Document ScrollPosition (#9731)

packages/flutter/lib/src/rendering/viewport_offset.dart

@@ -55,7 +55,8 @@ ScrollDirection flipScrollDirection(ScrollDirection direction) {
 /// select which part of its content to display. As the user scrolls the
 /// viewport, this value changes, which changes the content that is displayed.
 ///
-/// This object notifies its listeners when [pixels] changes.
+/// This object is a [Listable] that notifies its listeners when [pixels]
+/// changes.
 ///
 /// See also:
 ///
@@ -154,6 +155,9 @@ abstract class ViewportOffset extends ChangeNotifier {
   /// The direction in which the user is trying to change [pixels], relative to
   /// the viewport's [RenderViewport.axisDirection].
   ///
+  /// If the user is not scrolling, this will return [ScrollDirection.idle] even
+  /// if there is an [activity] currently animating the position.
+  ///
   /// This is used by some slivers to determine how to react to a change in
   /// scroll offset. For example, [RenderSliverFloatingPersistentHeader] will
   /// only expand a floating app bar when the [userScrollDirection] is in the

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

@@ -81,16 +81,15 @@ class ScrollPhysics {
 
   /// Determines the overscroll by applying the boundary conditions.
   ///
-  /// Called by [ScrollPositionWithSingleContext.applyBoundaryConditions], which
-  /// is called by [ScrollPositionWithSingleContext.setPixels] just before the
-  /// [ScrollPosition.pixels] value is updated, to determine how much of the
-  /// offset is to be clamped off and sent to
-  /// [ScrollPositionWithSingleContext.didOverscrollBy].
+  /// Called by [ScrollPosition.applyBoundaryConditions], which is called by
+  /// [ScrollPosition.setPixels] just before the [ScrollPosition.pixels] value
+  /// is updated, to determine how much of the offset is to be clamped off and
+  /// sent to [ScrollPosition.didOverscrollBy].
   ///
   /// The `value` argument is guaranteed to not equal [pixels] when this is
   /// called.
   ///
-  /// It is possible for this method to be called when the [position] describes
+  /// It is possible for this method to be called when the `position` describes
   /// an already-out-of-bounds position. In that case, the boundary conditions
   /// should usually only prevent a further increase in the extent to which the
   /// position is out of bounds, allowing a decrease to be applied successfully,

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

@@ -20,14 +20,46 @@ import 'scroll_physics.dart';
 
 export 'scroll_activity.dart' show ScrollHoldController;
 
-// ## Subclassing ScrollPosition
-//
-//  * Describe how to impelement [absorb]
-//     - May need to start an idle activity
-//     - May need to update the activity's idea of what the delegate is
-//     - etc
-//  * Need to call [didUpdateScrollDirection] when changing [userScrollDirection]
+/// Determines which portion of the content is visible in a scroll view.
+///
+/// The [pixels] value determines the scroll offset that the scroll view uses to
+/// select which part of its content to display. As the user scrolls the
+/// viewport, this value changes, which changes the content that is displayed.
+///
+/// The [ScrollPosition] applies [physics] to scrolling, and stores the
+/// [minScrollExtent] and [maxScrollExtent].
+///
+/// Scrolling is controlled by the current [activity], which is set by
+/// [beginActivity]. [ScrollPosition] itself does not start any activities.
+/// Instead, concrete subclasses, such as [ScrollPositionWithSingleContext],
+/// typically start activities in response to user input or instructions from a
+/// [ScrollController].
+///
+/// This object is a [Listable] that notifies its listeners when [pixels]
+/// changes.
+///
+/// ## Subclassing ScrollPosition
+///
+/// Over time, a [Scrollable] might have many different [ScrollPosition]
+/// objects. For example, if [Scrollable.physics] changes type, [Scrollable]
+/// creates a new [ScrollPosition] with the new physics. To transfer state from
+/// the old instance to the new instance, subclasses implement [absorb]. See
+/// [absorb] for more details.
+///
+/// Subclasses also need to call [didUpdateScrollDirection] whenever
+/// [userScrollDirection] changes values.
+///
+/// See also:
+///
+///  * [Scrollable], which uses a [ScrollPosition] to determine which portion of
+///    its content to display.
+///  * [ScrollController], which can be used with [ListView], [GridView] and
+///    other scrollable widgets to control a [ScrollPosition].
+///  * [ScrollPositionWithSingleContext], which is the most commonly used
+///    concrete subclass of [ScrollPosition].
 abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
+  /// Creates an object that determines which portion of the content is visible
+  /// in a scroll view.
   ScrollPosition({
     @required this.physics,
     @required this.context,
@@ -41,8 +73,19 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
       absorb(oldPosition);
   }
 
