# Control Flow and Collection Literals — Dart

Source: https://www.skillbyai.com/en/dart/n-control

> Use conditions, loops and collection if, for and spread elements.

## Familiar flow with useful extras

Dart has `if`/`else`, `for`, `for-in` (over any `Iterable`), `while`, `do-while`, `break`, `continue`, `assert` (checked in debug mode) and `switch`, which in Dart 3 supports **patterns** and does **not fall through** between non-empty cases. Dart adds expressive **collection literals**. **Collection `if`** includes an element conditionally: `[ 'Home', if (isAdmin) 'Admin', 'Logout' ]`. **Collection `for`** builds elements in place: `[ for (final p in products) p.title ]`. The **spread operator** `...` inserts all elements of another collection, and `...?` handles null. These features are used constantly in Flutter to build lists of widgets declaratively, without separate loops and temporary lists. The **cascade operator** `..` performs several operations on the same object (`paint..color = c..strokeWidth = 2`), and `?..` does so only when the object is not null. Use `for-in` for iteration and `forEach` sparingly, since `forEach` with an `async` callback does not wait for each call.

## Collection if, for, spread and cascades

Declarative list building, as used in Flutter widget trees.

```dart
class Product {
  const Product(this.title, this.price, {this.inStock = true});
  final String title;
  final double price;
  final bool inStock;
}

void main() {
  const products = [
    Product('Notebook', 120),
    Product('Pen', 49.5),
    Product('Ink', 199, inStock: false),
  ];
  const isAdmin = true;
  final extraLinks = <String>['Help'];

  final menu = [
    'Home',
    if (isAdmin) 'Admin',
    for (final p in products) if (p.inStock) 'Buy ${p.title}',
    ...extraLinks,
  ];
  print(menu);   // [Home, Admin, Buy Notebook, Buy Pen, Help]

  final buffer = StringBuffer()
    ..write('Order: ')
    ..write(products.length)
    ..write(' items');              // cascade: several calls on one object
  print(buffer.toString());
}
```

## Packing a bag with a checklist

Collection if and for are a packing checklist written as the bag itself: "umbrella, if it is monsoon; one shirt for each day; plus everything from the toiletry kit". The result is the packed bag, not a set of instructions.

**Quiz:** What does `[ 'Home', if (isAdmin) 'Admin' ]` produce when isAdmin is false?

- [ ] [Home, null]
- [x] [Home]
- [ ] [Home, Admin]
- [ ] A compile error

*Answer:* [Home]. Collection if omits the element entirely when the condition is false.