container.dart

// 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 'package:flutter/foundation.dart';
import 'package:flutter/painting.dart';
import 'package:flutter/rendering.dart';

import 'basic.dart';
import 'framework.dart';
import 'image.dart';

// Examples can assume:
// BuildContext context;

/// A widget that paints a [Decoration] either before or after its child paints.
///
/// [Container] insets its child by the widths of the borders; this widget does
/// not.
///
/// Commonly used with [BoxDecoration].
///
/// The [child] is not clipped. To clip a child to the shape of a particular
/// [ShapeDecoration], consider using a [ClipPath] widget.
///
/// {@tool snippet}
///
/// This sample shows a radial gradient that draws a moon on a night sky:
///
/// ```dart
/// DecoratedBox(
///   decoration: BoxDecoration(
///     gradient: RadialGradient(
///       center: const Alignment(-0.5, -0.6),
///       radius: 0.15,
///       colors: <Color>[
///         const Color(0xFFEEEEEE),
///         const Color(0xFF111133),
///       ],
///       stops: <double>[0.9, 1.0],
///     ),
///   ),
/// )
/// ```
/// {@end-tool}
///
/// See also:
///
///  * [Ink], which paints a [Decoration] on a [Material], allowing
///    [InkResponse] and [InkWell] splashes to paint over them.
///  * [DecoratedBoxTransition], the version of this class that animates on the
///    [decoration] property.
///  * [Decoration], which you can extend to provide other effects with
///    [DecoratedBox].
///  * [CustomPaint], another way to draw custom effects from the widget layer.
class DecoratedBox extends SingleChildRenderObjectWidget {
  /// Creates a widget that paints a [Decoration].
  ///
  /// The [decoration] and [position] arguments must not be null. By default the
  /// decoration paints behind the child.
  const DecoratedBox({
    Key? key,
    required this.decoration,
    this.position = DecorationPosition.background,
    Widget? child,
  }) : assert(decoration != null),
       assert(position != null),
       super(key: key, child: child);

  /// What decoration to paint.
  ///
  /// Commonly a [BoxDecoration].
  final Decoration decoration;

  /// Whether to paint the box decoration behind or in front of the child.
  final DecorationPosition position;

  @override
  RenderDecoratedBox createRenderObject(BuildContext context) {
    return RenderDecoratedBox(
      decoration: decoration,
      position: position,
      configuration: createLocalImageConfiguration(context),
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderDecoratedBox renderObject) {
    renderObject
      ..decoration = decoration
      ..configuration = createLocalImageConfiguration(context)
      ..position = position;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    final String label;
    switch (position) {
      case DecorationPosition.background:
        label = 'bg';
        break;
      case DecorationPosition.foreground:
        label = 'fg';
        break;
    }
    properties.add(EnumProperty<DecorationPosition>('position', position, level: DiagnosticLevel.hidden));
    properties.add(DiagnosticsProperty<Decoration>(label, decoration));
  }
}

