# Signals

## Overview

The purpose of the signals.t extension is to provide a means for one object to send signals to another (which can then respond to them) and to provide a mechanism for establishing and breaking signalling links between objects. This mechanism employs the Relations extension, which must also be present.

## New Classes, Objects and Methods

In addition to a number of methods intended purely for internal use, this extension defines the following new classes, objects and methods:

- Classes: Signal
- Objects: litSignal, unlitSignal, discoverSignal, undiscoverSignal, lockSignal, unlockSignal, onSignal, offSignal, wornSignal, doffSignal, moveSignal, seenSignal, examineSignal, takeSignal, dropSignal, openSignal, closeSignal, pushSignal, pullSignal, feelSignal
- Methods on Signal: emit().
- New methods on Thing: emit(), handle().

## Usage

Include the signals.t file after the library files but before your game source files. The Relations extensions (relations.t) must also be present.

The basic mechanism is that an object (the sender) sends a signal by calling its emit() method with the signal as the argument, for example:

    emit(openSignal);

Any interested objects can then handle this signal in their handle() method, which takes two arguments, the sender and the signal that's just been sent:

    handle(sender, signal)
    {
       if(sender == safeDoor && signal == openSignal)
         ...
    }

To register that an object (the receiver) is interested in receiving a particular signal from a particular sender, we establish a relation between them using the connect() function:

    connect(sender, signal, receiver);

The relation between sender and receiver can be severed using the unconnect()[1] function:

    unconnect(sender, signal, receiver);

A sender will send signals only to those receivers that have been related to it through the relevant Signal/Relation using the connect() function.

Defining a new signal is usually very straightforward. Since a Signal is a kind of Relation, it can be defined using the Relation template, e.g. to define a signal an object might send when it's cut:

    cutSignal: Signal 'cut';

Here the 'cut' in the template defines the signal's name property, which may be used in the connect() and unconnect() functions in place of the Signal's programmatic name. Thus these two statements do precisely the same thing:

    connect(wire, cutSignal, alarm);
    connect(wire, 'cut', alarm);

That all said, there is an easier and better way do define a Signal, using the DefSignal macro. Using this macro the foregoing definition cutSignal becomes:

     DefSignal(cut, cut);

This expands to:

    cutSignal: Signal
      name = 'cut'
      handleProp = &handle_cut
    ;

The purpose of the handleProp property will be explained below; in the meantime the point to remember is that the macro DefSignal(sig, nam) expands to:

    sigSignal: Signal
      name = 'nam'
      handleProp = &handle_sig

A Signal may be defined with additional properties which the sender can set to convey additional information to the sender. For example this extension defines:

     DefSignal(move, move) destination = nil;

This (the destination property) allows moveSignal to convey information about where the sender was moved to, as well as the fact that it was moved. How it does so will be discussed further below.

Note that there is no need to define the relationType of a Signal since this extension already defines it as manyToMany.

Note also that simply defining a signal doesn't make anything happen. Your code still has to emit it somewhere, and it won't be handled anywhere until you've related it to the relevant receivers using the connect() function. Some signals come predefinined in this extension, however, along with the code to emit them at appropriate points. These are described below.

[Note 1: The name 'unconnect' is used rather than the more normal 'disconnect' mainly because 'disconnect' is already used as a method name elsewhere in the library and so cannot also be used as the name of a function. Also, the use of the names connect() and unconnect() better parallels that of the names relate() and unrelate() to establish and break relations, which is more or less what these functions do as well. Indeed, in many instances, relate() and unrelate() could be used for Signals as well, but the connect() and unconnect() functions do some additional work that is needed in more complex cases, so it is probably best to stick to their use consistently in relation to Signals.]

## Signals Defined in this Extension

This extension defines the following signals and causes them to be emitted as appropriate:

- litSignal: Signal 'lit'; - emitted when an object becomes lit via a call to makeLit(true).
- unlitSignal: Signal 'unlit'; - emitted when an object becomes unlit via a call to makeLit(nil).
- discoverSignal: Signal 'discovered'; - emitted when an object becomes discovered via a call to discover(true).
- undiscoverSignal: Signal 'lost'; - emitted when an object becomes undiscovered via a call to discover(nil)
- lockSignal: Signal 'locked'; - emitted when an object becomes locked via a call to makeLocked(true).
- unlockSignal: Signal 'unlocked'; - emitted when an object becomes unlocked via a call to makeLocked(nil).
- onSignal: Signal 'turned on'; - emitted when an object becomes switched on via a call to makeOn(true).
- offSignal: Signal 'turned off'; - emitted when an object becomes switched on via a call to makeOn(nil).
- openSignal: Signal 'open'; - emitted when an object becomes open via a call to makeOpen(true).
- closeSignal: Signal 'closed'; - emitted when an object becomes closed via a call to makeOpen(nil).
- wornSignal: Signal 'worn' wearer = nil; - emitted when an object becomes worn via a call to makeWorn(wearer).
- doffSignal: Signal 'doffed'; - emitted when an object becomes not worn via a call to makeWorn(nil).
- moveSignal: Signal 'moved' destination = nil; - emitted when an object is moved via a call to moveInto(dest)
- actmoveSignal: Signal 'action moved' destination = nil; - emitted when an object is moved via a call to actionMoveInto(dest). Note that if an actmoveSignal is sent a moveSignal will also be sent, but the reverse is not necessarily the case. An actmoveSignal is only sent if the sender is moved as the direct result of some player or NPC action, whereas a moveSignal may also be sent if the sender is moved by authorial fiat for some other reason.
- seenSignal: Signal 'seen' location = nil; - emitted when noteSeen() is called on the object.
- examineSignal: Signal 'examine'; - emitted when the EXAMINE action is carried out on the object.
- takeSignal: Signal 'take'; - emitted when the TAKE action is carried out on the object.
- dropSignal: Signal 'drop'; - emitted when the DROP action is carried out on the object.
- pushSignal: Signal 'push'; - emitted when the PUSH action is carried out on the object.
- pullSignal: Signal 'pull'; - emitted when the PULL action is carried out on the object.
- feelSignal: Signal 'feel'; - emitted when the FEEL action is carried out on the object.

