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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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Mark,<br>
<br>
<blockquote type="cite">
<div class="">Still struggling with this. It seems like your
DuktapeObject will do this, but I can’t work out how it works.</div>
</blockquote>
<br>
I admit my documentation has some shortcomings. I'll try to fill
that gap first:<br>
<br>
<br>
<b><u>Concept #1: Reference counting</u></b><br>
<br>
A DuktapeObject is meant to be a C++/OS extension of a Javascript
object, for example to hold some system binding associated with the
JS object.<br>
<br>
The primary goal is to provide asynchronous operations on the system
side, initiating JS callbacks when finishing/failing. For example a
HTTP operation can be started by a script, passing some "done"
callback. The system then starts the network operation
asynchronously, the JS task can continue processing other scripts.
Once the network operation is done, the DuktapeObject sends a
request to the Duktape task to execute the "done" callback on
itself.<br>
<br>
So the DuktapeObject is normally shared by multiple contexts and
parallel operations. Asynchronous operation also means the JS object
or context may already be gone when the operation finishes. So the
DuktapeObject needs to stay locked in memory independent of the JS
context.<br>
<br>
That's implemented by counting the active references to the
DuktapeObject (methods Ref() / Unref()). The last Unref()
automatically deletes the DuktapeObject instance.<br>
<br>
For example: JS requests a network operation. The initial JS binding
(see coupling) sets the reference count to 1. The DuktapeObject
starts the network request, increasing the ref count to 2. If the JS
context now dies (decreasing the reference count), the DuktapeObject
will still remain valid until the network operation returns.<br>
<br>
As the Ref/Unref operations need a (recursive) mutex, that's also
part of the DuktapeObject and exposed by the API for other uses:
Lock() / Unlock().<br>
<br>
<br>
<b><u>Concept #2: Coupling</u></b><br>
<br>
Javascript does not have a destructor concept. JS objects get
deleted by heap destruction or by the garbage collector when no
reference to the JS object is left. In both cases, the actual object
deletion is called "finalization", and a special finalizer
method/callback can be installed on a JS object to be called just
before the object gets deallocated. That is done by the
DuktapeObject::Couple() method (implicitly called when constructed
directly with a JS object reference).<br>
<br>
There is no way to force finalization on a JS object. So a
DuktapeObject cannot tell Duktape to delete it's coupled object,
that means a DuktapeObject should normally not be deleted from
outside the Duktape context, at least not if still coupled to the JS
object. Coupling and decoupling can only be done in the Duktape
context.<br>
<br>
The standard finalizer DuktapeObject::Finalizer() simply decouples,
automatically deleting itself if the coupling was the last
reference. This is a virtual method, so can be overridden as
necessary.<br>
<br>
The coupling operation additionally adds a hidden pointer to the
DuktapeObject instance in the JS object. That allows to check for
and retreive associated DuktapeObject instances from any JS object,
which is provided by the GetInstance() call.<br>
<br>
<br>
<b><u>Concept #3: Registration</u></b><br>
<br>
For asynchronous operations, it's normally very convenient to have a
"fire & forget" API. Example from the documentation:<br>
<pre id="codecell13" style="box-sizing: border-box; font-family: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", Courier, monospace; font-size: 12px; white-space: pre; margin: 0px; padding: 12px; display: block; overflow: auto; line-height: 1.4; color: rgb(64, 64, 64); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><span class="nx" style="box-sizing: border-box;">VFS</span><span class="p" style="box-sizing: border-box;">.</span><span class="nx" style="box-sizing: border-box;">Save</span><span class="p" style="box-sizing: border-box;">({</span>
<span class="nx" style="box-sizing: border-box;">path</span><span class="o" style="box-sizing: border-box; color: rgb(102, 102, 102);">:</span> <span class="s2" style="box-sizing: border-box; color: rgb(186, 33, 33);">"/sd/mydata/telemetry.json"</span><span class="p" style="box-sizing: border-box;">,</span>
<span class="nx" style="box-sizing: border-box;">data</span><span class="o" style="box-sizing: border-box; color: rgb(102, 102, 102);">:</span> <span class="nx" style="box-sizing: border-box;">Duktape</span><span class="p" style="box-sizing: border-box;">.</span><span class="nx" style="box-sizing: border-box;">enc</span><span class="p" style="box-sizing: border-box;">(</span><span class="s1" style="box-sizing: border-box; color: rgb(186, 33, 33);">'jx'</span><span class="p" style="box-sizing: border-box;">,</span> <span class="nx" style="box-sizing: border-box;">telemetry</span><span class="p" style="box-sizing: border-box;">),</span>
