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

// Copyright 2015 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 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';

import 'framework.dart';

/// Delegate for configuring a [SliverPersistentHeader].
abstract class SliverPersistentHeaderDelegate {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const SliverPersistentHeaderDelegate();

  /// The widget to place inside the [SliverPersistentHeader].
  ///
  /// The `context` is the [BuildContext] of the sliver.
  ///
  /// The `shrinkOffset` is a distance from [maxExtent] towards [minExtent]
  /// representing the current amount by which the sliver has been shrunk. When
  /// the `shrinkOffset` is zero, the contents will be rendered with a dimension
  /// of [maxExtent] in the main axis. When `shrinkOffset` equals the difference
  /// between [maxExtent] and [minExtent] (a positive number), the contents will
  /// be rendered with a dimension of [minExtent] in the main axis. The
  /// `shrinkOffset` will always be a positive number in that range.
  ///
  /// The `overlapsContent` argument is true if subsequent slivers (if any) will
  /// be rendered beneath this one, and false if the sliver will not have any
  /// contents below it. Typically this is used to decide whether to draw a
  /// shadow to simulate the sliver being above the contents below it. Typically
  /// this is true when `shrinkOffset` is at its greatest value and false
  /// otherwise, but that is not guaranteed. See [NestedScrollView] for an
  /// example of a case where `overlapsContent`'s value can be unrelated to
  /// `shrinkOffset`.
  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent);

  /// The smallest size to allow the header to reach, when it shrinks at the
  /// start of the viewport.
  ///
  /// This must return a value equal to or less than [maxExtent].
  ///
  /// This value should not change over the lifetime of the delegate. It should
  /// be based entirely on the constructor arguments passed to the delegate. See
  /// [shouldRebuild], which must return true if a new delegate would return a
  /// different value.
  double get minExtent;

  /// The size of the header when it is not shrinking at the top of the
  /// viewport.
  ///
  /// This must return a value equal to or greater than [minExtent].
  ///
  /// This value should not change over the lifetime of the delegate. It should
  /// be based entirely on the constructor arguments passed to the delegate. See
  /// [shouldRebuild], which must return true if a new delegate would return a
  /// different value.
  double get maxExtent;

  /// Specifies how floating headers should animate in and out of view.
  ///
  /// If the value of this property is null, then floating headers will
  /// not animate into place.
  ///
  /// This is only used for floating headers (those with
  /// [SliverPersistentHeader.floating] set to true).
  ///
  /// Defaults to null.
  FloatingHeaderSnapConfiguration get snapConfiguration => null;

  /// Whether this delegate is meaningfully different from the old delegate.
  ///
  /// If this returns false, then the header might not be rebuilt, even though
  /// the instance of the delegate changed.
  ///
  /// This must return true if `oldDelegate` and this object would return
  /// different values for [minExtent], [maxExtent], [snapConfiguration], or
  /// would return a meaningfully different widget tree from [build] for the
  /// same arguments.
  bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate);
}

/// A sliver whose size varies when the sliver is scrolled to the leading edge
/// of the viewport.
///
/// This is the layout primitive that [SliverAppBar] uses for its
/// shrinking/growing effect.
class SliverPersistentHeader extends StatelessWidget {
  /// Creates a sliver that varies its size when it is scrolled to the start of
  /// a viewport.
  ///
  /// The [delegate], [pinned], and [floating] arguments must not be null.
  const SliverPersistentHeader({
    Key key,
    @required this.delegate,
    this.pinned: false,
    this.floating: false,
  }) : assert(delegate != null),
       assert(pinned != null),
       assert(floating != null),
       super(key: key);

  /// Configuration for the sliver's layout.
  ///
  /// The delegate provides the following information:
  ///
  ///  * The minimum and maximum dimensions of the sliver.
  ///
  ///  * The builder for generating the widgets of the sliver.
  ///
  ///  * The instructions for snapping the scroll offset, if [floating] is true.
  final SliverPersistentHeaderDelegate delegate;

  /// Whether to stick the header to the start of the viewport once it has
  /// reached its minimum size.
  ///
  /// If this is false, the header will continue scrolling off the screen after
  /// it has shrunk to its minimum extent.
  final bool pinned;

