automatic_keep_alive.dart

// Copyright 2017 The Chromium 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/foundation.dart';
import 'package:flutter/scheduler.dart';

import 'framework.dart';
import 'notification_listener.dart';
import 'sliver.dart';

/// Allows subtrees to request to be kept alive in lazy lists.
///
/// This widget is like [KeepAlive] but instead of being explicitly configured,
/// it listens to [KeepAliveNotification] messages from the [child] and other
/// descendants.
///
/// The subtree is kept alive whenever there is one or more descendant that has
/// sent a [KeepAliveNotification] and not yet triggered its
/// [KeepAliveNotification.handle].
///
/// To send these notifications, consider using [AutomaticKeepAliveClientMixin].
class AutomaticKeepAlive extends StatefulWidget {
  /// Creates a widget that listens to [KeepAliveNotification]s and maintains a
  /// [KeepAlive] widget appropriately.
  const AutomaticKeepAlive({
    Key key,
    this.child,
  }) : super(key: key);

  /// The widget below this widget in the tree.
  final Widget child;

  @override
  _AutomaticKeepAliveState createState() => new _AutomaticKeepAliveState();
}

class _AutomaticKeepAliveState extends State<AutomaticKeepAlive> {
  Map<Listenable, VoidCallback> _handles;
  Widget _child;
  bool _keepingAlive = false;

  @override
  void initState() {
    super.initState();
    _updateChild();
  }

  @override
  void didUpdateWidget(AutomaticKeepAlive oldWidget) {
    super.didUpdateWidget(oldWidget);
    _updateChild();
  }

  void _updateChild() {
    _child = new NotificationListener<KeepAliveNotification>(
      onNotification: _addClient,
      child: widget.child,
    );
  }

  @override
  void dispose() {
    if (_handles != null) {
      for (Listenable handle in _handles.keys)
        handle.removeListener(_handles[handle]);
    }
    super.dispose();
  }

  bool _addClient(KeepAliveNotification notification) {
    final Listenable handle = notification.handle;
    _handles ??= <Listenable, VoidCallback>{};
    assert(!_handles.containsKey(handle));
    _handles[handle] = _createCallback(handle);
    handle.addListener(_handles[handle]);
    if (!_keepingAlive) {
      _keepingAlive = true;
      // We use Element.visitChildren rather than context.visitChildElements
      // because we might be called during build, and context.visitChildElements
      // verifies that it is not called during build. Element.visitChildren does
      // not, instead it assumes that the caller will be careful. (See the
      // documentation for these methods for more details.)
      //
      // Here we know it's safe because we just received a notification, which
      // we wouldn't be able to do if we hadn't built our child and its child --
      // our build method always builds the same subtree and it always includes
      // the node we're looking for (KeepAlive) as the parent of the node that
      // reports the notifications (NotificationListener).
      //
      // (We're only going down one level, to get our direct child.)
      final Element element = context;
      element.visitChildren((Element child) {
        assert(child is ParentDataElement<SliverMultiBoxAdaptorWidget>);
        final ParentDataElement<SliverMultiBoxAdaptorWidget> childElement = child;
        childElement.applyWidgetOutOfTurn(build(context));
      });
    }
    return false;
  }