+  /// How the scroll position should respond to user input.
+  ///
+  /// For example, determines how the widget continues to animate after the
+  /// user stops dragging the scroll view.
   final ScrollPhysics physics;
+
+  /// Where the scrolling is taking place.
+  ///
+  /// Typically implemented by [ScrollableState].
   final ScrollContext context;
+
+  /// A label that is used in the [toString] output. Intended to aid with
+  /// identifying animation controller instances in debug output.
   final String debugLabel;
 
   @override
@@ -72,11 +115,30 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   /// Take any current applicable state from the given [ScrollPosition].
   ///
   /// This method is called by the constructor if it is given an `oldPosition`.
+  /// The `other` argument might not have the same [runtimeType] as this object.
   ///
   /// This method can be destructive to the other [ScrollPosition]. The other
   /// object must be disposed immediately after this call (in the same call
   /// stack, before microtask resolution, by whomever called this object's
   /// constructor).
+  ///
+  /// If the old [ScrollPosition] object is a different [runtimeType] than this
+  /// one, the [ScrollActivity.resetActivity] method is invoked on the newly
+  /// adopted [ScrollActivity].
+  ///
+  /// ## Overriding
+  ///
+  /// Overrides of this method must call `super.absorb` after setting any
+  /// metrics-related or activity-related state, since this method may restart
+  /// the activity and scroll activities tend to use those metrics when being
+  /// restarted.
+  ///
+  /// Overrides of this method might need to start an [IdleScrollActivity] if
+  /// they are unable to absorb the activity from the other [ScrollPosition].
+  ///
+  /// Overrides of this method might also need to update the delegates of
+  /// absorbed scroll activities if they use themselves as a
+  /// [ScrollActivityDelegate].
   @protected
   @mustCallSuper
   void absorb(ScrollPosition other) {
@@ -206,6 +268,12 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     notifyListeners();
   }
 
+  /// Returns the overscroll by applying the boundary conditions.
+  ///
+  /// If the given value is in bounds, returns 0.0. Otherwise, returns the
+  /// amount of value that cannot be applied to [pixels] as a result of the
+  /// boundary conditions. If the [physics] allow out-of-bounds scrolling, this
+  /// method always returns 0.0.
   @protected
   double applyBoundaryConditions(double value) {
     final double result = physics.applyBoundaryConditions(this, value);
@@ -256,6 +324,25 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     return true;
   }
 
+  /// Notifies the activity that the dimensions of the underlying viewport or
+  /// contents have changed.
+  ///
+  /// Called after [applyViewportDimension] or [applyContentDimensions] have
+  /// changed the [minScrollExtent], the [maxScrollExtent], or the
+  /// [viewportDimension]. When this method is called, it should be called
+  /// _after_ any corrections are applied to [pixels] using [correctPixels], not
+  /// before.
+  ///
+  /// The default implementation informs the [activity] of the new dimensions by
+  /// calling [ScrollActivityDelegate.applyNewDimensions].
+  ///
+  /// See also:
+  ///
+  /// * [applyViewportDimension], which is called when new
+  ///   viewport dimensions are established.
+  /// * [applyContentDimensions], which is called after new
+  ///   viewport dimensions are established, and also if new content dimensions
+  ///   are established, and which calls [ScrollPosition.applyNewDimensions].
   @protected
   @mustCallSuper
   void applyNewDimensions() {
@@ -294,21 +381,73 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   /// [State.dispose] method.
   final ValueNotifier<bool> isScrollingNotifier = new ValueNotifier<bool>(false);
 