  /// Whether the header should immediately grow again if the user reverses
  /// scroll direction.
  ///
  /// If this is false, the header only grows again once the user reaches the
  /// part of the viewport that contains the sliver.
  ///
  /// The [delegate]'s [SliverPersistentHeaderDelegate.snapConfiguration] is
  /// ignored unless [floating] is true.
  final bool floating;

  @override
  Widget build(BuildContext context) {
    if (floating && pinned)
      return new _SliverFloatingPinnedPersistentHeader(delegate: delegate);
    if (pinned)
      return new _SliverPinnedPersistentHeader(delegate: delegate);
    if (floating)
      return new _SliverFloatingPersistentHeader(delegate: delegate);
    return new _SliverScrollingPersistentHeader(delegate: delegate);
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('delegate: $delegate');
    final List<String> flags = <String>[];
    if (pinned)
      flags.add('pinned');
    if (floating)
      flags.add('floating');
    if (flags.isEmpty)
      flags.add('normal');
    description.add('mode: ${flags.join(", ")}');
  }
}

class _SliverPersistentHeaderElement extends RenderObjectElement {
  _SliverPersistentHeaderElement(_SliverPersistentHeaderRenderObjectWidget widget) : super(widget);

  @override
  _SliverPersistentHeaderRenderObjectWidget get widget => super.widget;

  @override
  _RenderSliverPersistentHeaderForWidgetsMixin get renderObject => super.renderObject;

  @override
  void mount(Element parent, dynamic newSlot) {
    super.mount(parent, newSlot);
    renderObject._element = this;
  }

  @override
  void unmount() {
    super.unmount();
    renderObject._element = null;
  }

  @override
  void update(_SliverPersistentHeaderRenderObjectWidget newWidget) {
    final _SliverPersistentHeaderRenderObjectWidget oldWidget = widget;
    super.update(newWidget);
    final SliverPersistentHeaderDelegate newDelegate = newWidget.delegate;
    final SliverPersistentHeaderDelegate oldDelegate = oldWidget.delegate;
    if (newDelegate != oldDelegate &&
        (newDelegate.runtimeType != oldDelegate.runtimeType || newDelegate.shouldRebuild(oldDelegate)))
      renderObject.triggerRebuild();
  }

  @override
  void performRebuild() {
    renderObject.triggerRebuild();
  }

  Element child;

  void _build(double shrinkOffset, bool overlapsContent) {
    owner.buildScope(this, () {
      child = updateChild(child, widget.delegate.build(this, shrinkOffset, overlapsContent), null);
    });
  }

  @override
  void forgetChild(Element child) {
    assert(child == this.child);
    this.child = null;
  }

  @override
  void insertChildRenderObject(covariant RenderObject child, Null slot) {
    assert(renderObject.debugValidateChild(child));
    renderObject.child = child;
  }

  @override
  void moveChildRenderObject(covariant RenderObject child, Null slot) {
    assert(false);
  }

  @override
  void removeChildRenderObject(covariant RenderObject child) {
    renderObject.child = null;
  }

  @override
  void visitChildren(ElementVisitor visitor) {
    if (child != null)
      visitor(child);
  }
}

abstract class _SliverPersistentHeaderRenderObjectWidget extends RenderObjectWidget {
  const _SliverPersistentHeaderRenderObjectWidget({
    Key key,
    @required this.delegate,
  }) : assert(delegate != null),
       super(key: key);

  final SliverPersistentHeaderDelegate delegate;

  @override
  _SliverPersistentHeaderElement createElement() => new _SliverPersistentHeaderElement(this);

  @override
  _RenderSliverPersistentHeaderForWidgetsMixin createRenderObject(BuildContext context);

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('delegate: $delegate');
  }
}

abstract class _RenderSliverPersistentHeaderForWidgetsMixin extends RenderSliverPersistentHeader {
  // This class is intended to be used as a mixin, and should not be
  // extended directly.
  factory _RenderSliverPersistentHeaderForWidgetsMixin._() => null;

  _SliverPersistentHeaderElement _element;

  @override
  double get minExtent => _element.widget.delegate.minExtent;

  @override
  double get maxExtent => _element.widget.delegate.maxExtent;

  @override
  void updateChild(double shrinkOffset, bool overlapsContent) {
    assert(_element != null);
    _element._build(shrinkOffset, overlapsContent);
  }

