Fix bug in FadeInImage causing changes in placeholder to be missed (#20356)

packages/flutter/lib/src/widgets/fade_in_image.dart

@@ -393,7 +393,7 @@ class _FadeInImageState extends State<FadeInImage> with TickerProviderStateMixin
   @override
   void didUpdateWidget(FadeInImage oldWidget) {
     super.didUpdateWidget(oldWidget);
-    if (widget.image != oldWidget.image || widget.placeholder != widget.placeholder)
+    if (widget.image != oldWidget.image || widget.placeholder != oldWidget.placeholder)
       _resolveImage();
   }
 

packages/flutter/test/widgets/fade_in_image_test.dart

@@ -13,6 +13,7 @@ Future<Null> main() async {
   // These must run outside test zone to complete
   final ui.Image targetImage = await createTestImage();
   final ui.Image placeholderImage = await createTestImage();
+  final ui.Image secondPlaceholderImage = await createTestImage();
 
   group('FadeInImage', () {
     testWidgets('animates uncached image and shows cached image immediately', (WidgetTester tester) async {
@@ -80,5 +81,40 @@ Future<Null> main() async {
       expect(stateAfterRecreate.phase, FadeInImagePhase.completed); // completes immediately
       expect(displayedImage().image, same(targetImage));
     });
+
+    testWidgets('handles a updating the placeholder image', (WidgetTester tester) async {
+      RawImage displayedImage() => tester.widget(find.byType(RawImage));
+
+      // The placeholder is expected to be already loaded
+      final TestImageProvider placeholderProvider = new TestImageProvider(placeholderImage);
+      final TestImageProvider secondPlaceholderProvider = new TestImageProvider(secondPlaceholderImage);
+
+      // Test case: long loading image
+      final TestImageProvider imageProvider = new TestImageProvider(targetImage);
+
+      await tester.pumpWidget(new FadeInImage(
+        placeholder: placeholderProvider,
+        image: imageProvider,
+        fadeOutDuration: const Duration(milliseconds: 50),
+        fadeInDuration: const Duration(milliseconds: 50),
+      ));
+      placeholderProvider.complete();
+      await tester.pump();
+
+      expect(displayedImage().image, same(placeholderImage)); // placeholder completed
+      expect(displayedImage().image, isNot(same(secondPlaceholderImage)));
+
+      await tester.pumpWidget(new FadeInImage(
+        placeholder: secondPlaceholderProvider,
+        image: imageProvider,
+        fadeOutDuration: const Duration(milliseconds: 50),
+        fadeInDuration: const Duration(milliseconds: 50),
+      ));
+      secondPlaceholderProvider.complete();
+      await tester.pump();
+
+      expect(displayedImage().image, isNot(same(placeholderImage))); // placeholder replaced
+      expect(displayedImage().image, same(secondPlaceholderImage));
+    });
   });
 }