Add the Form widget and get the fields working with it. Still need to pull the values into a usable object for saving.

lib/main.dart

@@ -166,59 +166,106 @@ class ExpenseInputDialog extends StatefulWidget {
 }
 
 class _ExpenseInputDialogState extends State<ExpenseInputDialog> {
+  final _expenseFormKey = GlobalKey<FormState>();
+
   @override
   Widget build(BuildContext context) {
     const inputWidth = 300.0;
     const inputHeight = 50.0;
 
-    const recurrenceValues = <DropdownMenuEntry>[
+    const recurrenceValues = <DropdownMenuItem>[
-      DropdownMenuEntry(value: Recurrence.daily, label: "Daily"),
+      DropdownMenuItem(value: Recurrence.daily, child: Text("Daily")),
-      DropdownMenuEntry(value: Recurrence.weekly, label: "Weekly"),
+      DropdownMenuItem(value: Recurrence.weekly, child: Text("Weekly")),
-      DropdownMenuEntry(value: Recurrence.biweekly, label: "Biweekly"),
+      DropdownMenuItem(value: Recurrence.biweekly, child: Text("Biweekly")),
-      DropdownMenuEntry(value: Recurrence.montly, label: "Monthly"),
+      DropdownMenuItem(value: Recurrence.montly, child: Text("Monthly")),
-      DropdownMenuEntry(value: Recurrence.yearly, label: "Yearly"),
+      DropdownMenuItem(
+          value: Recurrence.yearly,
+          child: Text(
+            "Yearly",
+          )),
     ];
 
     return AlertDialog(
       title: Center(child: Text("Add New Expense")),
-      content: Column(mainAxisSize: MainAxisSize.min, spacing: 10, children: [
+      content: Form(
-        SizedBox(
+        key: _expenseFormKey,
+        autovalidateMode: AutovalidateMode.onUserInteraction,
+        child: Column(mainAxisSize: MainAxisSize.min, spacing: 10, children: [
+          SizedBox(
+              width: inputWidth,
+              height: inputHeight,
+              child: TextFormField(
+                keyboardType: TextInputType.text,
+                decoration: InputDecoration(
+                  labelText: "Name",
+                  hintText: "Example: Red Pocket Phone Bill",
+                ),
+                validator: (value) {
+                  if (value!.isEmpty) {
+                    return "Name must be provided.";
+                  }
+                  return null;
+                },
+                onChanged: (newValue) {
+                  // _expenseFormKey.currentState!.reset();
+                },
+              )),
+          SizedBox(
             width: inputWidth,
             height: inputHeight,
-            child: TextField(
+            child: TextFormField(
-              keyboardType: TextInputType.text,
+              keyboardType: TextInputType.numberWithOptions(decimal: true),
               decoration: InputDecoration(
-                labelText: "Name",
+                  labelText: "Cost", hintText: "Example: 10.00"),
-                hintText: "Example: Red Pocket Phone Bill",
+              validator: (value) {
-              ),
+                if (value!.isEmpty) {
-              // https://docs.flutter.dev/cookbook/forms/retrieve-input
+                  return "Cost must be provided.";
-              //controller: nameFieldController,
+                }
-            )),
+                if (double.tryParse(value!) == null) {
-        SizedBox(
+                  return "Cost must be a valid number.";
-          width: inputWidth,
+                }
-          height: inputHeight,
+                return null;
-          child: TextField(
+              },
-            keyboardType: TextInputType.numberWithOptions(decimal: true),
+              onChanged: (newValue) {
-            decoration:
+                //_expenseFormKey.currentState!.reset();
-                InputDecoration(labelText: "Cost", hintText: "Example: 10.00"),
+              },
+            ),
           ),
-        ),
+          SizedBox(
-        DropdownMenu(
-          dropdownMenuEntries: recurrenceValues,
-          width: inputWidth,
-          label: Text("Recurrence"),
-          hintText: "Example: Monthly",
-        ),
-        SizedBox(
             width: inputWidth,
             height: inputHeight,
-            child: TextField(
+            child: DropdownButtonFormField(
-              keyboardType: TextInputType.text,
+              items: recurrenceValues,
               decoration: InputDecoration(
-                  labelText: "Description",
+                  labelText: "Recurrence", hintText: "Example: Monthly"),
-                  hintText: "Example: 1GB data with unlimited talk & text."),
+              validator: (value) {
-            )),
+                if (value == null) {
-      ]),
+                  return "Recurrence must be provided.";
+                }
+                return null;
+              },
+              onChanged: (newValue) {
+                //_expenseFormKey.currentState!.reset();
+              },
+            ),
+          ),
+          SizedBox(
+              width: inputWidth,
+              height: inputHeight,
+              child: TextFormField(
+                keyboardType: TextInputType.text,
+                decoration: InputDecoration(
+                    labelText: "Description",
+                    hintText: "Example: 1GB data with unlimited talk & text."),
+                validator: (value) {
+                  return null;
+                },
+                onChanged: (newValue) {
+                  //_expenseFormKey.currentState!.reset();
+                },
+              )),
+        ]),
+      ),
       actions: [
         SizedBox(
             width: inputWidth,
@@ -238,7 +285,10 @@ class _ExpenseInputDialogState extends State<ExpenseInputDialog> {
                 ElevatedButton.icon(
                   onPressed: () {
                     print("TODO: Save expense!");
-                    Navigator.of(context).pop();
+                    if (_expenseFormKey.currentState!.validate()) {
+                      _expenseFormKey.currentState!.save();
+                      Navigator.of(context).pop();
+                    }
                   },
                   icon: Icon(Icons.save),
                   label: Text('Submit'),