getNotifier method Null safety
- BuildContext context
Obtains a ValueNotifier from the TickerMode surrounding the context
,
which indicates whether tickers are enabled in the given subtree.
When that TickerMode enabled or disabled tickers, the notifier notifies its listeners.
While the ValueNotifier is stable for the lifetime of the surrounding
TickerMode, calling this method does not establish a dependency between
the context
and the TickerMode and the widget owning the context
does not rebuild when the ticker mode changes from true to false or vice
versa. This is preferable when the ticker mode does not impact what is
currently rendered on screen, e.g. because it is ony used to mute/unmute a
Ticker. Since no dependency is established, the widget owning the
context
is also not informed when it is moved to a new location in the
tree where it may have a different TickerMode ancestor. When this
happens, the widget must manually unsubscribe from the old notifier,
obtain a new one from the new ancestor TickerMode by calling this method
again, and re-subscribe to it. StatefulWidgets can, for example, do this
in State.activate, which is called after the widget has been moved to
a new location.
Alternatively, of can be used instead of this method to create a
dependency between the provided context
and the ancestor TickerMode.
In this case, the widget automatically rebuilds when the ticker mode
changes or when it is moved to a new TickerMode ancestor, which
simplifies the management cost in the widget at the expensive of some
potential unnecessary rebuilds.
In the absence of a TickerMode widget, this function returns a ValueNotifier, whose ValueNotifier.value is always true.
Implementation
static ValueNotifier<bool> getNotifier(BuildContext context) {
final _EffectiveTickerMode? widget = context.getElementForInheritedWidgetOfExactType<_EffectiveTickerMode>()?.widget as _EffectiveTickerMode?;
return widget?.notifier ?? ValueNotifier<bool>(true);
}