Add "QuantityField"

Juicy juicy refactoring

lib/widget/fields.dart

@@ -0,0 +1,26 @@
+
+import 'package:flutter/material.dart';
+
+class QuantityField extends TextFormField {
+
+  QuantityField({String label = "", String hint = "", double max = null, TextEditingController controller}) :
+      super(
+        decoration: InputDecoration(
+          labelText: label,
+          hintText: hint,
+        ),
+        controller: controller,
+        keyboardType: TextInputType.numberWithOptions(signed: false, decimal: true),
+        validator: (value) {
+          if (value.isEmpty) return "Quantity is empty";
+
+          double quantity = double.tryParse(value);
+
+          if (quantity == null) return "Invalid quantity";
+          if (quantity <= 0) return "Quantity must be positive";
+          if ((max != null) && (quantity > max)) return "Quantity must not exceed ${max}";
+
+          return null;
+        },
+      );
+}