Add onVisible callback to snackbar. (#42344)

packages/flutter/lib/src/material/snack_bar.dart

@@ -161,7 +161,7 @@ class _SnackBarActionState extends State<SnackBarAction> {
 ///  * [SnackBarThemeData], to configure the default property values for
 ///    [SnackBar] widgets.
 ///  * <https://material.io/design/components/snackbars.html>
-class SnackBar extends StatelessWidget {
+class SnackBar extends StatefulWidget {
   /// Creates a snack bar.
   ///
   /// The [content] argument must be non-null. The [elevation] must be null or
@@ -176,6 +176,7 @@ class SnackBar extends StatelessWidget {
     this.action,
     this.duration = _snackBarDisplayDuration,
     this.animation,
+    this.onVisible,
   }) : assert(elevation == null || elevation >= 0.0),
        assert(content != null),
        assert(duration != null),
@@ -245,10 +246,86 @@ class SnackBar extends StatelessWidget {
   /// The animation driving the entrance and exit of the snack bar.
   final Animation<double> animation;
 
+  /// Called the first time that the snackbar is visible within a [Scaffold].
+  final VoidCallback onVisible;
+
+  // API for Scaffold.showSnackBar():
+
+  /// Creates an animation controller useful for driving a snack bar's entrance and exit animation.
+  static AnimationController createAnimationController({ @required TickerProvider vsync }) {
+    return AnimationController(
+      duration: _snackBarTransitionDuration,
+      debugLabel: 'SnackBar',
+      vsync: vsync,
+    );
+  }
+
+  /// Creates a copy of this snack bar but with the animation replaced with the given animation.
+  ///
+  /// If the original snack bar lacks a key, the newly created snack bar will
+  /// use the given fallback key.
+  SnackBar withAnimation(Animation<double> newAnimation, { Key fallbackKey }) {
+    return SnackBar(
+      key: key ?? fallbackKey,
+      content: content,
+      backgroundColor: backgroundColor,
+      elevation: elevation,
+      shape: shape,
+      behavior: behavior,
+      action: action,
+      duration: duration,
+      animation: newAnimation,
+      onVisible: onVisible,
+    );
+  }
+
+  @override
+  State<SnackBar> createState() => _SnackBarState();
+}
+
+
+class _SnackBarState extends State<SnackBar> {
+  bool _wasVisible = false;
+
+  @override
+  void initState() {
+    super.initState();
+    widget.animation.addStatusListener(_onAnimationStatusChanged);
+  }
+
+  @override
+  void didUpdateWidget(SnackBar oldWidget) {
+    if (widget.animation != oldWidget.animation) {
+      oldWidget.animation.removeStatusListener(_onAnimationStatusChanged);
+      widget.animation.addStatusListener(_onAnimationStatusChanged);
+    }
+    super.didUpdateWidget(oldWidget);
+  }
+
+  @override
+  void dispose() {
+    widget.animation.removeStatusListener(_onAnimationStatusChanged);
+    super.dispose();
+  }
+
+  void _onAnimationStatusChanged(AnimationStatus animationStatus) {
+    switch (animationStatus) {
+      case AnimationStatus.dismissed:
+      case AnimationStatus.forward:
+      case AnimationStatus.reverse:
+        break;
+      case AnimationStatus.completed:
+        if (widget.onVisible != null && !_wasVisible) {
+          widget.onVisible();
+        }
+        _wasVisible = true;
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final MediaQueryData mediaQueryData = MediaQuery.of(context);
-    assert(animation != null);
+    assert(widget.animation != null);
     final ThemeData theme = Theme.of(context);
     final ColorScheme colorScheme = theme.colorScheme;
     final SnackBarThemeData snackBarTheme = theme.snackBarTheme;
@@ -284,14 +361,14 @@ class SnackBar extends StatelessWidget {
     );
 
     final TextStyle contentTextStyle = snackBarTheme.contentTextStyle ?? inverseTheme.textTheme.subhead;
-    final SnackBarBehavior snackBarBehavior = behavior ?? snackBarTheme.behavior ?? SnackBarBehavior.fixed;
+    final SnackBarBehavior snackBarBehavior = widget.behavior ?? snackBarTheme.behavior ?? SnackBarBehavior.fixed;
     final bool isFloatingSnackBar = snackBarBehavior == SnackBarBehavior.floating;
     final double snackBarPadding = isFloatingSnackBar ? 16.0 : 24.0;
 
