How do keys work and when do you need them?

Without a key, Flutter matches children by runtime type and position. When list items are inserted, removed or reordered, position-based matching can attach state to the wrong item. A key tells Flutter which element belongs to which logical item.

ListView.builder(
  itemCount: tasks.length,
  itemBuilder: (context, index) {
    final task = tasks[index];
    return Dismissible(
      key: ValueKey(task.id),
      onDismissed: (_) => removeTask(task.id),
      child: ListTile(title: Text(task.title)),
    );
  },
)

• ValueKey compares by value and is the default choice for list items
• ObjectKey compares by object identity and is useful for model instances
• UniqueKey forces a fresh element every build and is rarely appropriate
• GlobalKey allows state access and reparenting but is expensive and easy to misuse

Where keys are required

Keys matter when the children of the same parent can change order or identity, or when state must follow an item as it moves. They are unnecessary for a fixed, static list. Two siblings must not share the same key, because that causes a duplicate-key assertion.