<h1>Command Duration<aclass="headerlink"href="#command-duration"title="Permalink to this headline">¶</a></h1>
<p>Before reading this tutorial, if you haven’t done so already, you might want to
read <aclass="reference internal"href="../Components/Commands.html"><spanclass="doc std std-doc">the documentation on commands</span></a> to get a basic understanding of
how commands work in Evennia.</p>
<p>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
<h2>The simple way to pause commands with yield<aclass="headerlink"href="#the-simple-way-to-pause-commands-with-yield"title="Permalink to this headline">¶</a></h2>
<p>Evennia allows a shortcut in syntax to create simple pauses in commands. This
syntax uses the <codeclass="docutils literal notranslate"><spanclass="pre">yield</span></code> keyword. The <codeclass="docutils literal notranslate"><spanclass="pre">yield</span></code> 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 clear:</p>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"Before ten seconds..."</span><spanclass="p">)</span>
<div><p>Important: The <codeclass="docutils literal notranslate"><spanclass="pre">yield</span></code> functionality will <em>only</em> work in the <codeclass="docutils literal notranslate"><spanclass="pre">func</span></code> method of
Commands. It only works because Evennia has especially
catered for it in Commands. If you want the same functionality elsewhere you
must use the <aclass="reference internal"href="../Concepts/Async-Process.html#the-interactive-decorator"><spanclass="std std-doc">interactive decorator</span></a>.</p>
</div></blockquote>
<p>The important line is the <codeclass="docutils literal notranslate"><spanclass="pre">yield</span><spanclass="pre">10</span></code>. 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 <codeclass="docutils literal notranslate"><spanclass="pre">yield</span></code> several times in your command.</p>
<p>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.</p>
<blockquote>
<div><p>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
<h2>The more advanced way with utils.delay<aclass="headerlink"href="#the-more-advanced-way-with-utils-delay"title="Permalink to this headline">¶</a></h2>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">yield</span></code> 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.</p>
<p>Below is a simple command example for adding a duration for a command to finish.</p>
<spanclass="sd"> This is called at the initial shout. </span>
<spanclass="sd">"""</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">caller</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="sa">f</span><spanclass="s2">"You shout '</span><spanclass="si">{</span><spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">args</span><spanclass="si">}</span><spanclass="s2">' and wait for an echo ..."</span><spanclass="p">)</span>
<spanclass="c1"># this waits non-blocking for 10 seconds, then calls self.echo</span>
<spanclass="n">utils</span><spanclass="o">.</span><spanclass="n">delay</span><spanclass="p">(</span><spanclass="mi">10</span><spanclass="p">,</span><spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">echo</span><spanclass="p">)</span><spanclass="c1"># call echo after 10 seconds</span>
<spanclass="sa">f</span><spanclass="s2">"You hear an echo: </span><spanclass="si">{</span><spanclass="n">shout</span><spanclass="o">.</span><spanclass="n">upper</span><spanclass="p">()</span><spanclass="si">}</span><spanclass="s2"> ... </span><spanclass="si">{</span><spanclass="n">shout</span><spanclass="o">.</span><spanclass="n">capitalize</span><spanclass="p">()</span><spanclass="si">}</span><spanclass="s2"> ... </span><spanclass="si">{</span><spanclass="n">shout</span><spanclass="o">.</span><spanclass="n">lower</span><spanclass="p">()</span><spanclass="si">}</span><spanclass="s2">"</span>
<spanclass="p">)</span>
</pre></div>
</div>
<p>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
<em>non-blocking</em> 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.</p>
<sectionid="about-utils-delay">
<h3>About utils.delay()<aclass="headerlink"href="#about-utils-delay"title="Permalink to this headline">¶</a></h3>
<p><codeclass="docutils literal notranslate"><spanclass="pre">utils.delay(timedelay,</span><spanclass="pre">callback,</span><spanclass="pre">persistent=False,</span><spanclass="pre">*args,</span><spanclass="pre">**kwargs)</span></code> is a useful function. It will
wait <codeclass="docutils literal notranslate"><spanclass="pre">timedelay</span></code> seconds, then call the <codeclass="docutils literal notranslate"><spanclass="pre">callback</span></code> function, optionally passing to it the arguments
provided to utils.delay by way of *args and/or **kwargs`.</p>
<blockquote>
<div><p>Note: The callback argument should be provided with a python path to the desired function, for
instance <codeclass="docutils literal notranslate"><spanclass="pre">my_object.my_function</span></code> instead of <codeclass="docutils literal notranslate"><spanclass="pre">my_object.my_function()</span></code>. Otherwise my_function would
get called and run immediately upon attempting to pass it to the delay function.