  @protected
  void triggerRebuild() {
    markNeedsLayout();
  }
}

class _SliverScrollingPersistentHeader extends _SliverPersistentHeaderRenderObjectWidget {
  const _SliverScrollingPersistentHeader({
    Key key,
    @required SliverPersistentHeaderDelegate delegate,
  }) : super(key: key, delegate: delegate);

  @override
  _RenderSliverPersistentHeaderForWidgetsMixin createRenderObject(BuildContext context) {
    return new _RenderSliverScrollingPersistentHeaderForWidgets();
  }
}

// This class exists to work around https://github.com/dart-lang/sdk/issues/15101
abstract class _RenderSliverScrollingPersistentHeader extends RenderSliverScrollingPersistentHeader { }

class _RenderSliverScrollingPersistentHeaderForWidgets extends _RenderSliverScrollingPersistentHeader
  with _RenderSliverPersistentHeaderForWidgetsMixin { }

class _SliverPinnedPersistentHeader extends _SliverPersistentHeaderRenderObjectWidget {
  const _SliverPinnedPersistentHeader({
    Key key,
    @required SliverPersistentHeaderDelegate delegate,
  }) : super(key: key, delegate: delegate);

  @override
  _RenderSliverPersistentHeaderForWidgetsMixin createRenderObject(BuildContext context) {
    return new _RenderSliverPinnedPersistentHeaderForWidgets();
  }
}

// This class exists to work around https://github.com/dart-lang/sdk/issues/15101
abstract class _RenderSliverPinnedPersistentHeader extends RenderSliverPinnedPersistentHeader { }

class _RenderSliverPinnedPersistentHeaderForWidgets extends _RenderSliverPinnedPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin { }

class _SliverFloatingPersistentHeader extends _SliverPersistentHeaderRenderObjectWidget {
  const _SliverFloatingPersistentHeader({
    Key key,
    @required SliverPersistentHeaderDelegate delegate,
  }) : super(key: key, delegate: delegate);

  @override
  _RenderSliverPersistentHeaderForWidgetsMixin createRenderObject(BuildContext context) {
    // Not passing this snapConfiguration as a constructor parameter to avoid the
    // additional layers added due to https://github.com/dart-lang/sdk/issues/15101
    return new _RenderSliverFloatingPersistentHeaderForWidgets()
      ..snapConfiguration = delegate.snapConfiguration;
  }

  @override
  void updateRenderObject(BuildContext context, _RenderSliverFloatingPersistentHeaderForWidgets renderObject) {
    renderObject.snapConfiguration = delegate.snapConfiguration;
  }
}

// This class exists to work around https://github.com/dart-lang/sdk/issues/15101
abstract class _RenderSliverFloatingPinnedPersistentHeader extends RenderSliverFloatingPinnedPersistentHeader { }

class _RenderSliverFloatingPinnedPersistentHeaderForWidgets extends _RenderSliverFloatingPinnedPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin { }

class _SliverFloatingPinnedPersistentHeader extends _SliverPersistentHeaderRenderObjectWidget {
  const _SliverFloatingPinnedPersistentHeader({
    Key key,
    @required SliverPersistentHeaderDelegate delegate,
  }) : super(key: key, delegate: delegate);

  @override
  _RenderSliverPersistentHeaderForWidgetsMixin createRenderObject(BuildContext context) {
    // Not passing this snapConfiguration as a constructor parameter to avoid the
    // additional layers added due to https://github.com/dart-lang/sdk/issues/15101
    return new _RenderSliverFloatingPinnedPersistentHeaderForWidgets()
      ..snapConfiguration = delegate.snapConfiguration;
  }

  @override
  void updateRenderObject(BuildContext context, _RenderSliverFloatingPinnedPersistentHeaderForWidgets renderObject) {
    renderObject.snapConfiguration = delegate.snapConfiguration;
  }
}

// This class exists to work around https://github.com/dart-lang/sdk/issues/15101
abstract class _RenderSliverFloatingPersistentHeader extends RenderSliverFloatingPersistentHeader { }

class _RenderSliverFloatingPersistentHeaderForWidgets extends _RenderSliverFloatingPersistentHeader with _RenderSliverPersistentHeaderForWidgetsMixin { }