KeepAliveNotification class

Indicates that the subtree through which this notification bubbles must be kept alive even if it would normally be discarded as an optimization.

For example, a focused text field might fire this notification to indicate that it should not be disposed even if the user scrolls the field off screen.

Each KeepAliveNotification is configured with a handle that consists of a Listenable that is triggered when the subtree no longer needs to be kept alive.

The handle should be triggered any time the sending widget is removed from the tree (in State.deactivate). If the widget is then rebuilt and still needs to be kept alive, it should immediately send a new notification (possible with the very same Listenable) during build.

This notification is listened to by the AutomaticKeepAlive widget, which is added to the tree automatically by SliverList (and ListView) and SliverGrid (and GridView) widgets.

Failure to trigger the handle in the manner described above will likely cause the AutomaticKeepAlive to lose track of whether the widget should be kept alive or not, leading to memory leaks or lost data. For example, if the widget that requested keepalive is removed from the subtree but doesn't trigger its Listenable on the way out, then the subtree will continue to be kept alive until the list itself is disposed. Similarly, if the Listenable is triggered while the widget needs to be kept alive, but a new KeepAliveNotification is not immediately sent, then the widget risks being garbage collected while it wants to be kept alive.

It is an error to use the same handle in two KeepAliveNotifications within the same AutomaticKeepAlive without triggering that handle before the second notification is sent.

For a more convenient way to interact with AutomaticKeepAlive widgets, consider using AutomaticKeepAliveClientMixin, which uses KeepAliveNotification internally.