If you want to provide arguments for utils.delay to use, when calling your callback function, you
have to do it separatly, for instance using the utils.delay *args and/or **kwargs, as mentioned
above.</p>
</div></blockquote>
<blockquote>
<div><p>If you are not familiar with the syntax <codeclass="docutils literal notranslate"><spanclass="pre">*args</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">**kwargs</span></code>, <aclass="reference external"href="https://docs.python.org/2/tutorial/controlflow.html#arbitrary-argument-lists">see the Python documentation
here</a>.</p>
</div></blockquote>
<p>Looking at it you might think that <codeclass="docutils literal notranslate"><spanclass="pre">utils.delay(10,</span><spanclass="pre">callback)</span></code> in the code above is just an
alternative to some more familiar thing like <codeclass="docutils literal notranslate"><spanclass="pre">time.sleep(10)</span></code>. This is <em>not</em> the case. If you do
<codeclass="docutils literal notranslate"><spanclass="pre">time.sleep(10)</span></code> you will in fact freeze the <em>entire server</em> for ten seconds! The <codeclass="docutils literal notranslate"><spanclass="pre">utils.delay()</span></code>is
a thin wrapper around a Twisted
<aclass="reference external"href="https://twistedmatrix.com/documents/11.0.0/core/howto/defer.html">Deferred</a> 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).</p>
<p>The point to remember here is that the <codeclass="docutils literal notranslate"><spanclass="pre">delay()</span></code> call will not “pause” at that point when it is
called (the way <codeclass="docutils literal notranslate"><spanclass="pre">yield</span></code> does in the previous section). The lines after the <codeclass="docutils literal notranslate"><spanclass="pre">delay()</span></code> call will
actually execute <em>right away</em>. What you must do is to tell it which function to call <em>after the time
has passed</em> (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:</p>
<spanclass="s2">"This sets off a chain of delayed calls"</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">caller</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="sa">f</span><spanclass="s2">"You shout '</span><spanclass="si">{</span><spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">args</span><spanclass="si">}</span><spanclass="s2">', waiting for an echo ..."</span><spanclass="p">)</span>
<spanclass="c1"># wait 2 seconds before calling self.echo1</span>
<h2>Blocking commands<aclass="headerlink"href="#blocking-commands"title="Permalink to this headline">¶</a></h2>
<p>As mentioned, a great thing about the delay introduced by <codeclass="docutils literal notranslate"><spanclass="pre">yield</span></code> or <codeclass="docutils literal notranslate"><spanclass="pre">utils.delay()</span></code> 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.</p>
<p>The simplest way of implementing blocking is to use the technique covered in the <aclass="reference internal"href="Command-Cooldown.html"><spanclass="doc std std-doc">Command
Cooldown</span></a> 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 <em>very</em>
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.</p>
<p>Here is an example where we will use <codeclass="docutils literal notranslate"><spanclass="pre">utils.delay</span></code> to tell the player when the cooldown has passed:</p>
<spanclass="c1"># we are still off-balance.</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">caller</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You are off balance and need time to recover!"</span><spanclass="p">)</span>
<spanclass="k">return</span>
<spanclass="c1"># [attack/hit code goes here ...]</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">caller</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You swing big! You are off balance now."</span><spanclass="p">)</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">caller</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You regain your balance."</span><spanclass="p">)</span>
</pre></div>
</div>
<p>Note how, after the cooldown, the user will get a message telling them they are now ready for
another swing.</p>
<p>By storing the <codeclass="docutils literal notranslate"><spanclass="pre">off_balance</span></code> 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 <codeclass="docutils literal notranslate"><spanclass="pre">off_balance</span></code> status to gain bonuses, to