+  /// Animates the position from its current value to the given value.
+  ///
+  /// Any active animation is canceled. If the user is currently scrolling, that
+  /// action is canceled.
+  ///
+  /// The returned [Future] will complete when the animation ends, whether it
+  /// completed successfully or whether it was interrupted prematurely.
+  ///
+  /// An animation will be interrupted whenever the user attempts to scroll
+  /// manually, or whenever another activity is started, or whenever the
+  /// animation reaches the edge of the viewport and attempts to overscroll. (If
+  /// the [ScrollPosition] does not overscroll but instead allows scrolling
+  /// beyond the extents, then going beyond the extents will not interrupt the
+  /// animation.)
+  ///
+  /// The animation is indifferent to changes to the viewport or content
+  /// dimensions.
+  ///
+  /// Once the animation has completed, the scroll position will attempt to
+  /// begin a ballistic activity in case its value is not stable (for example,
+  /// if it is scrolled beyond the extents and in that situation the scroll
+  /// position would normally bounce back).
+  ///
+  /// The duration must not be zero. To jump to a particular value without an
+  /// animation, use [jumpTo].
+  ///
+  /// The animation is typically handled by an [DrivenScrollActivity].
   Future<Null> animateTo(double to, {
     @required Duration duration,
     @required Curve curve,
   });
 
+  /// Jumps the scroll position from its current value to the given value,
+  /// without animation, and without checking if the new value is in range.
+  ///
+  /// Any active animation is canceled. If the user is currently scrolling, that
+  /// action is canceled.
+  ///
+  /// If this method changes the scroll position, a sequence of start/update/end
+  /// scroll notifications will be dispatched. No overscroll notifications can
+  /// be generated by this method.
+  ///
+  /// If settle is true then, immediately after the jump, a ballistic activity
+  /// is started, in case the value was out of range.
   void jumpTo(double value);
 
   /// Deprecated. Use [jumpTo] or a custom [ScrollPosition] instead.
   @Deprecated('This will lead to bugs.')
   void jumpToWithoutSettling(double value);
 
+  /// Stop the current activity and start a [HoldScrollActivity].
   ScrollHoldController hold(VoidCallback holdCancelCallback);
 
+  /// Start a drag activity corresponding to the given [DragStartDetails].
+  ///
+  /// The `onDragCanceled` argument will be invoked if the drag is ended
+  /// prematurely (e.g. from another activity taking over). See
+  /// [ScrollDragController.onDragCanceled] for details.
   Drag drag(DragStartDetails details, VoidCallback dragCancelCallback);
 
+  /// The currently operative [ScrollActivity].
+  ///
+  /// If the scroll position is not performing any more specific activity, the
+  /// activity will be an [IdleScrollActivity]. To determine whether the scroll
+  /// position is idle, check the [isScrollingNotifier].
+  ///
+  /// Call [beginActivity] to change the current activity.
   @protected
   ScrollActivity get activity => _activity;
   ScrollActivity _activity;
@@ -335,9 +474,9 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
       wasScrolling = false;
     }
     _activity = newActivity;
-    isScrollingNotifier.value = activity.isScrolling;
     if (oldIgnorePointer != activity.shouldIgnorePointer)
       context.setIgnorePointer(activity.shouldIgnorePointer);
+    isScrollingNotifier.value = activity.isScrolling;
     if (!wasScrolling && _activity.isScrolling)
       didStartScroll();
   }

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

@@ -76,25 +76,6 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     correctPixels(pixels + correction);
   }
 
-  /// Take any current applicable state from the given [ScrollPosition].
-  ///
-  /// This method is called by the constructor, before calling [ensureActivity],
-  /// if it is given an `oldPosition`. It adopts the old position's current
-  /// [activity] as its own.
-  ///
-  /// This method is destructive to the other [ScrollPosition]. The other
-  /// object must be disposed immediately after this call (in the same call
-  /// stack, before microtask resolution, by whomever called this object's
-  /// constructor).
-  ///
-  /// If the old [ScrollPosition] object is a different [runtimeType] than this
-  /// one, the [ScrollActivity.resetActivity] method is invoked on the newly
-  /// adopted [ScrollActivity].
-  ///
-  /// When overriding this method, call `super.absorb` after setting any
-  /// metrics-related or activity-related state, since this method may restart
-  /// the activity and scroll activities tend to use those metrics when being
-  /// restarted.
   @override
   void absorb(ScrollPosition other) {
     super.absorb(other);
@@ -113,28 +94,12 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     }
   }
 
-  /// Notifies the activity that the dimensions of the underlying viewport or
-  /// contents have changed.
-  ///
-  /// When this method is called, it should be called _after_ any corrections
-  /// are applied to [pixels] using [correctPixels], not before.
-  ///
-  /// See also:
-  ///
-  /// * [ScrollPosition.applyViewportDimension], which is called when new
-  ///   viewport dimensions are established.
-  /// * [ScrollPosition.applyContentDimensions], which is called after new
-  ///   viewport dimensions are established, and also if new content dimensions
-  ///   are established, and which calls [ScrollPosition.applyNewDimensions].
   @override
   void applyNewDimensions() {
     super.applyNewDimensions();
     context.setCanDrag(physics.shouldAcceptUserOffset(this));
   }
 