Note that the last six signals are emitted as a result of actions being carried out, while all the rest are emitted by state changes. This difference is reflected in the (string) name properties assigned to each signal; state-change signals have names like 'closed' or 'locked' that reflect the state just attained, while action signals have (string) names like 'take' or 'drop' that reflect the name of the action just carried out.

Some of these signals have additional properties, like destination on moveSignal or location on seenSignal. These are ways of passing additional information via the signal. The recipient may want to know not simply that the sender has moved, but where it's moved to; the destination property supplies this information. In a similar way the location property of seenSignal specifies where the target has just been seen. The extension sets these properties when the relevant signals are emitted, but in the next section we look at how such properties are set so you can do the same on any additional signals you define in your own game.

Note that defining additional signals that are automatically emitted by TActions is very straightforward: you just define the new Signal and assign it to the signal property of the action in question, e.g.:

    DefSignal(clean, clean);

    modify Clean
      signal = cleanSignal
    ;

This will result in a cleanSignal being emitted by the direct object of a CLEAN action. Note, however, that this short-cut is only available for TActions, and not for any other kind of action (TIActions included). If you want signals to be triggered by any other kind of action you'll need insert your own call to the emit() method at some appropriate point in the code that handles the action. For anything other than a TAction it is probably better for game code to decide what the appropriate point is in individual cases (whereas for a TAction it makes good sense simply to emit the relevant signal from the direct object immediately after the action has taken place).

## Sending Additional Information via Signal Properties

In order to add information about where the sender is being moved to when the moveSignal is emitted from moveInto(), this extension defines moveInto() as follows:

     moveInto(newCont)
     {
            inherited(newCont);

            emit(moveSignal, newCont);
     }

newCont is the new container into which the object is being moved. This is the value that needs to be assigned to the destination property of moveSignal. But how does newSignal know which property to set to this value? This is defined in the propList property of the Signal. The propList property contains a list of property pointers, which define the properties to which successive arguments of the emit() method are to be assigned. For example, moveSignal defines propList = [&destination]; this means that the first argument of the emit() method following the signal name is assigned to the destination property. If fooSignal defined propList as [&foobar, &bar, &thingy], then a call to emit(fooSignal, x, y, z) would set fooSignal.foobar to x, fooSignal.bar to y, and fooSignal.thingy to z.

Since this relies on matching the argument list in the call to emit() to the order of properties defined in the Signal's propList property (which may not always be easy to remember), an alternative syntax is also available that allows values to be assigned to properties by supplying lists of property pointers followed by the value to be assigned to the corresponding property. So, for example, emit(moveSignal, newCont) could instead be written as emit(moveSignal, [&destination, newCont]), which makes it explicit that the value of newCont is to be assigned to the destination property, and which doesn't depend on getting the order of properties right. Thus the fooSignal example could be written as emit(fooSignal, [&thingy, z], [&bar, y], [&foo, x]) and the end result would be the same.

You can mix the two ways of assigning values to properties, but only if the lists come after the positional properties. Thus emit(fooSignal, x, [&thingy, z], [&bar, y]) would be fine, but not emit(fooSignal, [&thingy, z], [&bar, y], x). Also, you should not attempt to do something like this:

        fooSignal.foobar = x; //DON'T DO THIS!!
        fooSignal.bar = y;
        fooSignal.thingy = z;
        emit(fooSignal);

The reason for not doing this is that the call to emit() will overwrite all the properties in the propList list to null before assigning any values passed as parameters. This is to prevent spurious values left over from a previous emit() call being sent to the wrong handler. We use null rather than nil as the non-value, since in some circumstances a value of nil could be significant; for example a call to moveInto(nil) would cause a moveSignal to be emitted with a destination of nil, which might be important information for any handler that receives the signal.

## More Complex Handling

So far we have assumed that a signal will be handled by the handle() method on any interested recipient. In fact handle() is just the fall-back (or default) method that is used if no more specific handler method has been defined. In fact each Signal can have its own handler method. You may recall that when a Signal is defined with the DefSignal() macro, this automatically initializes its handleProp property with a property pointer like &handle_sig for a Signal called sigSignal (e.g. handleProp is &handle_move for the moveSignal). This means that when a receiver gets a moveSignal (say), it will call its handle_move() method to handle it if one is defined, and fall back on the handle() method otherwise.

