一、集合方法概述

Dart提供丰富的集合操作方法，使数据处理更简洁高效。

|| 方法 | 功能 | 返回类型 |
|------|------|----------|
| map | 转换元素 | Iterable |
| where | 过滤元素 | Iterable |
| fold | 聚合计算 | T |
| reduce | 归约计算 | T |
| sort | 原地排序 | void |

二、示例代码

class _Page09CollectionDemo extends StatefulWidget {
  const _Page09CollectionDemo();

  @override
  State<_Page09CollectionDemo> createState() => _Page09CollectionDemoState();
}

class _Page09CollectionDemoState extends State<_Page09CollectionDemo> {
  final List<int> _numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
  String _result = '';

  void _filterEven() {
    setState(() {
      final evenNumbers = _numbers.where((n) => n % 2 == 0).toList();
      _result = '偶数: ${evenNumbers.join(', ')}';
    });
  }

  void _squareAll() {
    setState(() {
      final squares = _numbers.map((n) => n * n).toList();
      _result = '平方: ${squares.join(', ')}';
    });
  }

  void _sumAll() {
    setState(() {
      final sum = _numbers.fold(0, (acc, n) => acc + n);
      _result = '总和: $sum';
    });
  }

  void _sortDesc() {
    setState(() {
      final sorted = List.from(_numbers)..sort((a, b) => b.compareTo(a));
      _result = '降序: ${sorted.join(', ')}';
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.indigo.shade50,
      padding: const EdgeInsets.all(20),
      child: Column(
        children: [
          Container(
            padding: const EdgeInsets.all(20),
            decoration: BoxDecoration(
              color: Colors.indigo.shade600,
              borderRadius: BorderRadius.circular(20),
            ),
            child: const Column(
              children: [
                Icon(Icons.data_array, size: 48, color: Colors.white),
                SizedBox(height: 16),
                Text(
                  '集合操作',
                  style: TextStyle(fontSize: 28, fontWeight: FontWeight.bold, color: Colors.white),
                ),
                SizedBox(height: 8),
                Text('数据转换 - 页面 9/10', style: TextStyle(color: Colors.white70)),
              ],
            ),
          ),
          const SizedBox(height: 24),
          Expanded(
            child: Container(
              padding: const EdgeInsets.all(20),
              decoration: BoxDecoration(color: Colors.white, borderRadius: BorderRadius.circular(20)),
              child: Column(
                children: [
                  Text(
                    _numbers.join(', '),
                    style: const TextStyle(fontSize: 16, color: Colors.grey),
                  ),
                  const SizedBox(height: 20),
                  if (_result.isNotEmpty)
                    Container(
                      padding: const EdgeInsets.all(16),
                      decoration: BoxDecoration(
                        color: Colors.indigo.shade100,
                        borderRadius: BorderRadius.circular(10),
                      ),
                      child: Text(_result, style: const TextStyle(fontSize: 18)),
                    ),
                  const Spacer(),
                  Column(
                    children: [
                      ElevatedButton(
                        onPressed: _filterEven,
                        style: ElevatedButton.styleFrom(backgroundColor: Colors.indigo.shade600),
                        child: const Text('过滤偶数', style: TextStyle(color: Colors.white)),
                      ),
                      const SizedBox(height: 8),
                      ElevatedButton(
                        onPressed: _squareAll,
                        style: ElevatedButton.styleFrom(backgroundColor: Colors.indigo.shade600),
                        child: const Text('平方计算', style: TextStyle(color: Colors.white)),
                      ),
                      const SizedBox(height: 8),
                      ElevatedButton(
                        onPressed: _sumAll,
                        style: ElevatedButton.styleFrom(backgroundColor: Colors.indigo.shade600),
                        child: const Text('求和', style: TextStyle(color: Colors.white)),
                      ),
                      const SizedBox(height: 8),
                      ElevatedButton(
                        onPressed: _sortDesc,
                        style: ElevatedButton.styleFrom(backgroundColor: Colors.indigo.shade600),
                        child: const Text('降序排列', style: TextStyle(color: Colors.white)),
                      ),
                    ],
                  ),
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }
}

三、常用方法

四、性能对比

五、最佳实践

• ✅ 使用链式调用
• ✅ 避免不必要的toList
• ✅ 合理使用延迟计算
• ❌ 注意性能影响

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