take another example.</p>
</section>
<sectionid="abortable-commands">
<h2>Abortable commands<aclass="headerlink"href="#abortable-commands"title="Permalink to this headline">¶</a></h2>
<p>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.</p>
<p>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:</p>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">caller</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You are already crafting!"</span><spanclass="p">)</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You create the first part of the armour."</span><spanclass="p">)</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You create the second part of the armour."</span><spanclass="p">)</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You finalize your armour."</span><spanclass="p">)</span>
<spanclass="c1"># example of a command that aborts crafting</span>
<p>The above code creates a delayed crafting command that will gradually create the armour. If the
<codeclass="docutils literal notranslate"><spanclass="pre">attack</span></code> 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.</p>
</section>
<sectionid="persistent-delays">
<h2>Persistent delays<aclass="headerlink"href="#persistent-delays"title="Permalink to this headline">¶</a></h2>
<p>In the latter examples above we used <codeclass="docutils literal notranslate"><spanclass="pre">.ndb</span></code> storage. This is fast and easy but it will reset all
cooldowns/blocks/crafting etc if you reload the server. If you don’t want that you can replace
<codeclass="docutils literal notranslate"><spanclass="pre">.ndb</span></code> with <codeclass="docutils literal notranslate"><spanclass="pre">.db</span></code>. But even this won’t help because the <codeclass="docutils literal notranslate"><spanclass="pre">yield</span></code> keyword is not persisent and nor is
the use of <codeclass="docutils literal notranslate"><spanclass="pre">delay</span></code> shown above. To resolve this you can use <codeclass="docutils literal notranslate"><spanclass="pre">delay</span></code> with the <codeclass="docutils literal notranslate"><spanclass="pre">persistent=True</span></code>
keyword. But wait! Making something persistent will add some extra complications, because now you
must make sure Evennia can properly store things to the database.</p>
<p>Here is the original echo-command reworked to function with persistence:</p>
<spanclass="sd"> This is called at the initial shout. </span>
<spanclass="sd">"""</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">caller</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="sa">f</span><spanclass="s2">"You shout '</span><spanclass="si">{</span><spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">args</span><spanclass="si">}</span><spanclass="s2">' and wait for an echo ..."</span><spanclass="p">)</span>
<spanclass="c1"># this waits non-blocking for 10 seconds, then calls echo(self.caller, self.args)</span>
<li><p>The callback (<codeclass="docutils literal notranslate"><spanclass="pre">echo</span></code>) was moved out of the class and became its own stand-alone function in the
outermost scope of the module. It also now takes <codeclass="docutils literal notranslate"><spanclass="pre">caller</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">args</span></code> as arguments (it doesn’t have
access to them directly since this is now a stand-alone function).</p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">utils.delay</span></code> specifies the <codeclass="docutils literal notranslate"><spanclass="pre">echo</span></code> function (not <codeclass="docutils literal notranslate"><spanclass="pre">self.echo</span></code> - it’s no longer a method!) and sends
<codeclass="docutils literal notranslate"><spanclass="pre">self.caller</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">self.args</span></code> as arguments for it to use. We also set <codeclass="docutils literal notranslate"><spanclass="pre">persistent=True</span></code>.</p></li>
</ul>
<p>The reason for this change is because Evennia needs to <codeclass="docutils literal notranslate"><spanclass="pre">pickle</span></code> the callback into storage and it
cannot do this correctly when the method sits on the command class. Now this behave the same as the
first version except if you reload (or even shut down) the server mid-delay it will still fire the
callback when the server comes back up (it will resume the countdown and ignore the downtime).</p>