<span class="nx" style="box-sizing: border-box;">fail</span><span class="o" style="box-sizing: border-box; color: rgb(102, 102, 102);">:</span> <span class="kd" style="box-sizing: border-box; color: rgb(0, 128, 0); font-weight: bold;">function</span><span class="p" style="box-sizing: border-box;">(</span><span class="nx" style="box-sizing: border-box;">error</span><span class="p" style="box-sizing: border-box;">)</span> <span class="p" style="box-sizing: border-box;">{</span>
<span class="nx" style="box-sizing: border-box;">print</span><span class="p" style="box-sizing: border-box;">(</span><span class="s2" style="box-sizing: border-box; color: rgb(186, 33, 33);">"Error saving telemetry: "</span> <span class="o" style="box-sizing: border-box; color: rgb(102, 102, 102);">+</span> <span class="nx" style="box-sizing: border-box;">error</span><span class="p" style="box-sizing: border-box;">);</span>
<span class="p" style="box-sizing: border-box;">}</span>
<span class="p" style="box-sizing: border-box;">});</span>
</pre>
I.e. you simply pass the operation arguments including the done/fail
callbacks to an API method and don't need to care about storing a
reference to some handle. In JS that normally means the object used
won't have any reference left after the call, so would be deleted by
the garbage collector on the next run.<br>
<br>
To avoid garbage collection and lock the JS object in memory, we
need to store a reference to it in a "public" place. Duktape
provides a special public place for this, hidden from scripts,
called the global stash. DuktapeObject maintains a dedicated global
object registry in that stash.<br>
<br>
Adding and removing the coupled object reference to/from that
registry is done by the Register() and Deregister() methods.<br>
<br>
So for asynchronous system operations, or system integrations that
shall be persistent, you normally do a Register() call together with
the coupling, unless some ressource isn't available. Deregistration
is then normally done when all pending JS callbacks have been
executed, or when the persistent system integration has been
unbound.<br>
<br>
Other API designs are possible here: if you'd rather like the script
needing to store a reference to your operation handle, you don't
need to do a registration. The object will then be deleted
(finalized) by the garbage collector automatically after the script
deletes the reference.<br>
<br>
<br>
<b><u>Concept #4: Callback invocation</u></b><br>
<br>
Triggers on the system side, for example a finished or failed
network operation, shall normally trigger a JS method execution.<br>
<br>
JS callback methods are simply passed as part of the arguments
object in modern JS APIs. This allows to pass simple function
definitions inline, as well as to reference a separately defined
general handler function. JS allows functions to be excuted in the
context of any object, and callbacks normally are executed in the
context of the API object. This adds even more convenience, as the
callbacks can easily access the other API arguments still stored in
the object, as well as additional data added by the call.<br>
<br>
JS callbacks cannot be executed directly from any system context,
they need to run in the Duktape context. So the DuktapeObject
callback invocation mechanism includes a general method to request a
callback execution by Duktape: RequestCallback()<br>
<br>
Note: RequestCallback() is an asynchronous operation. A synchronous
variant can be added if necessary (and probably will be for command
execution from a console).<br>
<br>
A pending callback automatically increments the reference count, so
the object is locked in memory until the callback has been executed
(or aborted) by the Duktape task.<br>
<br>
The callback invocation API provides a void* for simple data (e.g. a
fixed string) to be passed to the callback method, but for more
complex data, you will normally fill some DuktapeObject member
variables before invoking the callback.<br>
<br>
In Duktape context, the callback invocation translates the data
returned or provided by the system side into the Duktape callback
arguments and then runs the callback (if the object actually has the
requested callback set). The default implementation for this is
DuktapeObject::CallMethod(), which can be used directly for simple
callbacks without arguments. For more complex handling, override
this with your custom implementation.<br>
<br>
The callbacks are by default executed on the coupled JS object, so
data can also be transported by setting properties on that object.
