improve ContainerRenderObjectMixin error message when parentData …

…is not set up properly (flutter#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.
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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);
   }
 

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();