Show arrow indicator on toolbar (#245)

lib/src/widgets/toolbar.dart

@@ -6,6 +6,7 @@ import 'package:image_picker/image_picker.dart';
 
 import '../models/documents/attribute.dart';
 import 'controller.dart';
+import 'toolbar/arrow_indicated_button_list.dart';
 import 'toolbar/clear_format_button.dart';
 import 'toolbar/color_button.dart';
 import 'toolbar/history_button.dart';
@@ -316,21 +317,9 @@ class _QuillToolbarState extends State<QuillToolbar> {
   @override
   Widget build(BuildContext context) {
     return Container(
-      padding: const EdgeInsets.symmetric(horizontal: 8),
       constraints: BoxConstraints.tightFor(height: widget.preferredSize.height),
       color: Theme.of(context).canvasColor,
-      child: CustomScrollView(
-        scrollDirection: Axis.horizontal,
-        slivers: [
-          SliverFillRemaining(
-            hasScrollBody: false,
-            child: Row(
-              mainAxisAlignment: MainAxisAlignment.spaceEvenly,
-              children: widget.children,
-            ),
-          ),
-        ],
-      ),
+      child: ArrowIndicatedButtonList(buttons: widget.children),
     );
   }
 }

lib/src/widgets/toolbar/arrow_indicated_button_list.dart

@@ -0,0 +1,125 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+
+/// Scrollable list with arrow indicators.
+///
+/// The arrow indicators are automatically hidden if the list is not
+/// scrollable in the direction of the respective arrow.
+class ArrowIndicatedButtonList extends StatefulWidget {
+  const ArrowIndicatedButtonList({required this.buttons, Key? key})
+      : super(key: key);
+
+  final List<Widget> buttons;
+
+  @override
+  _ArrowIndicatedButtonListState createState() =>
+      _ArrowIndicatedButtonListState();
+}
+
+class _ArrowIndicatedButtonListState extends State<ArrowIndicatedButtonList>
+    with WidgetsBindingObserver {
+  final ScrollController _controller = ScrollController();
+  bool _showLeftArrow = false;
+  bool _showRightArrow = false;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller.addListener(_handleScroll);
+
+    // Listening to the WidgetsBinding instance is necessary so that we can
+    // hide the arrows when the window gets a new size and thus the toolbar
+    // becomes scrollable/unscrollable.
+    WidgetsBinding.instance!.addObserver(this);
+
+    // Workaround to allow the scroll controller attach to our ListView so that
+    // we can detect if overflow arrows need to be shown on init.
+    Timer.run(_handleScroll);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      children: <Widget>[
+        _buildLeftArrow(),
+        _buildScrollableList(),
+        _buildRightColor(),
+      ],
+    );
+  }
+
+  @override
+  void didChangeMetrics() => _handleScroll();
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    WidgetsBinding.instance!.removeObserver(this);
+    super.dispose();
+  }
+
+  void _handleScroll() {
+    setState(() {
+      _showLeftArrow =
+          _controller.position.minScrollExtent != _controller.position.pixels;
+      _showRightArrow =
+          _controller.position.maxScrollExtent != _controller.position.pixels;
+    });
+  }
+
+  Widget _buildLeftArrow() {
+    return SizedBox(
+      width: 8,
+      child: Transform.translate(
+        // Move the icon a few pixels to center it
+        offset: const Offset(-5, 0),
+        child: _showLeftArrow ? const Icon(Icons.arrow_left, size: 18) : null,
+      ),
+    );
+  }
+
+  Widget _buildScrollableList() {
+    return Expanded(
+      child: ScrollConfiguration(
+        // Remove the glowing effect, as we already have the arrow indicators
+        behavior: _NoGlowBehavior(),
+        // The CustomScrollView is necessary so that the children are not
+        // stretched to the height of the toolbar, https://bit.ly/3uC3bjI
+        child: CustomScrollView(
+          scrollDirection: Axis.horizontal,
+          controller: _controller,
+          physics: const ClampingScrollPhysics(),
+          slivers: [
+            SliverFillRemaining(
+              hasScrollBody: false,
+              child: Row(
+                mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+                children: widget.buttons,
+              ),
+            )
+          ],
+        ),
+      ),
+    );
+  }
+
+  Widget _buildRightColor() {
+    return SizedBox(
+      width: 8,
+      child: Transform.translate(
+        // Move the icon a few pixels to center it
+        offset: const Offset(-5, 0),
+        child: _showRightArrow ? const Icon(Icons.arrow_right, size: 18) : null,
+      ),
+    );
+  }
+}
+
+/// ScrollBehavior without the Material glow effect.
+class _NoGlowBehavior extends ScrollBehavior {
+  @override
+  Widget buildViewportChrome(BuildContext _, Widget child, AxisDirection __) {
+    return child;
+  }
+}