In some types of games a command should not start and finish immediately. Loading a crossbow might take a bit of time to do - time you don’t have when the enemy comes rushing at you. Crafting that armour will not be immediate either. For some types of games the very act of moving or changing pose all comes with a certain time associated with it.
The simple way to pause commands with yield¶
Evennia allows a shortcut in syntax to create simple pauses in commands.
This syntax uses the
yield keyword. The
yield keyword is used in
Python to create generators, although you don’t need to know what
generators are to use this syntax. A short example will probably make it
class CmdTest(Command): """ A test command just to test waiting. Usage: test """ key = "test" locks = "cmd:all()" def func(self): self.msg("Before ten seconds...") yield 10 self.msg("Afterwards.")
The important line is the
yield 10. It tells Evennia to “pause” the
command and to wait for 10 seconds to execute the rest. If you add this
command and run it, you’ll see the first message, then, after a pause of
ten seconds, the next message. You can use
yield several times in
This syntax will not “freeze” all commands. While the command is “pausing”, you can execute other commands (or even call the same command again). And other players aren’t frozen either.
Note: this will not save anything in the database. If you reload the game while a command is “paused”, it will not resume after the server has reloaded.
The more advanced way with utils.delay¶
yield syntax is easy to read, easy to understand, easy to use.
But it’s not that flexible if you want more advanced options. Learning
to use alternatives might be much worth it in the end.
Below is a simple command example for adding a duration for a command to finish.
from evennia import default_cmds, utils class CmdEcho(default_cmds.MuxCommand): """ wait for an echo Usage: echo <string> Calls and waits for an echo """ key = "echo" locks = "cmd:all()" def func(self): """ This is called at the initial shout. """ self.caller.msg("You shout '%s' and wait for an echo ..." % self.args) # this waits non-blocking for 10 seconds, then calls self.echo utils.delay(10, callback=self.echo) # call echo after 10 seconds def echo(self): "Called after 10 seconds." shout = self.args string = "You hear an echo: %s ... %s ... %s" string = string % (shout.upper(), shout.capitalize(), shout.lower()) self.caller.msg(string)
Import this new echo command into the default command set and reload the server. You will find that it will take 10 seconds before you see your shout coming back. You will also find that this is a non-blocking effect; you can issue other commands in the interim and the game will go on as usual. The echo will come back to you in its own time.
utils.delay(delay, callback=None, *args, **kwargs) is a useful
function. It will wait
delay seconds, then call a function you give
If you are not familiar with the syntax
**kwargs, see the Python documentation here.
Looking at it you might think that
utils.delay(10, callback) in the
code above is just an alternative to some more familiar thing like
time.sleep(10). This is not the case. If you do
you will in fact freeze the entire server for ten seconds! The
utils.delay()is a thin wrapper around a Twisted Deferred that
will delay execution until 10 seconds have passed, but will do so
asynchronously, without bothering anyone else (not even you - you can
continue to do stuff normally while it waits to continue).
The point to remember here is that the
delay() call will not “pause”
at that point when it is called. The lines after the
will actually execute right away. What you must do is to tell it which
function to call after the time has passed (its “callback”). This may
sound strange at first, but it is normal practice in asynchronous
systems. You can also link such calls together as seen below:
from evennia import default_cmds, utils class CmdEcho(default_cmds.MuxCommand): """ waits for an echo Usage: echo <string> Calls and waits for an echo """ key = "echo" locks = "cmd:all()" def func(self): "This sets off a chain of delayed calls" self.caller.msg("You shout '%s', waiting for an echo ..." % self.args) # wait 2 seconds before calling self.echo1 utils.delay(2, callback=self.echo1) # callback chain, started above def echo1(self): "First echo" self.caller.msg("... %s" % self.args.upper()) # wait 2 seconds for the next one utils.delay(2, callback=self.echo2) def echo2(self): "Second echo" self.caller.msg("... %s" % self.args.capitalize()) # wait another 2 seconds utils.delay(2, callback=self.echo3) def echo3(self): "Last echo" self.caller.msg("... %s ..." % self.args.lower())
The above version will have the echoes arrive one after another, each separated by a two second delay.
> echo Hello! ... HELLO! ... Hello! ... hello! ...