  VoidCallback _createCallback(Listenable handle) {
    return () {
      assert(() {
        if (!mounted) {
          throw new FlutterError(
            'AutomaticKeepAlive handle triggered after AutomaticKeepAlive was disposed.'
            'Widgets should always trigger their KeepAliveNotification handle when they are '
            'deactivated, so that they (or their handle) do not send spurious events later '
            'when they are no longer in the tree.'
          );
        }
        return true;
      }());
      _handles.remove(handle);
      if (_handles.isEmpty) {
        if (SchedulerBinding.instance.schedulerPhase.index < SchedulerPhase.persistentCallbacks.index) {
          // Build/layout haven't started yet so let's just schedule this for
          // the next frame.
          setState(() { _keepingAlive = false; });
        } else {
          // We were probably notified by a descendant when they were yanked out
          // of our subtree somehow. We're probably in the middle of build or
          // layout, so there's really nothing we can do to clean up this mess
          // short of just scheduling another build to do the cleanup. This is
          // very unfortunate, and means (for instance) that garbage collection
          // of these resources won't happen for another 16ms.
          //
          // The problem is there's really no way for us to distinguish these
          // cases:
          //
          //  * We haven't built yet (or missed out chance to build), but
          //    someone above us notified our descendant and our descendant is
          //    disconnecting from us. If we could mark ourselves dirty we would
          //    be able to clean everything this frame. (This is a pretty
          //    unlikely scenario in practice. Usually things change before
          //    build/layout, not during build/layout.)
          //
          //  * Our child changed, and as our old child went away, it notified
          //    us. We can't setState, since we _just_ built. We can't apply the
          //    parent data information to our child because we don't _have_ a
          //    child at this instant. We really want to be able to change our
          //    mind about how we built, so we can give the KeepAlive widget a
          //    new value, but it's too late.
          //
          //  * A deep descendant in another build scope just got yanked, and in
          //    the process notified us. We could apply new parent data
          //    information, but it may or may not get applied this frame,
          //    depending on whether said child is in the same layout scope.
          //
          //  * A descendant is being moved from one position under us to
          //    another position under us. They just notified us of the removal,
          //    at some point in the future they will notify us of the addition.
          //    We don't want to do anything. (This is why we check that
          //    _handles is still empty below.)
          //
          //  * We're being notified in the paint phase, or even in a post-frame
          //    callback. Either way it is far too late for us to make our
          //    parent lay out again this frame, so the garbage won't get
          //    collected this frame.
          //
          //  * We are being torn out of the tree ourselves, as is our
          //    descendant, and it notified us while it was being deactivated.
          //    We don't need to do anything, but we don't know yet because we
          //    haven't been deactivated yet. (This is why we check mounted
          //    below before calling setState.)
          //
          // Long story short, we have to schedule a new frame and request a
          // frame there, but this is generally a bad practice, and you should
          // avoid it if possible.
          _keepingAlive = false;
          scheduleMicrotask(() {
            if (mounted && _handles.isEmpty) {
              // If mounted is false, we went away as well, so there's nothing to do.
              // If _handles is no longer empty, then another client (or the same
              // client in a new place) registered itself before we had a chance to
              // turn off keep-alive, so again there's nothing to do.
              setState(() {
                assert(!_keepingAlive);
              });
            }
          });
        }
      }
    };
  }

  @override
  Widget build(BuildContext context) {
    assert(_child != null);
    return new KeepAlive(
      keepAlive: _keepingAlive,
      child: _child,
    );
  }


  @override
  void debugFillProperties(DiagnosticPropertiesBuilder description) {
    super.debugFillProperties(description);
    description.add(new FlagProperty('_keepingAlive', value: _keepingAlive, ifTrue: 'keeping subtree alive'));
    description.add(new DiagnosticsProperty<Map<Listenable, VoidCallback>>(
      'handles',
      _handles,
      description: _handles != null ?
        '${_handles.length} active client${ _handles.length == 1 ? "" : "s" }' :
        null,
      ifNull: 'no notifications ever received',
    ));
  }
}

/// Indicates that the subtree through which this notification bubbles must be
/// kept alive even if it would normally be discarded as an optimisation.
///
/// For example, a focused text field might fire this notification to indicate
/// that it should not be disposed even if the user scrolls the field off
/// screen.
///
/// Each [KeepAliveNotification] is configured with a [handle] that consists of
/// a [Listenable] that is triggered when the subtree no longer needs to be kept
/// alive.
///
/// The [handle] should be triggered any time the sending widget is removed from
/// the tree (in [State.deactivate]). If the widget is then rebuilt and still
/// needs to be kept alive, it should immediately send a new notification
/// (possible with the very same [Listenable]) during build.
///
/// This notification is listened to by the [AutomaticKeepAlive] widget, which
/// is added to the tree automatically by [SliverList] (and [ListView]) and
/// [SliverGrid] (and [GridView]) widgets.
///
/// Failure to trigger the [handle] in the manner described above will likely
/// cause the [AutomaticKeepAlive] to lose track of whether the widget should be
/// kept alive or not, leading to memory leaks or lost data. For example, if the
/// widget that requested keep-alive is removed from the subtree but doesn't
/// trigger its [Listenable] on the way out, then the subtree will continue to
/// be kept alive until the list itself is disposed. Similarly, if the
/// [Listenable] is triggered while the widget needs to be kept alive, but a new
/// [KeepAliveNotification] is not immediately sent, then the widget risks being
/// garbage collected while it wants to be kept alive.
///
/// It is an error to use the same [handle] in two [KeepAliveNotification]s
/// within the same [AutomaticKeepAlive] without triggering that [handle] before
/// the second notification is sent.
///
/// For a more convenient way to interact with [AutomaticKeepAlive] widgets,
/// consider using [AutomaticKeepAliveClientMixin], which uses
/// [KeepAliveNotification] internally.
class KeepAliveNotification extends Notification {
  /// Creates a notification to indicate that a subtree must be kept alive.
  ///
  /// The [handle] must not be null.
  const KeepAliveNotification(this.handle) : assert(handle != null);