-
-  // SCROLL ACTIVITIES
-
   @override
   void beginActivity(ScrollActivity newActivity) {
     if (newActivity == null)
@@ -153,8 +118,6 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     setPixels(pixels - physics.applyPhysicsToUserOffset(this, delta));
   }
 
-  /// End the current [ScrollActivity], replacing it with an
-  /// [IdleScrollActivity].
   @override
   void goIdle() {
     beginActivity(new IdleScrollActivity(this));
@@ -180,10 +143,6 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     }
   }
 
-  /// The direction that the user most recently began scrolling in.
-  ///
-  /// If the user is not scrolling, this will return [ScrollDirection.idle] even
-  /// if there is an [activity] currently animating the position.
   @override
   ScrollDirection get userScrollDirection => _userScrollDirection;
   ScrollDirection _userScrollDirection = ScrollDirection.idle;
@@ -200,35 +159,6 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     didUpdateScrollDirection(value);
   }
 
-  // FEATURES USED BY SCROLL CONTROLLERS
-
-  /// Animates the position from its current value to the given value.
-  ///
-  /// Any active animation is canceled. If the user is currently scrolling, that
-  /// action is canceled.
-  ///
-  /// The returned [Future] will complete when the animation ends, whether it
-  /// completed successfully or whether it was interrupted prematurely.
-  ///
-  /// An animation will be interrupted whenever the user attempts to scroll
-  /// manually, or whenever another activity is started, or whenever the
-  /// animation reaches the edge of the viewport and attempts to overscroll. (If
-  /// the [ScrollPosition] does not overscroll but instead allows scrolling
-  /// beyond the extents, then going beyond the extents will not interrupt the
-  /// animation.)
-  ///
-  /// The animation is indifferent to changes to the viewport or content
-  /// dimensions.
-  ///
-  /// Once the animation has completed, the scroll position will attempt to
-  /// begin a ballistic activity in case its value is not stable (for example,
-  /// if it is scrolled beyond the extents and in that situation the scroll
-  /// position would normally bounce back).
-  ///
-  /// The duration must not be zero. To jump to a particular value without an
-  /// animation, use [jumpTo].
-  ///
-  /// The animation is handled by an [DrivenScrollActivity].
   @override
   Future<Null> animateTo(double to, {
     @required Duration duration,
@@ -246,18 +176,6 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     return activity.done;
   }
 
-  /// Jumps the scroll position from its current value to the given value,
-  /// without animation, and without checking if the new value is in range.
-  ///
-  /// Any active animation is canceled. If the user is currently scrolling, that
-  /// action is canceled.
-  ///
-  /// If this method changes the scroll position, a sequence of start/update/end
-  /// scroll notifications will be dispatched. No overscroll notifications can
-  /// be generated by this method.
-  ///
-  /// If settle is true then, immediately after the jump, a ballistic activity
-  /// is started, in case the value was out of range.
   @override
   void jumpTo(double value) {
     goIdle();
@@ -272,7 +190,6 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     goBallistic(0.0);
   }
 
-  /// Deprecated. Use [jumpTo] or a custom [ScrollPosition] instead.
   @Deprecated('This will lead to bugs.')
   @override
   void jumpToWithoutSettling(double value) {
@@ -287,8 +204,6 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     }
   }
 
-  ScrollDragController _currentDrag;
-
   @override
   ScrollHoldController hold(VoidCallback holdCancelCallback) {
     final HoldScrollActivity activity = new HoldScrollActivity(
@@ -299,11 +214,8 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     return activity;
   }
 
-  /// Start a drag activity corresponding to the given [DragStartDetails].
-  ///
-  /// The `onDragCanceled` argument will be invoked if the drag is ended
-  /// prematurely (e.g. from another activity taking over). See
-  /// [ScrollDragController.onDragCanceled] for details.
+  ScrollDragController _currentDrag;
+
   @override
   Drag drag(DragStartDetails details, VoidCallback onDragCanceled) {
     final ScrollDragController drag = new ScrollDragController(