/// A convenience widget that combines common painting, positioning, and sizing
/// widgets.
///
/// {@youtube 560 315 https://www.youtube.com/watch?v=c1xLMaTUWCY}
///
/// A container first surrounds the child with [padding] (inflated by any
/// borders present in the [decoration]) and then applies additional
/// [constraints] to the padded extent (incorporating the `width` and `height`
/// as constraints, if either is non-null). The container is then surrounded by
/// additional empty space described from the [margin].
///
/// During painting, the container first applies the given [transform], then
/// paints the [decoration] to fill the padded extent, then it paints the child,
/// and finally paints the [foregroundDecoration], also filling the padded
/// extent.
///
/// Containers with no children try to be as big as possible unless the incoming
/// constraints are unbounded, in which case they try to be as small as
/// possible. Containers with children size themselves to their children. The
/// `width`, `height`, and [constraints] arguments to the constructor override
/// this.
///
/// By default, containers return false for all hit tests. If the [color]
/// property is specified, the hit testing is handled by [ColoredBox], which
/// always returns true. If the [decoration] or [foregroundDecoration] properties
/// are specified, hit testing is handled by [Decoration.hitTest].
///
/// ## Layout behavior
///
/// _See [BoxConstraints] for an introduction to box layout models._
///
/// Since [Container] combines a number of other widgets each with their own
/// layout behavior, [Container]'s layout behavior is somewhat complicated.
///
/// Summary: [Container] tries, in order: to honor [alignment], to size itself
/// to the [child], to honor the `width`, `height`, and [constraints], to expand
/// to fit the parent, to be as small as possible.
///
/// More specifically:
///
/// If the widget has no child, no `height`, no `width`, no [constraints],
/// and the parent provides unbounded constraints, then [Container] tries to
/// size as small as possible.
///
/// If the widget has no child and no [alignment], but a `height`, `width`, or
/// [constraints] are provided, then the [Container] tries to be as small as
/// possible given the combination of those constraints and the parent's
/// constraints.
///
/// If the widget has no child, no `height`, no `width`, no [constraints], and
/// no [alignment], but the parent provides bounded constraints, then
/// [Container] expands to fit the constraints provided by the parent.
///
/// If the widget has an [alignment], and the parent provides unbounded
/// constraints, then the [Container] tries to size itself around the child.
///
/// If the widget has an [alignment], and the parent provides bounded
/// constraints, then the [Container] tries to expand to fit the parent, and
/// then positions the child within itself as per the [alignment].
///
/// Otherwise, the widget has a [child] but no `height`, no `width`, no
/// [constraints], and no [alignment], and the [Container] passes the
/// constraints from the parent to the child and sizes itself to match the
/// child.
///
/// The [margin] and [padding] properties also affect the layout, as described
/// in the documentation for those properties. (Their effects merely augment the
/// rules described above.) The [decoration] can implicitly increase the
/// [padding] (e.g. borders in a [BoxDecoration] contribute to the [padding]);
/// see [Decoration.padding].
///
/// ## Example
///
/// {@tool snippet}
/// This example shows a 48x48 amber square (placed inside a [Center] widget in
/// case the parent widget has its own opinions regarding the size that the
/// [Container] should take), with a margin so that it stays away from
/// neighboring widgets:
///
/// ![An amber colored container with the dimensions of 48 square pixels.](https://flutter.github.io/assets-for-api-docs/assets/widgets/container_a.png)
///
/// ```dart
/// Center(
///   child: Container(
///     margin: const EdgeInsets.all(10.0),
///     color: Colors.amber[600],
///     width: 48.0,
///     height: 48.0,
///   ),
/// )
/// ```
/// {@end-tool}
///
/// {@tool snippet}
///
/// This example shows how to use many of the features of [Container] at once.
/// The [constraints] are set to fit the font size plus ample headroom
/// vertically, while expanding horizontally to fit the parent. The [padding] is
/// used to make sure there is space between the contents and the text. The
/// [color] makes the box blue. The [alignment] causes the [child] to be
/// centered in the box. Finally, the [transform] applies a slight rotation to the
/// entire contraption to complete the effect.
///
/// ![A blue rectangular container with 'Hello World' in the center, rotated
/// slightly in the z axis.](https://flutter.github.io/assets-for-api-docs/assets/widgets/container_b.png)
///
/// ```dart
/// Container(
///   constraints: BoxConstraints.expand(
///     height: Theme.of(context).textTheme.headline4.fontSize * 1.1 + 200.0,
///   ),
///   padding: const EdgeInsets.all(8.0),
///   color: Colors.blue[600],
///   alignment: Alignment.center,
///   child: Text('Hello World',
///     style: Theme.of(context)
///         .textTheme
///         .headline4
///         .copyWith(color: Colors.white)),
///   transform: Matrix4.rotationZ(0.1),
/// )
/// ```
/// {@end-tool}
///
/// See also:
///
///  * [AnimatedContainer], a variant that smoothly animates the properties when
///    they change.
///  * [Border], which has a sample which uses [Container] heavily.
///  * [Ink], which paints a [Decoration] on a [Material], allowing
///    [InkResponse] and [InkWell] splashes to paint over them.
///  * Cookbook: [Animate the properties of a container](https://flutter.dev/docs/cookbook/animation/animated-container)
///  * The [catalog of layout widgets](https://flutter.dev/widgets/layout/).
class Container extends StatelessWidget {
  /// Creates a widget that combines common painting, positioning, and sizing widgets.
  ///
  /// The `height` and `width` values include the padding.
  ///
  /// The `color` and `decoration` arguments cannot both be supplied, since
  /// it would potentially result in the decoration drawing over the background
  /// color. To supply a decoration with a color, use `decoration:
  /// BoxDecoration(color: color)`.
  Container({
    Key? key,
    this.alignment,
    this.padding,
    this.color,
    this.decoration,
    this.foregroundDecoration,
    double? width,
    double? height,
    BoxConstraints? constraints,
    this.margin,
    this.transform,
    this.transformAlignment,
    this.child,
    this.clipBehavior = Clip.none,
  }) : assert(margin == null || margin.isNonNegative),
       assert(padding == null || padding.isNonNegative),
       assert(decoration == null || decoration.debugAssertIsValid()),
       assert(constraints == null || constraints.debugAssertIsValid()),
       assert(clipBehavior != null),
       assert(decoration != null || clipBehavior == Clip.none),
       assert(color == null || decoration == null,
         'Cannot provide both a color and a decoration\n'
         'To provide both, use "decoration: BoxDecoration(color: color)".'
       ),
       constraints =
        (width != null || height != null)
          ? constraints?.tighten(width: width, height: height)
            ?? BoxConstraints.tightFor(width: width, height: height)
          : constraints,
       super(key: key);

