slider to its own file

Also de-couple it from RevenueCat specific APIs.

lib/widgets/purchase_slider.dart

@@ -0,0 +1,60 @@
+import 'package:flutter/material.dart';
+import 'package:equatable/equatable.dart';
+
+class PaymentInfo extends Equatable {
+  final double value;
+  final String text;
+
+  PaymentInfo(this.value, this.text);
+
+  @override
+  List<Object> get props => [value, text];
+}
+
+typedef PaymentSliderChanged = Function(PaymentInfo);
+
+class PurchaseSlider extends StatelessWidget {
+  final List<PaymentInfo> values;
+  final PaymentInfo selectedValue;
+  final PaymentSliderChanged onChanged;
+
+  PurchaseSlider({
+    @required this.values,
+    @required this.selectedValue,
+    @required this.onChanged,
+  }) {
+    values.sort((a, b) => a.value.compareTo(b.value));
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Slider(
+      min: values.first.value,
+      max: values.last.value + 0.50,
+      value: selectedValue.value,
+      onChanged: (double val) {
+        int i = -1;
+        for (i = 1; i < values.length; i++) {
+          var prev = values[i - 1].value;
+          var cur = values[i].value;
+
+          if (prev < val && val <= cur) {
+            i--;
+            break;
+          }
+        }
+        if (val == values.first.value) {
+          i = 0;
+        } else if (val >= values.last.value) {
+          i = values.length - 1;
+        }
+
+        if (i != -1) {
+          onChanged(values[i]);
+        }
+      },
+      label: selectedValue.text,
+      divisions: values.length,
+    );
+  }
+}

lib/widgets/purchase_widget.dart

@@ -7,6 +7,7 @@ import 'package:gitjournal/analytics.dart';
 import 'package:gitjournal/.env.dart';
 import 'package:gitjournal/iap.dart';
 import 'package:gitjournal/settings.dart';
+import 'package:gitjournal/widgets/purchase_slider.dart';
 
 import 'package:purchases_flutter/purchases_flutter.dart';
 
@@ -163,36 +164,30 @@ class _PurchaseWidgetState extends State<PurchaseWidget> {
         : buildBody(context);
   }
 
+  PaymentInfo _fromOffering(Offering o) {
+    var prod = o.monthly.product;
+    return PaymentInfo(prod.price, prod.priceString);
+  }
+
+  Offering _fromPaymentInfo(PaymentInfo info) {
+    for (var o in _offerings) {
+      if (o.monthly.product.priceString == info.text) {
+        return o;
+      }
+    }
+    assert(false);
+    return null;
+  }
+
   Widget buildBody(BuildContext context) {
-    var slider = Slider(
-      min: _offerings.first.monthly.product.price,
-      max: _offerings.last.monthly.product.price + 0.50,
-      value: _selectedOffering.monthly.product.price,
-      onChanged: (double val) {
-        int i = -1;
-        for (i = 1; i < _offerings.length; i++) {
-          var prev = _offerings[i - 1].monthly.product;
-          var cur = _offerings[i].monthly.product;
-
-          if (prev.price < val && val <= cur.price) {
-            i--;
-            break;
-          }
-        }
-        if (val == _offerings.first.monthly.product.price) {
-          i = 0;
-        } else if (val >= _offerings.last.monthly.product.price) {
-          i = _offerings.length - 1;
-        }
-
-        if (i != -1) {
-          setState(() {
-            _selectedOffering = _offerings[i];
-          });
-        }
+    var slider = PurchaseSlider(
+      values: _offerings.map(_fromOffering).toList(),
+      selectedValue: _fromOffering(_selectedOffering),
+      onChanged: (PaymentInfo info) {
+        setState(() {
+          _selectedOffering = _fromPaymentInfo(info);
+        });
       },
-      label: _selectedOffering.monthly.product.priceString,
-      divisions: _offerings.length,
     );
 
     return Column(