[vector_graphics] Allow transition between placeholder and loaded image to have an animation (flutter#8195)

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue.*
[Issue #158857](flutter/flutter#158857)

Author: raju-muliyashiya

packages/vector_graphics/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.1.18
+
+* Allow transition between placeholder and loaded image to have an animation.
+
 ## 1.1.17
 
 * Reverts leaker tracker changes that caused runtime exceptions.

packages/vector_graphics/lib/src/vector_graphics.dart

@@ -61,6 +61,7 @@ VectorGraphic createCompatVectorGraphic({
   String? semanticsLabel,
   bool excludeFromSemantics = false,
   Clip clipBehavior = Clip.hardEdge,
+  Duration? transitionDuration,
   WidgetBuilder? placeholderBuilder,
   VectorGraphicsErrorWidget? errorBuilder,
   ColorFilter? colorFilter,
@@ -79,6 +80,7 @@ VectorGraphic createCompatVectorGraphic({
     semanticsLabel: semanticsLabel,
     excludeFromSemantics: excludeFromSemantics,
     clipBehavior: clipBehavior,
+    transitionDuration: transitionDuration,
     placeholderBuilder: placeholderBuilder,
     errorBuilder: errorBuilder,
     colorFilter: colorFilter,
@@ -118,6 +120,7 @@ class VectorGraphic extends StatefulWidget {
     this.semanticsLabel,
     this.excludeFromSemantics = false,
     this.clipBehavior = Clip.hardEdge,
+    this.transitionDuration,
     this.placeholderBuilder,
     this.errorBuilder,
     this.colorFilter,
@@ -137,6 +140,7 @@ class VectorGraphic extends StatefulWidget {
     this.semanticsLabel,
     this.excludeFromSemantics = false,
     this.clipBehavior = Clip.hardEdge,
+    this.transitionDuration,
     this.placeholderBuilder,
     this.errorBuilder,
     this.colorFilter,
@@ -218,6 +222,9 @@ class VectorGraphic extends StatefulWidget {
   /// A callback that fires if some exception happens during data acquisition or decoding.
   final VectorGraphicsErrorWidget? errorBuilder;
 
+  /// Set transition duration while switching from placeholder to url image
+  final Duration? transitionDuration;
+
   /// If provided, a color filter to apply to the vector graphic when painting.
   ///
   /// For example, `ColorFilter.mode(Colors.red, BlendMode.srcIn)` to give the vector
@@ -517,6 +524,19 @@ class _VectorGraphicWidgetState extends State<VectorGraphic> {
           );
     }
 
+    if (widget.transitionDuration != null) {
+      child = AnimatedSwitcher(
+        duration: widget.transitionDuration!,
+        child: child,
+        transitionBuilder: (Widget child, Animation<double> animation) {
+          return FadeTransition(
+            opacity: animation,
+            child: child,
+          );
+        },
+      );
+    }
+
     if (!widget.excludeFromSemantics) {
       child = Semantics(
         container: widget.semanticsLabel != null,

packages/vector_graphics/pubspec.yaml

@@ -2,7 +2,7 @@ name: vector_graphics
 description: A vector graphics rendering package for Flutter using a binary encoding.
 repository: https://github.com/flutter/packages/tree/main/packages/vector_graphics
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+vector_graphics%22
-version: 1.1.17
+version: 1.1.18
 
 environment:
   sdk: ^3.4.0

packages/vector_graphics/test/vector_graphics_test.dart

@@ -314,6 +314,32 @@ void main() {
     expect(debugLastTextDirection, TextDirection.ltr);
   });
 
+  testWidgets('Test animated switch between placeholder and image',
+      (WidgetTester tester) async {
+    final TestAssetBundle testBundle = TestAssetBundle();
+    const Text placeholderWidget = Text('Placeholder');
+
+    await tester.pumpWidget(DefaultAssetBundle(
+      bundle: testBundle,
+      child: Directionality(
+        textDirection: TextDirection.rtl,
+        child: VectorGraphic(
+          loader: const AssetBytesLoader('bar.svg'),
+          placeholderBuilder: (BuildContext context) => placeholderWidget,
+          transitionDuration: const Duration(microseconds: 500),
+        ),
+      ),
+    ));
+
+    expect(find.text('Placeholder'), findsOneWidget);
+    expect(find.byType(Container), findsNothing); // No image yet
+
+    await tester.pumpAndSettle(const Duration(microseconds: 500));
+
+    expect(find.text('Placeholder'), findsNothing);
+    expect(testBundle.loadKeys, <String>['bar.svg']);
+  });
+
   testWidgets('Can exclude from semantics', (WidgetTester tester) async {
     final TestAssetBundle testBundle = TestAssetBundle();