The callback can then simply access them via "this".<br>
<br>
To simplify callback invocation from code parts that may run outside
or inside Duktape, it's convenient to allow calling CallMethod()
without a Duktape context, and let CallMethod() translate that into
a RequestCallback() call as necessary. Pattern:<br>
<br>
<tt>duk_ret_t DuktapeHTTPRequest::CallMethod(duk_context *ctx, const
char* method, void* data /*=NULL*/)</tt><tt><br>
</tt><tt> {</tt><tt><br>
</tt><tt> if (!ctx)</tt><tt><br>
</tt><tt> {</tt><tt><br>
</tt><tt> RequestCallback(method, data);</tt><tt><br>
</tt><tt> return 0;</tt><tt><br>
</tt><tt> }</tt><tt><br>
…<br>
</tt><br>
A CallMethod() implementation isn't limited to executing a single
callback. A common example is an API defining "done" & "fail"
callbacks, as well as a general final "always" callback. <tt>DuktapeHTTPRequest::CallMethod()</tt>
also serves as an example implementation for this.<br>
<br>
<br>
<br>
Wow… that's become more to write & read than I expected. Please
provide some feedback: is that explanation sufficient & clear?
I'll refine it for the developer docs then.<br>
<br>
Regards,<br>
Michael<br>
<br>
<br>
<div class="moz-cite-prefix">Am 01.09.20 um 19:52 schrieb Michael
Balzer:<br>
</div>
<blockquote type="cite"
cite="mid:c19950a1-cf6e-928f-9ddd-90c444bb3d15@expeedo.de">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
Mark,<br>
<br>
I'll have a look.<br>
<br>
Regards,<br>
Michael<br>
<br>
<br>
<div class="moz-cite-prefix">Am 01.09.20 um 07:30 schrieb Mark
Webb-Johnson:<br>
</div>
<blockquote type="cite"
cite="mid:9393E6BF-E1C9-495C-88CE-16082D7678A0@webb-johnson.net">
<meta http-equiv="Content-Type" content="text/html;
charset=UTF-8">
Michael,
<div class=""><br class="">
</div>
<div class="">Still struggling with this. It seems like your
DuktapeObject will do this, but I can’t work out how it works.</div>
<div class=""><br class="">
</div>
<div class="">Here are some notes one what I have done so far:</div>
<div class=""><br class="">
</div>
<div class="">
<ol class="MailOutline">
<li class="">Created a stub DuktapeConsoleCommand (derived
from DuktapeObject) in ovms_duktape.{h,cpp}. This should
hold enough to be able to call the javascript callback
method for that object. It also stores the module filename
(so the registration can be removed when the module is
unloaded).<br class="">
<br class="">
</li>
<li class="">Provide a DuktapeCommandMap m_cmdmap in
ovms_duktape.{h,cpp} OvmsDuktape class that stores a
mapping from OvmsCommand to DuktapeConsoleCommand.<br
class="">
<br class="">
</li>
<li class="">Created
a OvmsDuktape::RegisterDuktapeConsoleCommand in
ovms_duktape.{h,cpp) that (a) creates the OvmsCommand()
object, (b) registers it, (c) creates
the DuktapeConsoleCommand() object, and (d) updates a map
from OvmsCommand->DuktapeConsoleCommand. There Is also
a single callback DukOvmsCommandRegisterRun designed to be
run by all.<br class="">
<br class="">
</li>
<li class="">Created
hooks NotifyDuktapeModuleLoad, NotifyDuktapeModuleUnload,
and NotifyDuktapeModuleUnloadAll in OvmsDuktape. The
javascript module is identified by filename (path to
module or script on vfs, usually, but may also be an
internal module). The Unload functions look through the
m_cmdmap and unregister commands for javascript modules
being unloaded.<br class="">
<br class="">
</li>
<li class="">Provide an implementation for
ovms_command DukOvmsCommandRegister to support registering
commands from Javascript modules. This should extract the
details, and then call
OvmsDuktape::RegisterDuktapeConsoleCommand to do the
actual registration. This has been implemented, except for
the callback method (and somehow passing that method from
Javascript in the OvmsCommand.Register javascript call).<br
class="">
<br class="">
</li>
<li class="">Provide a stub implementation for <span
style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);"
class="">DukOvmsCommandRegisterRun. This uses m_cmdmap
to lookup the </span><font class="" color="#000000">DuktapeConsoleCommand
object for the command to be run. It should execute the
callback method (but that part is not yet implemented).</font></li>
</ol>
</div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">I still need help with #5 and #6. What needs to be
implemented in DuktapeConsoleCommand, and how is the parameter
in <span style="caret-color: rgb(0, 0, 0); color: rgb(0, 0,
0);" class="">OvmsCommand.Register used to store the
callback (#5)? Then how to callback the command method from</span> <span
style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);"
class="">DukOvmsCommandRegisterRun</span><span
