flutter divider: How could I add divider between each line in my code?

There are a number of ways to do the same thing. Let me compare them here.

For a short static list

Use ListTile.divideTiles

ListView(
  children: ListTile.divideTiles( //          <-- ListTile.divideTiles
      context: context,
      tiles: [
        ListTile(
          title: Text('Horse'),
        ),
        ListTile(
          title: Text('Cow'),
        ),
        ListTile(
          title: Text('Camel'),
        ),
        ListTile(
          title: Text('Sheep'),
        ),
        ListTile(
          title: Text('Goat'),
        ),
      ]
  ).toList(),
)

For a long dynamic list

Use ListView.separated.

ListView.separated(
  itemCount: 100,
  itemBuilder: (context, index) {
    return ListTile(
      title: Text('$index sheep'),
    );
  },
  separatorBuilder: (context, index) {
    return Divider();
  },
)

This returns two widgets for every item, except for the last item. The separatorBuilder is used to add the divider.

For adding a divider after the last item

Create a custom item widget that uses a Divider or BoxDecoration.

Using Divider

final items = ['Horse', 'Cow', 'Camel', 'Sheep', 'Goat'];

@override
Widget build(BuildContext context) {
  return ListView.builder(
    itemCount: items.length,
    itemBuilder: (context, index) {
      return Column(
        children: <Widget>[
          ListTile(
            title: Text(items[index]),
          ),
          Divider(), //                           <-- Divider
        ],
      );
    },
  );
}

Using BoxDecoration

final items = ['Horse', 'Cow', 'Camel', 'Sheep', 'Goat'];

@override
Widget build(BuildContext context) {
  return ListView.builder(
    itemCount: items.length,
    itemBuilder: (context, index) {
      return Container(
        decoration: BoxDecoration( //                    <-- BoxDecoration
          border: Border(bottom: BorderSide()),
        ),
        child: ListTile(
          title: Text(items[index]),
        ),
      );
    },
  );
}

Both Divider and BoxDecoration are customizable as far as the line height and color go. Divider also has an indent option, but you could get a BoxDecoration to do the same thing with some padding.

For more style

Use a Card

final items = ['Horse', 'Cow', 'Camel', 'Sheep', 'Goat'];

@override
Widget build(BuildContext context) {
  return ListView.builder(
    itemCount: items.length,
    itemBuilder: (context, index) {
      return Card( //                           <-- Card
        child: ListTile(
          title: Text(items[index]),
        ),
      );
    },
  );
}

The most correct way is to use ListView.separated

ListView.separated(
     itemCount: 25,
     separatorBuilder: (BuildContext context, int index) => Divider(height: 1),
     itemBuilder: (BuildContext context, int index) {
       return ListTile(
         title: Text('item $index'),
       );
     },
);

Put your widget inside container with BoxDecoration as

Container(
   child: YourWidgetHere(),
   decoration: BoxDecoration(
      border: Border(bottom: BorderSide(color: Colors.black26))),
);