  /// The [child] contained by the container.
  ///
  /// If null, and if the [constraints] are unbounded or also null, the
  /// container will expand to fill all available space in its parent, unless
  /// the parent provides unbounded constraints, in which case the container
  /// will attempt to be as small as possible.
  ///
  /// {@macro flutter.widgets.ProxyWidget.child}
  final Widget? child;

  /// Align the [child] within the container.
  ///
  /// If non-null, the container will expand to fill its parent and position its
  /// child within itself according to the given value. If the incoming
  /// constraints are unbounded, then the child will be shrink-wrapped instead.
  ///
  /// Ignored if [child] is null.
  ///
  /// See also:
  ///
  ///  * [Alignment], a class with convenient constants typically used to
  ///    specify an [AlignmentGeometry].
  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
  ///    relative to text direction.
  final AlignmentGeometry? alignment;

  /// Empty space to inscribe inside the [decoration]. The [child], if any, is
  /// placed inside this padding.
  ///
  /// This padding is in addition to any padding inherent in the [decoration];
  /// see [Decoration.padding].
  final EdgeInsetsGeometry? padding;

  /// The color to paint behind the [child].
  ///
  /// This property should be preferred when the background is a simple color.
  /// For other cases, such as gradients or images, use the [decoration]
  /// property.
  ///
  /// If the [decoration] is used, this property must be null. A background
  /// color may still be painted by the [decoration] even if this property is
  /// null.
  final Color? color;

  /// The decoration to paint behind the [child].
  ///
  /// Use the [color] property to specify a simple solid color.
  ///
  /// The [child] is not clipped to the decoration. To clip a child to the shape
  /// of a particular [ShapeDecoration], consider using a [ClipPath] widget.
  final Decoration? decoration;

  /// The decoration to paint in front of the [child].
  final Decoration? foregroundDecoration;

  /// Additional constraints to apply to the child.
  ///
  /// The constructor `width` and `height` arguments are combined with the
  /// `constraints` argument to set this property.
  ///
  /// The [padding] goes inside the constraints.
  final BoxConstraints? constraints;

  /// Empty space to surround the [decoration] and [child].
  final EdgeInsetsGeometry? margin;

  /// The transformation matrix to apply before painting the container.
  final Matrix4? transform;

