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// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'dart:async';
import 'package:flutter/gestures.dart';
import 'package:flutter/physics.dart';
import 'package:flutter/rendering.dart';
import 'basic.dart';
import 'framework.dart';
import 'gesture_detector.dart';
import 'scroll_activity.dart';
import 'scroll_context.dart';
import 'scroll_notification.dart';
import 'scroll_physics.dart';
import 'scroll_position.dart';
/// A scroll position that manages scroll activities for a single
/// [ScrollContext].
///
/// This class is a concrete subclass of [ScrollPosition] logic that handles a
/// single [ScrollContext], such as a [Scrollable]. An instance of this class
/// manages [ScrollActivity] instances, which change what content is visible in
/// the [Scrollable]'s [Viewport].
///
/// See also:
///
/// * [ScrollPosition], which defines the underlying model for a position
/// within a [Scrollable] but is agnostic as to how that position is
/// changed.
/// * [ScrollView] and its subclasses such as [ListView], which use
/// [ScrollPositionWithSingleContext] to manage their scroll position.
/// * [ScrollController], which can manipulate one or more [ScrollPosition]s,
/// and which uses [ScrollPositionWithSingleContext] as its default class for
/// scroll positions.
class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollActivityDelegate {
/// Create a [ScrollPosition] object that manages its behavior using
/// [ScrollActivity] objects.
///
/// The `initialPixels` argument can be null, but in that case it is
/// imperative that the value be set, using [correctPixels], as soon as
/// [applyNewDimensions] is invoked, before calling the inherited
/// implementation of that method.
///
/// If [keepScrollOffset] is true (the default), the current scroll offset is
/// saved with [PageStorage] and restored it if this scroll position's scrollable
/// is recreated.
ScrollPositionWithSingleContext({
@required ScrollPhysics physics,
@required ScrollContext context,
double initialPixels = 0.0,
bool keepScrollOffset = true,
ScrollPosition oldPosition,
String debugLabel,
}) : super(
physics: physics,
context: context,
keepScrollOffset: keepScrollOffset,
oldPosition: oldPosition,
debugLabel: debugLabel,
) {
// If oldPosition is not null, the superclass will first call absorb(),
// which may set _pixels and _activity.
if (pixels == null && initialPixels != null)
correctPixels(initialPixels);
if (activity == null)
goIdle();
assert(activity != null);
}
/// Velocity from a previous activity temporarily held by [hold] to potentially
/// transfer to a next activity.
double _heldPreviousVelocity = 0.0;
@override
AxisDirection get axisDirection => context.axisDirection;
@override
double setPixels(double newPixels) {
assert(activity.isScrolling);
return super.setPixels(newPixels);
}
@override
void absorb(ScrollPosition other) {
super.absorb(other);
if (other is! ScrollPositionWithSingleContext) {
goIdle();
return;
}
activity.updateDelegate(this);
final ScrollPositionWithSingleContext typedOther = other as ScrollPositionWithSingleContext;
_userScrollDirection = typedOther._userScrollDirection;
assert(_currentDrag == null);
if (typedOther._currentDrag != null) {
_currentDrag = typedOther._currentDrag;
_currentDrag.updateDelegate(this);
typedOther._currentDrag = null;
}
}
@override
void applyNewDimensions() {
super.applyNewDimensions();
context.setCanDrag(physics.shouldAcceptUserOffset(this));
}
@override
void beginActivity(ScrollActivity newActivity) {
_heldPreviousVelocity = 0.0;
if (newActivity == null)
return;
assert(newActivity.delegate == this);
super.beginActivity(newActivity);
_currentDrag?.dispose();
_currentDrag = null;
if (!activity.isScrolling)
updateUserScrollDirection(ScrollDirection.idle);
}
@override
void applyUserOffset(double delta) {
updateUserScrollDirection(delta > 0.0 ? ScrollDirection.forward : ScrollDirection.reverse);
setPixels(pixels - physics.applyPhysicsToUserOffset(this, delta));
}
@override
void goIdle() {
beginActivity(IdleScrollActivity(this));
}
/// Start a physics-driven simulation that settles the [pixels] position,
/// starting at a particular velocity.
///
/// This method defers to [ScrollPhysics.createBallisticSimulation], which
/// typically provides a bounce simulation when the current position is out of
/// bounds and a friction simulation when the position is in bounds but has a
/// non-zero velocity.
///
/// The velocity should be in logical pixels per second.
@override
void goBallistic(double velocity) {
assert(pixels != null);
final Simulation simulation = physics.createBallisticSimulation(this, velocity);
if (simulation != null) {
beginActivity(BallisticScrollActivity(this, simulation, context.vsync));
} else {
goIdle();
}
}
@override
ScrollDirection get userScrollDirection => _userScrollDirection;
ScrollDirection _userScrollDirection = ScrollDirection.idle;
/// Set [userScrollDirection] to the given value.
///
/// If this changes the value, then a [UserScrollNotification] is dispatched.
@protected
@visibleForTesting
void updateUserScrollDirection(ScrollDirection value) {
assert(value != null);
if (userScrollDirection == value)
return;
_userScrollDirection = value;
didUpdateScrollDirection(value);
}
@override
Future<void> animateTo(
double to, {
@required Duration duration,
@required Curve curve,
}) {
if (nearEqual(to, pixels, physics.tolerance.distance)) {
// Skip the animation, go straight to the position as we are already close.
jumpTo(to);
return Future<void>.value();
}
final DrivenScrollActivity activity = DrivenScrollActivity(
this,
from: pixels,
to: to,
duration: duration,
curve: curve,
vsync: context.vsync,
);
beginActivity(activity);
return activity.done;
}
@override
void jumpTo(double value) {
goIdle();
if (pixels != value) {
final double oldPixels = pixels;
forcePixels(value);
notifyListeners();
didStartScroll();
didUpdateScrollPositionBy(pixels - oldPixels);
didEndScroll();
}
goBallistic(0.0);
}
@Deprecated('This will lead to bugs.') // ignore: flutter_deprecation_syntax, https://github.com/flutter/flutter/issues/44609
@override
void jumpToWithoutSettling(double value) {
goIdle();
if (pixels != value) {
final double oldPixels = pixels;
forcePixels(value);
notifyListeners();
didStartScroll();
didUpdateScrollPositionBy(pixels - oldPixels);
didEndScroll();
}
}
@override
ScrollHoldController hold(VoidCallback holdCancelCallback) {
final double previousVelocity = activity.velocity;
final HoldScrollActivity holdActivity = HoldScrollActivity(
delegate: this,
onHoldCanceled: holdCancelCallback,
);
beginActivity(holdActivity);
_heldPreviousVelocity = previousVelocity;
return holdActivity;
}
ScrollDragController _currentDrag;
@override
Drag drag(DragStartDetails details, VoidCallback dragCancelCallback) {
final ScrollDragController drag = ScrollDragController(
delegate: this,
details: details,
onDragCanceled: dragCancelCallback,
carriedVelocity: physics.carriedMomentum(_heldPreviousVelocity),
motionStartDistanceThreshold: physics.dragStartDistanceMotionThreshold,
);
beginActivity(DragScrollActivity(this, drag));
assert(_currentDrag == null);
_currentDrag = drag;
return drag;
}
@override
void dispose() {
_currentDrag?.dispose();
_currentDrag = null;
super.dispose();
}
@override
void debugFillDescription(List<String> description) {
super.debugFillDescription(description);
description.add('${context.runtimeType}');
description.add('$physics');
description.add('$activity');
description.add('$userScrollDirection');
}
}