From a0ba2decab156c88708c4261d40660ab8f60da5f Mon Sep 17 00:00:00 2001
From: PurplePolyhedron <120297255+PurplePolyhedron@users.noreply.github.com>
Date: Fri, 29 Nov 2024 18:20:31 +1000
Subject: [PATCH] improve `ContainerRenderObjectMixin` error message when
 `parentData` is not set up properly (#157846)

Previously when subclassing `MultiChildRenderObjectWidget` and
`RenderObject with ContainerRenderObjectMixin`, if one forgot to set up
parent data, the error message does not give hint that `setupParentData`
need to be implemented by the `RenderObject`.

This PR add assertion that check parent data type before type cast and
give hints if it is was not properly set.
## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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---
 .../flutter/lib/src/rendering/object.dart     |  6 ++++
 .../flutter/test/rendering/object_test.dart   | 29 +++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/packages/flutter/lib/src/rendering/object.dart b/packages/flutter/lib/src/rendering/object.dart
index 1c3b3c051e..9c0df3882c 100644
--- a/packages/flutter/lib/src/rendering/object.dart
+++ b/packages/flutter/lib/src/rendering/object.dart
@@ -4353,6 +4353,12 @@ mixin ContainerRenderObjectMixin<ChildType extends RenderObject, ParentDataType
     assert(child != _firstChild);
     assert(child != _lastChild);
     adoptChild(child);
+    assert(
+      child.parentData is ParentDataType,
+      'A child of $runtimeType has parentData of type ${child.parentData.runtimeType}, '
+      'which does not conform to $ParentDataType. Class using ContainerRenderObjectMixin '
+      'should override setupParentData() to set parentData to type $ParentDataType.',
+    );
     _insertIntoChildList(child, after: after);
   }
 
diff --git a/packages/flutter/test/rendering/object_test.dart b/packages/flutter/test/rendering/object_test.dart
index e685b90757..ae9b38c31d 100644
--- a/packages/flutter/test/rendering/object_test.dart
+++ b/packages/flutter/test/rendering/object_test.dart
@@ -377,6 +377,25 @@ void main() {
     root.buildScene(ui.SceneBuilder()).dispose();
     expect(calledBack, true);
   });
+
+  test('ContainerParentDataMixin asserts parentData type', () {
+    final TestRenderObject renderObject = TestRenderObjectWithoutSetupParentData();
+    final TestRenderObject child = TestRenderObject();
+    expect(
+      () => renderObject.add(child),
+      throwsA(
+        isA<AssertionError>().having(
+          (AssertionError error) => error.toString(),
+          'description',
+          contains(
+            'A child of TestRenderObjectWithoutSetupParentData has parentData of type ParentData, '
+            'which does not conform to TestRenderObjectParentData. Class using ContainerRenderObjectMixin '
+            'should override setupParentData() to set parentData to type TestRenderObjectParentData.'
+          ),
+        ),
+      ),
+    );
+  });
 }
 
 
@@ -485,6 +504,16 @@ class TestRenderObject extends RenderObject with ContainerRenderObjectMixin<Test
   }
 }
 
+class TestRenderObjectWithoutSetupParentData extends TestRenderObject {
+  @override
+  void setupParentData(RenderObject child) {
+    // Use a mismatched parent data type.
+    if (child.parentData is! ParentData) {
+      child.parentData = ParentData();
+    }
+  }
+}
+
 class LeaderLayerRenderObject extends RenderObject {
   LeaderLayerRenderObject();