style="caret-color: rgb(0, 0, 0); color: rgb(0, 0, 0);"
class=""> (#6)? If you have time, it is probably much
quicker for you to simply make those changes.</span></div>
<div class=""><span style="caret-color: rgb(0, 0, 0); color:
rgb(0, 0, 0);" class=""><br class="">
</span></div>
<div class=""><font class="" color="#000000">An alternative
implementation would be to do something like the pubsub
framework, where the mapping command->callback is done
from within a javascript module. That I could do, but it
seems your </font><span style="caret-color: rgb(0, 0, 0);
color: rgb(0, 0, 0);" class="">DuktapeObject can do it
better.</span><font class="" color="#000000"> </font></div>
<div class=""><br class="">
</div>
<div class="">Thanks, Mark.<br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On 15 Jul 2020, at 3:34 PM, Michael Balzer
<<a href="mailto:dexter@expeedo.de" class=""
moz-do-not-send="true">dexter@expeedo.de</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<meta http-equiv="Content-Type" content="text/html;
charset=UTF-8" class="">
<div class=""> Mark,<br class="">
<br class="">
yes, I needed that persistence for the HTTP and VFS
classes, but I also needed to be able to couple a
dynamic C++ instance with a JS object and have a
mechanism to prevent garbage collection while the C++
side is still in use. If the C++ side is no longer
needed, the JS finalizer also needs to imply the C++
instance can be deleted.<br class="">
<br class="">
That is all implemented by DuktapeObject.
DuktapeObject also provides JS method invocation on
the coupled JS object and a mutex for concurrency
protection.<br class="">
<br class="">
We probably need some more framework documentation
than the header comments (applies to all of our
framework classes…):<br class="">
<br class="">
<tt class="">/***************************************************************************************************</tt><tt
class=""><br class="">
</tt><tt class=""> * DuktapeObject: coupled C++ / JS
object</tt><tt class=""><br class="">
</tt><tt class=""> * </tt><tt class=""><br class="">
</tt><tt class=""> * Intended for API methods to
attach internal API state to a JS object and provide</tt><tt
class=""><br class="">
</tt><tt class=""> * a standard callback invocation
interface for JS objects in local scopes.</tt><tt
class=""><br class="">
</tt><tt class=""> * </tt><tt class=""><br class="">
</tt><tt class=""> * - Override CallMethod() to
implement specific method calls</tt><tt class=""><br
class="">
</tt><tt class=""> * - Override Finalize() for
specific destruction in JS context (garbage
collection)</tt><tt class=""><br class="">
</tt><tt class=""> * - call Register() to prevent
normal garbage collection (but not heap destruction)</tt><tt
class=""><br class="">
</tt><tt class=""> * - call Ref() to protect against
deletion (reference count)</tt><tt class=""><br
class="">
</tt><tt class=""> * - call Lock() to protect
concurrent access (recursive mutex)</tt><tt class=""><br
class="">
</tt><tt class=""> * </tt><tt class=""><br class="">
</tt><tt class=""> * - GetInstance() retrieves the
DuktapeObject associated with a JS object if any</tt><tt
class=""><br class="">
</tt><tt class=""> * - Push() pushes the JS object
onto the Duktape stack</tt><tt class=""><br class="">
</tt><tt class=""> * </tt><tt class=""><br class="">
</tt><tt class=""> * Note: the DuktapeObject may
persist after the JS object has been finalized, e.g.</tt><tt
class=""><br class="">
</tt><tt class=""> * if some callbacks are pending
after the Duktape heap has been destroyed.</tt><tt
class=""><br class="">
</tt><tt class=""> * Use IsCoupled() to check if
the JS object is still available.</tt><tt class=""><br
class="">
</tt><tt class=""> * </tt><tt class=""><br class="">
</tt><tt class=""> * Ref/Unref:</tt><tt class=""><br
class="">
</tt><tt class=""> * Normal life cycle is from
construction to finalization. Pending callbacks
extend</tt><tt class=""><br class="">
</tt><tt class=""> * the life until the last
callback has been processed. A subclass may extend
the life</tt><tt class=""><br class="">
</tt><tt class=""> * by calling Ref(), which
increases the reference count. Unref() deletes the
instance</tt><tt class=""><br class="">
</tt><tt class=""> * if no references are left.</tt><tt
class=""><br class="">
</tt><tt class=""> */</tt><tt class=""><br class="">
</tt><br class="">
You normally just need to use Register/Deregister
& Ref/Unref, and to implement the constructor and
CallMethod. Coupling of the instances normally is done
on construction, as a JS object is normally already
needed for the parameters and can simply be attached
to.<br class="">
<br class="">