  /// A [Listenable] that will inform its clients when the widget that fired the
  /// notification no longer needs to be kept alive.
  ///
  /// The [Listenable] should be triggered any time the sending widget is
  /// removed from the tree (in [State.deactivate]). If the widget is then
  /// rebuilt and still needs to be kept alive, it should immediately send a new
  /// notification (possible with the very same [Listenable]) during build.
  ///
  /// See also:
  ///
  ///  * [KeepAliveHandle], a convenience class for use with this property.
  final Listenable handle;
}

/// A [Listenable] which can be manually triggered.
///
/// Used with [KeepAliveNotification] objects as their
/// [KeepAliveNotification.handle].
///
/// For a more convenient way to interact with [AutomaticKeepAlive] widgets,
/// consider using [AutomaticKeepAliveClientMixin], which uses a
/// [KeepAliveHandle] internally.
class KeepAliveHandle extends ChangeNotifier {
  /// Trigger the listeners to indicate that the widget
  /// no longer needs to be kept alive.
  void release() {
    notifyListeners();
  }
}

/// A mixin with convenience methods for clients of [AutomaticKeepAlive].
///
/// Subclasses must implement [wantKeepAlive], and their [build] methods must
/// call `super.build` (which will always return null).
///
/// Then, whenever [wantKeepAlive]'s value changes (or might change), the
/// subclass should call [updateKeepAlive].
///
/// See also:
///
///  * [AutomaticKeepAlive], which listens to messages from this mixin.
///  * [KeepAliveNotification], the notifications sent by this mixin.
abstract class AutomaticKeepAliveClientMixin extends State<StatefulWidget> {
  // This class is intended to be used as a mixin, and should not be
  // extended directly.
  factory AutomaticKeepAliveClientMixin._() => null;

  KeepAliveHandle _keepAliveHandle;

  void _ensureKeepAlive() {
    assert(_keepAliveHandle == null);
    _keepAliveHandle = new KeepAliveHandle();
    new KeepAliveNotification(_keepAliveHandle).dispatch(context);
  }

  void _releaseKeepAlive() {
    _keepAliveHandle.release();
    _keepAliveHandle = null;
  }

  /// Whether the current instance should be kept alive.
  ///
  /// Call [updateKeepAlive] whenever this getter's value changes.
  @protected
  bool get wantKeepAlive;

  /// Ensures that any [AutomaticKeepAlive] ancestors are in a good state, by
  /// firing a [KeepAliveNotification] or triggering the [KeepAliveHandle] as
  /// appropriate.
  @protected
  void updateKeepAlive() {
    if (wantKeepAlive) {
      if (_keepAliveHandle == null)
        _ensureKeepAlive();
    } else {
      if (_keepAliveHandle != null)
        _releaseKeepAlive();
    }
  }

  @override
  void initState() {
    super.initState();
    if (wantKeepAlive)
      _ensureKeepAlive();
  }

  @override
  void deactivate() {
    if (_keepAliveHandle != null)
      _releaseKeepAlive();
    super.deactivate();
  }

  @mustCallSuper
  @override
  Widget build(BuildContext context) {
    if (wantKeepAlive && _keepAliveHandle == null)
      _ensureKeepAlive();
    return null;
  }
}