Mark,
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.
That is all implemented by DuktapeObject. DuktapeObject
also provides JS method invocation on the coupled JS
object and a mutex for concurrency protection.
We probably need some more framework documentation than
the header comments (applies to all of our framework
classes…):
/***************************************************************************************************
* DuktapeObject: coupled C++ / JS
object
*
* Intended for API methods to attach
internal API state to a JS object and provide
* a standard callback invocation
interface for JS objects in local scopes.
*
* - Override CallMethod() to
implement specific method calls
* - Override Finalize() for specific
destruction in JS context (garbage collection)
* - call Register() to prevent
normal garbage collection (but not heap destruction)
* - call Ref() to protect against
deletion (reference count)
* - call Lock() to protect
concurrent access (recursive mutex)
*
* - GetInstance() retrieves the
DuktapeObject associated with a JS object if any
* - Push() pushes the JS object onto
the Duktape stack
*
* Note: the DuktapeObject may
persist after the JS object has been finalized, e.g.
* if some callbacks are pending
after the Duktape heap has been destroyed.
* Use IsCoupled() to check if the
JS object is still available.
*
* Ref/Unref:
* Normal life cycle is from
construction to finalization. Pending callbacks extend
* the life until the last callback
has been processed. A subclass may extend the life
* by calling Ref(), which
increases the reference count. Unref() deletes the
instance
* if no references are left.
*/
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.
Have a look at DuktapeHTTPRequest, DuktapeVFSLoad and
DuktapeVFSSave, these are the current subclasses using
this.
For the command registration I would probably couple the
OvmsCommand instance with a JS command object providing
an execution method.
Tell me if you need more info.
Regards,
Michael
Am 15.07.20 um 08:12
schrieb Mark Webb-Johnson:
@Michael this is probably for you.
I am trying to implement javascript
command registration. The idea is that a javascript
module can call something like:
OvmsCommand.Register(basecommand,
name, title, callbackfn, usage, min, max)
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.
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).
To
implement this, I need to store the callbackfn as a
persistent reference to a duktape javascript
function.
The issue with callback function
references in duktape is summarised here:
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.
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.
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.
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).
So @Michael, any suggestions for this? I
don’t want to reinvent the wheel...
Regards, Mark.
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