widget tests: Add checkErrorDialog

test/widgets/dialog_checks.dart

@@ -0,0 +1,26 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+/// In a widget test, check that showErrorDialog was called with the right text.
+///
+/// Checks for an error dialog matching an expected title
+/// and, optionally, matching an expected message. Fails if none is found.
+///
+/// On success, returns the widget's "OK" button.
+/// Dismiss the dialog by calling `tester.tap(find.byWidget(okButton))`.
+Widget checkErrorDialog(WidgetTester tester, {
+  required String expectedTitle,
+  String? expectedMessage,
+}) {
+  final dialog = tester.widget<AlertDialog>(find.byType(AlertDialog));
+  tester.widget(find.descendant(matchRoot: true,
+    of: find.byWidget(dialog.title!), matching: find.text(expectedTitle)));
+  if (expectedMessage != null) {
+    tester.widget(find.descendant(matchRoot: true,
+      of: find.byWidget(dialog.content!), matching: find.text(expectedMessage)));
+  }
+
+  return tester.widget(
+    find.descendant(of: find.byWidget(dialog),
+      matching: find.widgetWithText(TextButton, 'OK')));
+}