Have a look at DuktapeHTTPRequest, DuktapeVFSLoad and
DuktapeVFSSave, these are the current subclasses using
this.<br class="">
<br class="">
For the command registration I would probably couple
the OvmsCommand instance with a JS command object
providing an execution method.<br class="">
<br class="">
Tell me if you need more info.<br class="">
<br class="">
Regards,<br class="">
Michael<br class="">
<br class="">
<br class="">
<div class="moz-cite-prefix">Am 15.07.20 um 08:12
schrieb Mark Webb-Johnson:<br class="">
</div>
<blockquote type="cite"
cite="mid:B291C8AF-8382-4F45-87E3-3DA555E068BE@webb-johnson.net"
class="">
<meta http-equiv="Content-Type" content="text/html;
charset=UTF-8" class="">
<div class=""><br class="">
</div>
<div class="">@Michael this is probably for you.</div>
<div class=""><br class="">
</div>
<div class="">I am trying to implement javascript
command registration. The idea is that a
javascript module can call something like:</div>
<div class=""><br class="">
</div>
<div class="">
<blockquote style="caret-color: rgb(0, 0, 0);
margin: 0px 0px 0px 40px; border: none; padding:
0px;" class="">OvmsCommand.Register(basecommand,
name, title, callbackfn, usage, min, max)</blockquote>
<div style="caret-color: rgb(0, 0, 0);" class=""><br
class="">
</div>
</div>
<div style="caret-color: rgb(0, 0, 0);" class="">Then
we reflect that into MyCommandApp.RegisterCommand,
and keep a track of which command is for which
javascript callbackfn. When the command is
executed, we pass it into duktape.</div>
<div style="caret-color: rgb(0, 0, 0);" class=""><br
class="">
</div>
<div style="caret-color: rgb(0, 0, 0);" class="">I
also have tracking for javascript module loading
and unloading, so I can DeregisterCommand() if
duktape is reloaded (and also protected against
commands being registered in short-lived scripts
run from the command line).</div>
<div style="caret-color: rgb(0, 0, 0);" class=""><br
class="">
</div>
<div style="caret-color: rgb(0, 0, 0);" class="">To
implement this, I need to store the callbackfn as
a persistent reference to a duktape javascript
function.</div>
<div class=""><br class="">
</div>
<div class="">The issue with callback function
references in duktape is summarised here:</div>
<div class=""><br class="">
</div>
<blockquote style="margin: 0 0 0 40px; border: none;
padding: 0px;" class="">
<div class=""><a
href="https://wiki.duktape.org/howtonativepersistentreferences"
class="" moz-do-not-send="true">https://wiki.duktape.org/howtonativepersistentreferences</a></div>
<div class=""><br class="">
</div>
<div class=""><i class="">When a Duktape/C
function is called, Duktape places the call
arguments on the value stack. While the
arguments are on the value stack,
they're guaranteed to be reachable and the
Duktape/C function can safely work with the
arguments.<br class="">
<br class="">
However, when the Duktape/C function returns,
the value stack is unwound and references in
the function's value stack frame are lost. If
the last reference to a particular value was
in the function's value stack frame, the value
will be garbage collected when the function
return is processed.</i></div>
</blockquote>
<div class=""><br class="">
</div>
<div class="">The standard approach is to store the
reference back in the
duktape duk_push_global_stash so it won’t get
garbage-collected. But, that seems messy.</div>
<div class=""><br class="">
</div>
<div class="">I see that Michael has already
implemented something that seems similar in
ovms_script.{h, cpp}, for the async http
callbacks. Presumably to avoid this issue. But,
the approach seems very different, and I am not
sure if it is stopping _all_ garbage collection
for the duration of the async query, or just that
particular object being garbage collected. The
work seems extensive (quite a few objects
involved).</div>
<div class=""><br class="">
</div>
<div class="">So @Michael, any suggestions for this?
I don’t want to reinvent the wheel...</div>
<div class=""><br class="">
</div>
<div class="">Regards, Mark.</div>
<br class="">
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</blockquote>
<br class="">
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Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26
</pre>
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class="">
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<pre class="moz-signature" cols="72">--
Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26</pre>
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<pre class="moz-signature" cols="72">--
Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26</pre>
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