widget tests: Add checkErrorDialog

Author: chrisbobbe

test/widgets/dialog_checks.dart

@@ -0,0 +1,54 @@
+import 'package:checks/checks.dart';
+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 alertDialogList = tester.widgetList<AlertDialog>(find.byWidgetPredicate((widget) {
+    if (widget is! AlertDialog) {
+      return false;
+    }
+    if (widget.title is! Text || (widget.title as Text).data != expectedTitle) {
+      return false;
+    }
+    if (
+      expectedMessage != null
+      && (
+         widget.content == null
+         || tester.widgetList(
+              find.descendant(of: find.byWidget(widget.content!),
+              matching: find.text(expectedMessage))
+            ).isEmpty
+      )
+    ) {
+      return false;
+    }
+    return true;
+  }));
+  // TODO: Can we give a more helpful, to-the-point error message than this:
+  //
+  //   The following TestFailure was thrown running a test:
+  //   Expected: a Iterable<AlertDialog> that:
+  //     has length that:
+  //       equals <1>
+  //   Actual: a Iterable<AlertDialog> that:
+  //     has length that:
+  //     Actual: <0>
+  //     Which: are not equal
+  check(alertDialogList).length.equals(1);
+
+  return tester.widget(
+    find.descendant(of: find.byWidget(alertDialogList.single),
+    matching: find.byWidgetPredicate((widget) {
+      return widget is TextButton && widget.child is Text && (widget.child as Text).data == 'OK';
+    })));
+}