samples-dev/swarm/swarm_ui_lib/touch/ScrollWatcher.dart

// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// @dart = 2.9

part of touch;

abstract class ScrollListener {
  /// The callback invoked for a scroll event.
  /// [decelerating] specifies whether or not the content is moving due
  /// to deceleration. It should be false if the content is moving because the
  /// user is dragging the content.
  void onScrollerMoved(double scrollX, double scrollY, bool decelerating);
}

/// The scroll watcher is intended to provide a single way to
/// listen for scroll events from instances of Scroller.
/// TODO(jacobr): this class is obsolete.
class ScrollWatcher {
  final Scroller _scroller;

  final List<ScrollListener> _listeners;

  Element _scrollerEl;

  ScrollWatcher(Scroller scroller)
      : _scroller = scroller,
        _listeners = <ScrollListener>[];

  void addListener(ScrollListener listener) {
    _listeners.add(listener);
  }

  /// Send the scroll event to all listeners.
  /// [decelerating] is true if the offset is changing because of deceleration.
  void _dispatchScroll(num scrollX, num scrollY, [bool decelerating = false]) {
    for (final listener in _listeners) {
      listener.onScrollerMoved(scrollX, scrollY, decelerating);
    }
  }

  /// Initializes elements and event handlers. Must be called after construction
  /// and before usage.
  void initialize() {
    _scrollerEl = _scroller.getElement();
    _scroller.onContentMoved.listen((e) {
      _onContentMoved(e);
    });
  }

  /// This callback is invoked any time the scroller content offset changes.
  void _onContentMoved(Event e) {
    num scrollX = _scroller.getHorizontalOffset();
    num scrollY = _scroller.getVerticalOffset();
    _dispatchScroll(scrollX, scrollY);
  }
}