Add directionality to all adpaters

example/lib/examples/adapter_view.dart

@@ -110,7 +110,7 @@ class AdapterView extends StatelessWidget {
             .animate(
               adapter: ScrollAdapter(
                 scrollController,
-                animated: true, // smooth the inputs
+                animated: true, // smooth the scroll input
               ),
             )
             .scaleXY(end: 0.5, curve: Curves.easeIn)

lib/src/adapters/adapter.dart

@@ -8,24 +8,29 @@ import 'package:flutter/widgets.dart';
 /// an animation with a slider input, or progressing an animation based on
 /// the time of day.
 ///
-/// Adapters must expose an `attach` method which accepts the
+/// [animated] specifies that the adapter should animate to new values. If `false`, it
+/// will jump to the new value, if `true` it will animate to the value using a
+/// duration calculated from the animation's total duration and the value change.
+/// Defaults to `false`.
+/// 
+/// Setting [direction] to [Direction.forward] or [Direction.reverse] will cause
+/// the adapter to only update if the new value is greater than or less than the
+/// current value respectively.
+///
+/// Adapter implementations must expose an [attach] method which accepts the
 /// [AnimationController] used by an [Animate] instance, and adds the logic
 /// to drive it from an external source by updating its `value` (0-1). See the
 /// included adapters for implementation examples.
 abstract class Adapter {
-  Adapter({bool? animated}) : animated = animated ?? false;
+  Adapter({bool? animated, this.direction}) : animated = animated ?? false;
 
-  /// Indicates whether the adapter should animate to new values. If `false`, it
-  /// will jump to the new value, if `true` it will animate to the value using a
-  /// duration calculated from the animation's total duration and the value change.
-  /// Defaults to `false`.
   final bool animated;
 
-  AnimationController? _controller;
+  final Direction? direction;
 
-  // properties to support animated:
+  AnimationController? _controller;
   Ticker? _ticker;
-  double _target = 0;
+  double _value = 0;
   int _prevT = 0;
 
   // this is called by Animate to associate the AnimationController.
@@ -43,19 +48,24 @@ abstract class Adapter {
   void config(AnimationController controller, double value) {
     assert(_controller == null, 'An adapter was assigned twice.');
     _controller = controller;
-    _controller?.value = value;
+    _controller?.value = _value = value;
     _ticker = Ticker(_tick);
   }
 
-  // called by implementers to update the value.
+  // called by implementers to update the value. Manages direction and animated.
   void updateValue(double value) {
     AnimationController controller = _controller!;
+    if (_value == value ||
+        (direction == Direction.forward && value < _value) ||
+        (direction == Direction.reverse && value > _value)) {
+      return;
+    }
+    _value = value;
 
     if (!animated) {
       controller.value = value;
     } else if (value != controller.value) {
       Ticker ticker = _ticker!;
-      _target = value;
       _prevT = DateTime.now().microsecondsSinceEpoch;
       if (!ticker.isActive) ticker.start();
     }
@@ -71,14 +81,16 @@ abstract class Adapter {
     double d = (t - _prevT) / controller.duration!.inMicroseconds;
     double val = controller.value;
 
-    if (val < _target) {
-      val = min(_target, val + d);
+    if (val < _value) {
+      val = min(_value, val + d);
     } else {
-      val = max(_target, val - d);
+      val = max(_value, val - d);
     }
 
     _prevT = t;
     controller.value = val;
-    if (val == _target) _ticker!.stop();
+    if (val == _value) _ticker!.stop();
   }
 }
+
+enum Direction { forward, reverse }

lib/src/adapters/change_notifier_adapter.dart

@@ -4,9 +4,15 @@ import '../../flutter_animate.dart';
 
 /// Drives an [Animate] animation from a [ChangeNotifier]. The [valueGetter]
 /// should provide a value in the range `0-1` when a change occurs.
+///
+/// See [Adapter] for information on [direction] and [animated].
 class ChangeNotifierAdapter extends Adapter {
-  ChangeNotifierAdapter(this.notifier, this.valueGetter, {bool? animated})
-      : super(animated: animated);
+  ChangeNotifierAdapter(
+    this.notifier,
+    this.valueGetter, {
+    bool? animated,
+    Direction? direction,
+  }) : super(animated: animated, direction: direction);
 
   final ChangeNotifier notifier;
   final ValueGetter<double> valueGetter;

lib/src/adapters/scroll_adapter.dart

@@ -28,34 +28,28 @@ import '../../flutter_animate.dart';
 ///   )
 /// ).fadeIn().slide();
 /// ```
+/// 
+/// See [Adapter] for information on [direction] and [animated].
 class ScrollAdapter extends Adapter {
   ScrollAdapter(
     this.scrollController, {
     this.begin,
     this.end,
-    this.direction,
     bool? animated,
-  }) : super(animated: animated);
+    Direction? direction,
+  }) : super(animated: animated, direction: direction);
 
   final ScrollController scrollController;
   final double? begin;
   final double? end;
-  final ScrollDirection? direction;
-  double? _value;
 
   @override
   void attach(AnimationController controller) {
-    _value = _getValue();
-    config(controller, _value ?? 0);
+    config(controller, _getValue() ?? 0);
     scrollController.addListener(() {
-      double? old = _value, val = _getValue();
-      if (val == null || old == val) return; // no clients or no change
-      if (old == null ||
-          direction == null ||
-          (direction == ScrollDirection.forward && val > old) ||
-          (direction == ScrollDirection.reverse && val < old)) {
-        updateValue(_value = val);
-      }
+      double? value = _getValue();
+      if (value == null) return; // no clients
+      updateValue(value);
     });
   }
 

lib/src/adapters/value_adapter.dart

@@ -15,9 +15,15 @@ import '../../flutter_animate.dart';
 /// Text("Hello").animate(adapter: ValueAdapter(_sliderVal))
 ///   .fadeIn().slide();
 /// ```
+///
+/// See [Adapter] for information on [direction] and [animated].
 class ValueAdapter extends ValueNotifierAdapter {
-  ValueAdapter(double value, {bool? animated})
-      : super(ValueNotifier<double>(value), animated: animated);
+  ValueAdapter(double value, {bool? animated, Direction? direction})
+      : super(
+          ValueNotifier<double>(value),
+          animated: animated,
+          direction: direction,
+        );
 
   set value(double value) => notifier.value = value;
 }

lib/src/adapters/value_notifier_adapter.dart

@@ -4,9 +4,11 @@ import '../../flutter_animate.dart';
 
 /// Drives an [Animate] animation from a [ValueNotifier]. The value from the
 /// notifier should be in the range `0-1`.
+///
+/// See [Adapter] for information on [direction] and [animated].
 class ValueNotifierAdapter extends Adapter {
-  ValueNotifierAdapter(this.notifier, {bool? animated})
-      : super(animated: animated);
+  ValueNotifierAdapter(this.notifier, {bool? animated, Direction? direction})
+      : super(animated: animated, direction: direction);
 
   final ValueNotifier<double> notifier;