when method Null safety

VoidCallback when(
  1. void callback(
    1. Object action
    )
)

Implementation

VoidCallback when(void Function(Object action) callback) {
  var active = true;
  _dispatchEmitter.on(callback);
  return () {
    if (!active) return;
    active = false;
    _dispatchEmitter.off(callback);
  };
}