  /// The alignment of the origin, relative to the size of the container, if [transform] is specified.
  ///
  /// When [transform] is null, the value of this property is ignored.
  ///
  /// See also:
  ///
  ///  * [Transform.alignment], which is set by this property.
  final AlignmentGeometry? transformAlignment;

  /// The clip behavior when [Container.decoration] is not null.
  ///
  /// Defaults to [Clip.none]. Must be [Clip.none] if [decoration] is null.
  ///
  /// If a clip is to be applied, the [Decoration.getClipPath] method
  /// for the provided decoration must return a clip path. (This is not
  /// supported by all decorations; the default implementation of that
  /// method throws an [UnsupportedError].)
  final Clip clipBehavior;

  EdgeInsetsGeometry? get _paddingIncludingDecoration {
    if (decoration == null || decoration!.padding == null)
      return padding;
    final EdgeInsetsGeometry? decorationPadding = decoration!.padding;
    if (padding == null)
      return decorationPadding;
    return padding!.add(decorationPadding!);
  }

  @override
  Widget build(BuildContext context) {
    Widget? current = child;

    if (child == null && (constraints == null || !constraints!.isTight)) {
      current = LimitedBox(
        maxWidth: 0.0,
        maxHeight: 0.0,
        child: ConstrainedBox(constraints: const BoxConstraints.expand()),
      );
    }

    if (alignment != null)
      current = Align(alignment: alignment!, child: current);

    final EdgeInsetsGeometry? effectivePadding = _paddingIncludingDecoration;
    if (effectivePadding != null)
      current = Padding(padding: effectivePadding, child: current);

    if (color != null)
      current = ColoredBox(color: color!, child: current);

    if (clipBehavior != Clip.none) {
      assert(decoration != null);
      current = ClipPath(
        clipper: _DecorationClipper(
          textDirection: Directionality.maybeOf(context),
          decoration: decoration!,
        ),
        clipBehavior: clipBehavior,
        child: current,
      );
    }

    if (decoration != null)
      current = DecoratedBox(decoration: decoration!, child: current);

    if (foregroundDecoration != null) {
      current = DecoratedBox(
        decoration: foregroundDecoration!,
        position: DecorationPosition.foreground,
        child: current,
      );
    }

    if (constraints != null)
      current = ConstrainedBox(constraints: constraints!, child: current);

    if (margin != null)
      current = Padding(padding: margin!, child: current);

    if (transform != null)
      current = Transform(transform: transform!, child: current, alignment: transformAlignment);

    return current!;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<AlignmentGeometry>('alignment', alignment, showName: false, defaultValue: null));
    properties.add(DiagnosticsProperty<EdgeInsetsGeometry>('padding', padding, defaultValue: null));
    properties.add(DiagnosticsProperty<Clip>('clipBehavior', clipBehavior, defaultValue: Clip.none));
    if (color != null)
      properties.add(DiagnosticsProperty<Color>('bg', color));
    else
      properties.add(DiagnosticsProperty<Decoration>('bg', decoration, defaultValue: null));
    properties.add(DiagnosticsProperty<Decoration>('fg', foregroundDecoration, defaultValue: null));
    properties.add(DiagnosticsProperty<BoxConstraints>('constraints', constraints, defaultValue: null));
    properties.add(DiagnosticsProperty<EdgeInsetsGeometry>('margin', margin, defaultValue: null));
    properties.add(ObjectFlagProperty<Matrix4>.has('transform', transform));
  }
}

/// A clipper that uses [Decoration.getClipPath] to clip.
class _DecorationClipper extends CustomClipper<Path> {
  _DecorationClipper({
    TextDirection? textDirection,
    required this.decoration
  }) : assert(decoration != null),
       textDirection = textDirection ?? TextDirection.ltr;

  final TextDirection textDirection;
  final Decoration decoration;

  @override
  Path getClip(Size size) {
    return decoration.getClipPath(Offset.zero & size, textDirection);
  }

  @override
  bool shouldReclip(_DecorationClipper oldClipper) {
    return oldClipper.decoration != decoration
        || oldClipper.textDirection != textDirection;
  }
}