examples/api/lib/widgets/basic/custom_multi_child_layout.0.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.

// Flutter code sample for CustomMultiChildLayout

import 'package:flutter/material.dart';

void main() => runApp(const ExampleApp());

class ExampleApp extends StatelessWidget {
  const ExampleApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: Directionality(
        // TRY THIS: Try changing the direction here and hot-reloading to
        // see the layout change.
        textDirection: TextDirection.ltr,
        child: Scaffold(
          body: ExampleWidget(),
        ),
      ),
    );
  }
}

/// Lays out the children in a cascade, where the top corner of the next child
/// is a little above (`overlap`) the lower end corner of the previous child.
///
/// Will relayout if the text direction changes.
class _CascadeLayoutDelegate extends MultiChildLayoutDelegate {
  _CascadeLayoutDelegate({
    required this.colors,
    required this.overlap,
    required this.textDirection,
  });

  final Map<String, Color> colors;
  final double overlap;
  final TextDirection textDirection;

  // Perform layout will be called when re-layout is needed.
  @override
  void performLayout(Size size) {
    final double columnWidth = size.width / colors.length;
    Offset childPosition = Offset.zero;
    switch (textDirection) {
      case TextDirection.rtl:
        childPosition += Offset(size.width, 0);
        break;
      case TextDirection.ltr:
        break;
    }
    for (final String color in colors.keys) {
      // layoutChild must be called exactly once for each child.
      final Size currentSize = layoutChild(
        color,
        BoxConstraints(maxHeight: size.height, maxWidth: columnWidth),
      );
      // positionChild must be called to change the position of a child from
      // what it was in the previous layout. Each child starts at (0, 0) for the
      // first layout.
      switch (textDirection) {
        case TextDirection.rtl:
          positionChild(color, childPosition - Offset(currentSize.width, 0));
          childPosition += Offset(-currentSize.width, currentSize.height - overlap);
          break;
        case TextDirection.ltr:
          positionChild(color, childPosition);
          childPosition += Offset(currentSize.width, currentSize.height - overlap);
          break;
      }
    }
  }

  // shouldRelayout is called to see if the delegate has changed and requires a
  // layout to occur. Should only return true if the delegate state itself
  // changes: changes in the CustomMultiChildLayout attributes will
  // automatically cause a relayout, like any other widget.
  @override
  bool shouldRelayout(_CascadeLayoutDelegate oldDelegate) {
    return oldDelegate.textDirection != textDirection
        || oldDelegate.overlap != overlap;
  }
}

class ExampleWidget extends StatelessWidget {
  const ExampleWidget({Key? key}) : super(key: key);

  static const Map<String, Color> _colors = <String, Color>{
    'Red': Colors.red,
    'Green': Colors.green,
    'Blue': Colors.blue,
    'Cyan': Colors.cyan,
  };

  @override
  Widget build(BuildContext context) {
    return CustomMultiChildLayout(
      delegate: _CascadeLayoutDelegate(
        colors: _colors,
        overlap: 30.0,
        textDirection: Directionality.of(context),
      ),
      children: <Widget>[
        // Create all of the colored boxes in the colors map.
        for (MapEntry<String, Color> entry in _colors.entries)
          // The "id" can be any Object, not just a String.
          LayoutId(
            id: entry.key,
            child: Container(
              color: entry.value,
              width: 100.0,
              height: 100.0,
              alignment: Alignment.center,
              child: Text(entry.key),
            ),
          ),
      ],
    );
  }
}