-    final CurvedAnimation heightAnimation = CurvedAnimation(parent: animation, curve: _snackBarHeightCurve);
-    final CurvedAnimation fadeInAnimation = CurvedAnimation(parent: animation, curve: _snackBarFadeInCurve);
+    final CurvedAnimation heightAnimation = CurvedAnimation(parent: widget.animation, curve: _snackBarHeightCurve);
+    final CurvedAnimation fadeInAnimation = CurvedAnimation(parent: widget.animation, curve: _snackBarFadeInCurve);
     final CurvedAnimation fadeOutAnimation = CurvedAnimation(
-      parent: animation,
+      parent: widget.animation,
       curve: _snackBarFadeOutCurve,
       reverseCurve: const Threshold(0.0),
     );
@@ -308,16 +385,16 @@ class SnackBar extends StatelessWidget {
               padding: const EdgeInsets.symmetric(vertical: _singleLineVerticalPadding),
               child: DefaultTextStyle(
                 style: contentTextStyle,
-                child: content,
+                child: widget.content,
               ),
             ),
           ),
-          if (action != null)
+          if (widget.action != null)
             ButtonTheme(
               textTheme: ButtonTextTheme.accent,
               minWidth: 64.0,
               padding: EdgeInsets.symmetric(horizontal: snackBarPadding),
-              child: action,
+              child: widget.action,
             )
           else
             SizedBox(width: snackBarPadding),
@@ -325,9 +402,9 @@ class SnackBar extends StatelessWidget {
       ),
     );
 
-    final double elevation = this.elevation ?? snackBarTheme.elevation ?? 6.0;
-    final Color backgroundColor = this.backgroundColor ?? snackBarTheme.backgroundColor ?? inverseTheme.backgroundColor;
-    final ShapeBorder shape = this.shape
+    final double elevation = widget.elevation ?? snackBarTheme.elevation ?? 6.0;
+    final Color backgroundColor = widget.backgroundColor ?? snackBarTheme.backgroundColor ?? inverseTheme.backgroundColor;
+    final ShapeBorder shape = widget.shape
       ?? snackBarTheme.shape
       ?? (isFloatingSnackBar ? RoundedRectangleBorder(borderRadius: BorderRadius.circular(4.0)) : null);
 
@@ -394,33 +471,4 @@ class SnackBar extends StatelessWidget {
 
     return ClipRect(child: snackBarTransition);
   }
-
-  // API for Scaffold.addSnackBar():
-
-  /// Creates an animation controller useful for driving a snack bar's entrance and exit animation.
-  static AnimationController createAnimationController({ @required TickerProvider vsync }) {
-    return AnimationController(
-      duration: _snackBarTransitionDuration,
-      debugLabel: 'SnackBar',
-      vsync: vsync,
-    );
-  }
-
-  /// Creates a copy of this snack bar but with the animation replaced with the given animation.
-  ///
-  /// If the original snack bar lacks a key, the newly created snack bar will
-  /// use the given fallback key.
-  SnackBar withAnimation(Animation<double> newAnimation, { Key fallbackKey }) {
-    return SnackBar(
-      key: key ?? fallbackKey,
-      content: content,
-      backgroundColor: backgroundColor,
-      elevation: elevation,
-      shape: shape,
-      behavior: behavior,
-      action: action,
-      duration: duration,
-      animation: newAnimation,
-    );
-  }
 }

packages/flutter/test/material/snack_bar_test.dart

@@ -1065,4 +1065,85 @@ void main() {
     await tester.tap(find.byKey(tapTarget));
     expect(tester.takeException(), isNotNull);
   });
+
+  testWidgets('Snackbar calls onVisible once', (WidgetTester tester) async {
+    const Key tapTarget = Key('tap-target');
+    int called = 0;
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                Scaffold.of(context).showSnackBar(SnackBar(
+                  content: const Text('hello'),
+                  duration: const Duration(seconds: 1),
+                  onVisible: () {
+                    called += 1;
+                  },
+                ));
+              },
+              behavior: HitTestBehavior.opaque,
+              child: Container(
+                height: 100.0,
+                width: 100.0,
+                key: tapTarget,
+              ),
+            );
+          },
+        ),
+      ),
+    ));
+
+    await tester.tap(find.byKey(tapTarget));
+    await tester.pump(); // start animation
+    await tester.pumpAndSettle();
+
+    expect(find.text('hello'), findsOneWidget);
+    expect(called, 1);
+  });
+
+  testWidgets('Snackbar does not call onVisible when it is queued', (WidgetTester tester) async {
+    const Key tapTarget = Key('tap-target');
+    int called = 0;
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                Scaffold.of(context).showSnackBar(SnackBar(
+                  content: const Text('hello'),
+                  duration: const Duration(seconds: 1),
+                  onVisible: () {
+                    called += 1;
+                  },
+                ));
+                Scaffold.of(context).showSnackBar(SnackBar(
+                  content: const Text('hello 2'),
+                  duration: const Duration(seconds: 1),
+                  onVisible: () {
+                    called += 1;
+                  },
+                ));
+              },
+              behavior: HitTestBehavior.opaque,
+              child: Container(
+                height: 100.0,
+                width: 100.0,
+                key: tapTarget,
+              ),
+            );
+          },
+        ),
+      ),
+    ));
+
+    await tester.tap(find.byKey(tapTarget));
+    await tester.pump(); // start animation
+    await tester.pumpAndSettle();
+
+    expect(find.text('hello'), findsOneWidget);
+    expect(called, 1);
+  });
 }