Reland InteractiveViewer.builder (#79287)

packages/flutter/test/widgets/interactive_viewer_test.dart

@@ -1161,6 +1161,89 @@ void main() {
         findsOneWidget,
       );
     });
+
+    testWidgets('builder can change widgets that are off-screen', (WidgetTester tester) async {
+      final TransformationController transformationController = TransformationController();
+      const double childHeight = 10.0;
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: Center(
+              child: SizedBox(
+                height: 50.0,
+                child: InteractiveViewer.builder(
+                  transformationController: transformationController,
+                  scaleEnabled: false,
+                  boundaryMargin: const EdgeInsets.all(double.infinity),
+                  // Build visible children green, off-screen children red.
+                  builder: (BuildContext context, Quad viewportQuad) {
+                    final Rect viewport = _axisAlignedBoundingBox(viewportQuad);
+                    final List<Container> children = <Container>[];
+                    for (int i = 0; i < 10; i++) {
+                      final double childTop = i * childHeight;
+                      final double childBottom = childTop + childHeight;
+                      final bool visible = (childBottom >= viewport.top && childBottom <= viewport.bottom)
+                          || (childTop >= viewport.top && childTop <= viewport.bottom);
+                      children.add(Container(
+                        height: childHeight,
+                        color: visible ? Colors.green : Colors.red,
+                      ));
+                    }
+                    return Column(
+                      children: children,
+                    );
+                  },
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      expect(transformationController.value, equals(Matrix4.identity()));
+
+      // The first six are partially visible and therefore green.
+      int i = 0;
+      for (final Element element in find.byType(Container, skipOffstage: false).evaluate()) {
+        final Container container = element.widget as Container;
+        if (i < 6) {
+          expect(container.color, Colors.green);
+        } else {
+          expect(container.color, Colors.red);
+        }
+        i++;
+      }
+
+      // Drag to pan down past the first child.
+      final Offset childOffset = tester.getTopLeft(find.byType(SizedBox));
+      const double translationY = 15.0;
+      final Offset childInterior = Offset(
+        childOffset.dx,
+        childOffset.dy + translationY,
+      );
+      final TestGesture gesture = await tester.startGesture(childInterior);
+      addTearDown(gesture.removePointer);
+      await tester.pump();
+      await gesture.moveTo(childOffset);
+      await tester.pump();
+      await gesture.up();
+      await tester.pumpAndSettle();
+      expect(transformationController.value, isNot(Matrix4.identity()));
+      expect(transformationController.value.getTranslation().y, -translationY);
+
+      // After scrolling down a bit, the first child is not visible, the next
+      // six are, and the final three are not.
+      i = 0;
+      for (final Element element in find.byType(Container, skipOffstage: false).evaluate()) {
+        final Container container = element.widget as Container;
+        if (i > 0 && i < 7) {
+          expect(container.color, Colors.green);
+        } else {
+          expect(container.color, Colors.red);
+        }
+        i++;
+      }
+    });
   });
 
   group('getNearestPointOnLine', () {
@@ -1370,3 +1453,27 @@ void main() {
     });
   });
 }
+
+// Returns the axis aligned bounding box for the given Quad, which might not
+// be axis aligned.
+Rect _axisAlignedBoundingBox(Quad quad) {
+  double? xMin;
+  double? xMax;
+  double? yMin;
+  double? yMax;
+  for (final Vector3 point in <Vector3>[quad.point0, quad.point1, quad.point2, quad.point3]) {
+    if (xMin == null || point.x < xMin) {
+      xMin = point.x;
+    }
+    if (xMax == null || point.x > xMax) {
+      xMax = point.x;
+    }
+    if (yMin == null || point.y < yMin) {
+      yMin = point.y;
+    }
+    if (yMax == null || point.y > yMax) {
+      yMax = point.y;
+    }
+  }
+  return Rect.fromLTRB(xMin!, yMin!, xMax!, yMax!);
+}