There's also a third possibility: the handler to be used can be overridden by the call to the connect() function. If connect() is called with a fourth argument (which should then be a property pointer), the method specified by that fourth argument will be used as the signal handler. A call to connect(sender, signal, receiver, &special_handler) will cause signal to be handled on receiver by receiver.special_handler(sender, signal), provided that receiver defines the special_handler() method. The receiver's dispatchSignal() method takes care of assigning a handler to a signal, and what it does may be summarized as follows:

1. If the call to connect() has established a special handler for this signal on this receiver, assign that special handler to prop.
2. Otherwise, if the signal has a property pointer assigned to its handleProp property (as any Signal defined with the DefSignal() macro will have), then assign that property to prop.
3. Otherwise, assign the default handler &handle to prop.
4. If prop now points to a method that's actually defined on the receiver, then call prop (with sender and signal as its arguments); otherwise call handle(sender, signal).

Note that this means that the special_handler method passed as the optional fourth argument to connect() can be either an existing standard handler or a new custom one, but it must be defined with two parameters corresponding to sender and signal. So, for example, you can't call something like connect(redLever, pullSignal, trapdoor, &makeOpen), since instead of opening the trapdoor it will simply cause a run-time error due to argument mismatch. You would instead need to call makeOpen(true) from within the trapdoor's handle() or handle_pull() method.

## Example

Suppose that somewhere in our game there's a big red switch (in the hall cupboard, say) that's meant to control a light in another location (the cellar, say), except that before it will work some cable needs to be reconnected. In the code for reconnecting the cable we might include the statements:

     connect(redSwitch, onSignal, cellarLight);
     connect(redSwitch, offSignal, cellarLight);

Since onSignal and offSignal will be emitted by the switch in any case, the only other step is to handle them on the cellar light. If we know that these are the only signals the cellar light is ever going to handle, we could simply do this:

    + cellarLight: Fixture 'light'
      ...
      handle(sender, signal)
      {
          if(signal == onSignal)
             makeLit(true);
          if(signal == offSignal)
             makeLit(nil);
      }
    ;

But suppose it's possible to cut the cable after it's been reconnected. Presumably that would cause the light to go out again (if it was on). Also we might want to describe the light going out differently if the cable is cut from the way we describe it if the switch is turned off. First we need to define a cutSignal:

     defSignal(cut, cut);

     modify Cut
       signal = cutSignal
     ;

Then, somewhere in the code that handles the cutting action on the cable, we need to register both the sending of the cutSignal from the cable to the light, and at the same time sever the sending of any signal from the switch to the light:

     cable: Fixture 'cable'
        ...

        isCut = nil

        dobjFor(Cut)
        {
           verify()
           {
              if(isCut)
                illogicalNow('The cable has already been cut. ');
           }

           action()
           {
              connect(self, cutSignal, cellarLamp);
              unconnect(redSwitch, onSignal, cellarLamp);
              unconnect(redSwitch, offSignal, cellarLamp);

              "You cut through the cable with your trusty knife. "
              isCut = true;
           }

        }
    ;

Then we have to write a rather more complicated handler on the cellar light:

    + cellarLight: Fixture 'light'
      ...
      handle(sender, signal)
      {
          if(signal == onSignal)
          {
             makeLit(true);
             senseSay('The light comes on. ', self);
          }
          if(signal == offSignal)
          {
             senseSay('The light suddenly goes out. ', self);
             makeLit(nil);
          }
          if(signal == cutSignal && isLit)      {

             senseSay('The light flickers and goes out. ');
             makeLit(nil);
          }
      }
    ;

The senseSay() function is used here to ensure that the message about the light going on or off is only displayed if the player character can see the cellar light. But the main point here is that the handle() method is beginning to become a little cumbersome. At this point it might be better to split the handling up between the various specialized handlers rather than using the catch-all default handle() method:

    + cellarLight: Fixture 'light'
      ...
      handle_on(sender, signal)
      {
         makeLit(true);
         senseSay('The light comes on. ', self);
      }

      handle_off(sender, signal)
      {
         senseSay('The light suddenly goes out. ', self);
         makeLit(nil);
      }

      handle_cut(sender, signal)
      {
         if(isLit)
         {
             senseSay('The light flickers and goes out. ');
             makeLit(nil);
         }
      }
    ;

Finally, suppose that it's possible to reconnect the cable after it's been cut, but that this reconnects things the wrong way round so that turning on the switch turns off the light and vice versa. If we've split the handlers into separate methods as above, we can then just write the relevant part of the re-connection code like so:

       connect(redSwitch, onSignal, cellarLight, &handle_off);
       connect(redSwitch, offSignal, cellarLight, &handle_on);

This will then make turning the switch on turn off the light, and turning the switch off turn on the light, sincce we've swapped over the normal handlers.

This covers most of what you need to know to use this extension. For additional information see the source code and comments in the signals.t file.

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