SenseFuse : Fuse
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One problem you may have with a Fuse is that it could go off when the player character is not near enough to perceive the result, but that any text defined in the method that's executed when the Fuse fires will be displayed whether it describes something the player character could witness or not. This wasn't a problem with the Daemon and Fuse we defined on the gold tablet, but in other situations in might be. In such situations what you need is a SenseFuse - a special type of Fuse that won't display any messages if the player character isn't there to see (or hear, or smell) what happens.

The way to set up a SenseFuse is very similar to the way you set up a Fuse, except that there are a couple of extra properties:

new SenseFuse(obj, &prop, turns, source, sense);

This definition will cause obj.prop to be executed after turns turns, as with a Fuse. The difference is that the player will only see any messages displayed by obj.prop if, at that point in time, the player character can sense the source object (which in practice may often be the same as the obj object, but need not be) using the sense sense (which could be sight, sound, smell or touch - most likely one of the first two).

As an example we'll put a SenseFuse on an exploded bomb hidden under a pile of rubble. Once the player finds the bomb in the rubble, a new fuse is created that will cause the bomb to explode in three turns (killing the player if s/he is still rash enough to be around). Obviously, we'll also need to create an environment for the bomb, so we'll start by adding a new location the Tardis can reach:

+ tardisDestinations : SecretFixture, PreinitObject
  destinations = static new LookupTable
    destinations['A0'] = hold;
    destinations['A2'] = spaceStation;
    destinations['C9'] = redDesert;
    destinations['T5'] = outsideCave;
    destinations['Q7'] = londonStreet;

Then we can proceed to define a couple of locations, some rubble, and a bomb:

/* London - 1940 */

londonStreet : OutdoorRoom 'City Street' 'the city street'
   "Several burned-out and half-destroyed buildings line this section of 
    the city street along with the ones that are still standing. The destruction
    seems to have been fairly recent, since the rubble of fallen masonry still
    spills out onto the street, blocking the way south. The road continues to
    the north. "
    south : NoTravelMessage { "The rubble blocks your path. "; }
    east : NoTravelMessage { "The houses directly to the east are burned-out
     shells; they don't look safe to enter. "; }
    north = streetJunction 

+ rubble : Immovable 'pile rubble' 'rubble'
  "The largest pile of rubble spills out into the street and blocks progress south. 
   <<bomb.isIn(nil) ? specialDesc : nil>> "
        "There's nothing much there but rubble. ";
      else if(bomb.discovered)
        "The bomb is still there. ";
        "You find a metal cylinder buried among the rubble. It looks horribly
         like a bomb. ";
  dobjFor(LookIn) asDobjFor(LookUnder)
  specialDesc = "Pieces of rubble have been blown all over the street, surrounding
    a fresh bomb crater. "
  specialDescOrder = 70
  useSpecialDesc() { return bomb.isIn(nil); }

+ bomb : Hidden, Immovable 'unexploded bomb/cylinder' 'bomb'
  "It's a fat, round-nosed cylinder with tail fins, on a couple of which
    are painted tiny swastikas. "
    new SenseFuse(self, &explode, 3, self, sight);
    "The bomb explodes, the blast sending chunks of masonry flying in all
     directions, one piece of which strikes you squarely on the head. ";
  cannotTakeMsg = 'You must be joking! '
  cannotPushMsg = 'That might set it off. '
  cannotMoveMsg = 'It\'s probably safest to leave it just where it is. '

streetJunction : OutdoorRoom 'Street Junction' 'the junction'
   "The street from the south meets another running east-west. A short way down
    to the street to the east a fire crew is fighting a blazing fire. "
   south = londonStreet
   east : FakeConnector { "After taking a few steps east you recall that 
     discretion is the better part of valour and decide to keep out of the
     way of the fire crew. "}
   atmosphereList : ShuffledEventList
     [ 'The drone of aircraft engines can be heard overhead. ',
       'From somewhere across the city comes the wail of a distant siren. ',
       'From somewhere to the <<dirn>> comes the bark of anti-aircraft fire. ',
       'Off to the <<dirn>> you hear the blast of a whistle and the sound 
        of running feet. ',
       'A fire engine races down a street somewhere to the <<dirn>> .',
       'There\'s a sudden explosion somewhere off to the <<dirn>>, as 
        another bomb finds a target. '    
     dirn = (rand('north' , 'south' , 'east' , 'west'))

Note that this is a case where we can safely use an embedded expression within a single-quoted string in an EventList. Although it will result in the dirn property being evaluated several times per turn (somewhere around six), the fact that there are six calls to rand() for every time we see one of these atmosphere strings won't matter much in practice.

We override the discover() method of the bomb to set up the new SenseFuse, in such a way that the player character must be in a position to see it for the messages in explode() to be displayed to the player. In this case we could have achieved much the same effect by incorporating the message that's displayed into the group of statements governed by the
if(gPlayerChar.isIn(location)) statement, but that would not always be so convenient, and we're trying to illustrate a SenseFuse!

Later on we'll make this bomb a bit more interesting, for example by adding a Noise object to make it tick. In the meantime note the use of the specialDesc property on the rubble; we override useSpecialDesc so that this specialDesc is displayed only after the bomb has gone off, and we set specialDescOrder to 70 (lower than the default of 100) so that the description of the new bomb crater etc. will come earlier in the listing of the room contents than other specialDescs and initDescs.