title: Flutter. Grouped list by a field which is a list
body: <p>I have a list of items with field groups. Groups is also list. I want to display this list of that items grouped by groups. The item can be the part of two or more group.</p>
  <pre><code>List&lt;Product&gt; _items = [
      Product(
          id: '1',
          title: 'Apple and more more thing', description: 'Some data to describe',
          group: ['Picnic', 'New'], price: 25, bonus: 1, quantity: 1.5, measure: 'кг', measureStep: 1.5,
          imageUrl: 'assets/fruits/orange.jpg'),
      Product(
          id: '2',
          title: 'Apple and more more thing', description: 'Some data to describe',
          price: 24.50, bonus: 1, group: ['Picnic', 'New'], quantity: 1.5, measure: 'кг', measureStep: 1.5,
          imageUrl: 'assets/fruits/orange.jpg'),
      Product(
          id: '3',
          title: 'Apple and more more thing', description: 'Some data to describe',
          price: 5.00, bonus: 1, group: ['New'], quantity: 1, measure: 'шт', measureStep: 1,
          imageUrl: 'assets/fruits/orange.jpg'),
      Product(
          id: '4',
          title: 'Apple and more more thing', description: 'Some data to describe',
          price: 24.00, bonus: 1, group: ['New'], quantity: 1.250, measure: 'кг', measureStep: 1.250,
          imageUrl: 'assets/fruits/orange.jpg'),
      Product(
          id: '5',
          title: 'Apple and more more thing', description: 'Some data to describe',
          price: 15.75, bonus: 1, group: ['Picnic'], quantity: 1, measure: 'шт', measureStep: 1,
          imageUrl: 'assets/fruits/orange.jpg'),
      Product(
          id: '6',
          title: 'Apple and more more thing', description: 'Some data to describe',
          price: 9.50, bonus: 1, group: ['For children', 'New'], quantity: 0.5, measure: 'лт', measureStep: 0.5,
          imageUrl: 'assets/fruits/orange.jpg'),
      Product(
          id: '7',
          title: 'Apple and more more thing', description: 'Some data to describe',
          price: 40.00, bonus: 1, group: ['For children'], quantity: 1, measure: 'шт', measureStep: 1,
          imageUrl: 'assets/fruits/orange.jpg'),
      Product(
          id: '8',
          title: 'Apple and more more thing', description: 'Some data to describe',
          price: 32.50, bonus: 1, group: ['For children', 'New'], quantity: 1.75, measure: 'кг', measureStep: 1.75,
          imageUrl: 'assets/fruits/orange.jpg')
  </code></pre>
  <p>I am new in flutter, so can you give some idea how it is possible or code will be perfect.</p>
  <p><a href="https://i.stack.imgur.com/L7TfA.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/L7TfA.png" alt="I made grouped list by one string, but now I have there a list." /></a></p>