Preconditions

Boxer gives you a way to short-circuit box rendering via the precondition block within a box. This block is always executed before any views of the box and is meant to provide an alternate rendering of the box, e.g. in case a primary object is nil.

Boxer.box(:user) do |box, user|
  box.precondition {|resp| resp.emit({}) if user.nil? }
  box.view(:base) { {:name => user.name} }
end

In this example, rendering the :base view would fail when user is nil, so the precondition preemptively returns {} via the emit method. As soon as the emit method is called, rendering of the current box is abandoned and the passed value is returned from ship.

>> Boxer.ship(:user, nil)
=> {}

>> Boxer.ship(:user, User.first)
=> {:name => "Bob Jones"}