The offset threshold the item has to be dragged in order to be considered dismissed.
Represented as a fraction, e.g. if it is 0.4 (the default), then the item has to be dragged at least 40% towards one direction to be considered dismissed. Clients can define different thresholds for each dismiss direction.
Flinging is treated as being equivalent to dragging almost to 1.0, so flinging can dismiss an item past any threshold less than 1.0.
See also direction, which controls the directions in which the items can be dismissed. Setting a threshold of 1.0 (or greater) prevents a drag in the given DismissDirection even if it would be allowed by the direction property.
final Map<DismissDirection, double> dismissThresholds