As mentioned, a great thing about the delay introduced by
utils.delay() is that it does not block. It just goes on in the
background and you are free to play normally in the interim. In some
cases this is not what you want however. Some commands should simply
“block” other commands while they are running. If you are in the process
of crafting a helmet you shouldn’t be able to also start crafting a
shield at the same time, or if you just did a huge power-swing with your
weapon you should not be able to do it again immediately.
The simplest way of implementing blocking is to use the technique covered in the Command Cooldown tutorial. In that tutorial we implemented cooldowns by having the Command store the current time. Next time the Command was called, we compared the current time to the stored time to determine if enough time had passed for a renewed use. This is a very efficient, reliable and passive solution. The drawback is that there is nothing to tell the Player when enough time has passed unless they keep trying.
Here is an example where we will use
utils.delay to tell the player
when the cooldown has passed:
from evennia import utils, default_cmds class CmdBigSwing(default_cmds.MuxCommand): """ swing your weapon in a big way Usage: swing <target> Makes a mighty swing. Doing so will make you vulnerable to counter-attacks before you can recover. """ key = "bigswing" locks = "cmd:all()" def func(self): "Makes the swing" if self.caller.ndb.off_balance: # we are still off-balance. self.caller.msg("You are off balance and need time to recover!") return # [attack/hit code goes here ...] self.caller.msg("You swing big! You are off balance now.") # set the off-balance flag self.caller.ndb.off_balance = True # wait 8 seconds before we can recover. During this time # we won't be able to swing again due to the check at the top. utils.delay(8, callback=self.recover) def recover(self): "This will be called after 8 secs" del self.caller.ndb.off_balance self.caller.msg("You regain your balance.")
Note how, after the cooldown, the user will get a message telling them they are now ready for another swing.
By storing the
off_balance flag on the character (rather than on,
say, the Command instance itself) it can be accessed by other Commands
too. Other attacks may also not work when you are off balance. You could
also have an enemy Command check your
off_balance status to gain
bonuses, to take another example.
One can imagine that you will want to abort a long-running command before it has a time to finish. If you are in the middle of crafting your armor you will probably want to stop doing that when a monster enters your smithy.
You can implement this in the same way as you do the “blocking” command above, just in reverse. Below is an example of a crafting command that can be aborted by starting a fight:
from evennia import utils, default_cmds class CmdCraftArmour(default_cmds.MuxCommand): """ Craft armour Usage: craft <name of armour> This will craft a suit of armour, assuming you have all the components and tools. Doing some other action (such as attacking someone) will abort the crafting process. """ key = "craft" locks = "cmd:all()" def func(self): "starts crafting" if self.caller.ndb.is_crafting: self.caller.msg("You are already crafting!") return if self._is_fighting(): self.caller.msg("You can't start to craft " "in the middle of a fight!") return # [Crafting code, checking of components, skills etc] # Start crafting self.caller.ndb.is_crafting = True self.caller.msg("You start crafting ...") utils.delay(60, callback=self.step1) def _is_fighting(self): "checks if we are in a fight." if self.caller.ndb.is_fighting: del self.caller.ndb.is_crafting return True def step1(self): "first step of armour construction" if self._is_fighting(): return self.msg("You create the first part of the armour.") utils.delay(60, callback=self.step2) def step2(self): "second step of armour construction" if self._is_fighting(): return self.msg("You create the second part of the armour.") utils.delay(60, callback=step3) def step3(self): "last step of armour construction" if self._is_fighting(): return # [code for creating the armour object etc] del self.caller.ndb.is_crafting self.msg("You finalize your armour.") # example of a command that aborts crafting class CmdAttack(default_cmds.MuxCommand): """ attack someone Usage: attack <target> Try to cause harm to someone. This will abort eventual crafting you may be currently doing. """ key = "attack" aliases = ["hit", "stab"] locks = "cmd:all()" def func(self): "Implements the command" self.caller.ndb.is_fighting = True # [...]
The above code creates a delayed crafting command that will gradually
create the armour. If the
attack command is issued during this
process it will set a flag that causes the crafting to be quietly
canceled next time it tries to update.
In these examples we only used
utils.delay(), which is a very simple
wrapper around Twisted’s
reactor.callLater(). If you know your
Twisted one might imagine using more advanced features such as
callback/errback chains to more efficiently handle various command
states and conditions.