Widget: Keys

  • Use keys to control which widgets the framework matches up with other widgets when a widget rebuilds.
  • By default, the framework matches widgets in the current and previous build according to their runtimeType and the order in which they appear. With keys, the framework requires that the two widgets have the same key as well as the same runtimeType.
  • Keys are most useful in widgets that build many instances of the same type of widget. For example, the ShoppingList widget, which builds just enough ShoppingListItem instances to fill its visible region:.
  • Without keys, the first entry in the current build would always sync with the first entry in the previous build, even if, semantically, the first entry in the list just scrolled off screen and is no longer visible in the viewport.
  • By assigning each entry in the list a “semantic” key, the infinite list can be more efficient because the framework syncs entries with matching semantic keys and therefore similar (or identical) visual appearances. Moreover, syncing the entries semantically means that state retained in stateful child widgets remains attached to the same semantic entry rather than the entry in the same numerical position in the viewport.

    • Related concepts

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    Keys
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    Widget: Keys — Structure map

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    Widget: Keys — Related pages:

    Introduction to widgets