Retrieving a context pointer in C++ function callback

[ericzundel]

(reposted from v8-dev to this group)

I'm working on a Chrome plugin that uses a separate context for V8
for
several entities. I've been using the sample shell.cc as an example
of how to bind a top level function to a C++ method and have read
through the embedder's guide. Here's what I have so far:

[CODE]
MyClass::init() {
v8::HandleScope handle_scope; // For allocation of local handles.
// Create a template for the global object.
v8::Handle<v8::ObjectTemplate> global_template =
v8::ObjectTemplate::New();
// Bind the global '__hLoad' function to the C++ Load callback.
global_template->Set(v8::String::New("__hLoad"),
v8::FunctionTemplate::New(&MyClass::Load));
...
}

// This method is declared static
v8::Handle<v8::Value> MyClass::Load(const v8::Arguments& args) {
...
return v8::Undefined();
}
[/CODE]


This works fine - My JavaScript methods can call __hLoad() and
MyClass::Load() is invoked. However, my question is, how do I get
the
instance pointer of type MyClass from init() into the MyClass::Load
static method? The examples in the docs don't seem to apply (I've
gotten many crashes trying to work it out.)
-Eric.

[Ondrej Zara]

Hi,

from what I know, it is not a good ideat to provide a _method_
callback. Use a function callback instead and set the reference to
your instance as an internal field of a JS class. In your callback
function, you then load the reference from internal field and do
whatever you want with it...


Ondrej

[Eric Ayers]

Sorry, I've been using the term method & function interchangeably and
I'm not sure what your point about the difference between 'method' and
'function' is.

There needs to be just one copy the function "__hLoad" and
"__hSendData" in this v8 context. I want to match up the function
calls with the C++ class instance associated with the v8 context so I
can route the data out of this v8 context to the correct destination.

My JS code looks like this:

myapi = {
prop1 : "value" // an example API property
};

/**
* Load external JavaScript from another file.
* path (String) - path relative to the config directory.
*/
myapi.load = function(path) {
// Instruct the plugin to load the specified file.
__hLoad(path);
}

/**
* Send a data string to the plugin, encoding the arguments as JSON.
*/
myapi.sendData = function(rule, tabId, timestamp, description, color,
refRecord) {
if (timestamp == null) {
throw new Error( rule + ": timestamp must be defined");
}
var value = {'rule' : rule,
'tabId' : tabId,
'timestamp' : timestamp,
'description' : description,
'color' : color,
'refRecord' : refRecord };
// Encode the object as JSON and send it to the plugin.
var jsonString = JSON.stringify(value);
__hSendData(jsonString);
}

Could you provide more details or a pointer on how to "set the
reference to your instance as an internal field of a JS class?" My
attempts to do this have resulted in crashes.

[Alex Iskander]

Difference between method and function:

class MyClass

{

public:

    void myMethod() { doSomething(); }

    static void MyFunction() { DoSomething(); }

}

void SomeFunction() { DoSomething(); }

Basically, methods belong to instances of a class. Functions do not. Methods are passed an invisible extra argument ("this"), functions are not.

What it looks like you want to do is actually create the "myapi" class in C++. In this case, myapi would be a FunctionTemplate that also has an InstanceTemplate, and perhaps a PrototypeTemplate.

Have you looked thoroughly through the Embedder's Guide, especially the part about using ObjectTemplates and internal fields?

http://code.google.com/apis/v8/embed.html

Alex

Alex Iskander

Web and Marketing

TPSi

[Eric Ayers]

I appreciate the pointers thus far.

I re-cast my code to create an object template, then create an
instance of the object to add my C++ instance pointer too. I keep
getting a crash in the ObjectTemplate->NewInstance() method. I've
tried moving the NewInstance() call around to different places in the
code and re-ordering the steps of creating the templates and calling
Set(), but the crash always stays in the same place.

v8 version r1040:

[CODE]
// C++ Header
class MyClass {
public:
// Private constructor for singleton class.
MyClass();
~MyClass();

private:
// Initialize the singleton
void Init();

// Callbacks for functions registered as built-ins.
static v8::Handle<v8::Value> AddData(const v8::Arguments& args);
static v8::Handle<v8::Value> Load(const v8::Arguments& args);
static v8::Handle<v8::Value> Log(const v8::Arguments& args);

// Execution environment containing built-in functions.
v8::Persistent<v8::Context> execution_context_;
};



// C++ code for MyClass::Init
void MyClass::Init() {

v8::HandleScope handle_scope; // For allocation of local handles.

// Create a template for the global object.
v8::Handle<v8::ObjectTemplate> global_template = v8::ObjectTemplate::New();

v8::Handle<v8::ObjectTemplate> myApi_template = v8::ObjectTemplate::New();

// Bind C++ funtions as callbacks
myApi_template->SetInternalFieldCount(1);

// Bind the global '__hLog' function to the C++ Print callback.
myApi_template->Set(v8::String::New("__hLog"),
v8::FunctionTemplate::New(&apu::MyClass::Log));

// Bind the global '__hLoad' function to the C++ Load callback.

myApi_template->Set(v8::String::New("__hLoad"),
v8::FunctionTemplate::New(&apu::MyClass::Load));
// Bind the '__hAddData' function
myApi_template->Set(v8::String::New("__hAddData"),
v8::FunctionTemplate::New(&apu::MyClass::AddData));

// *** myApi_template->NewInstance crashes
v8::Local<v8::Object> myApi_instance = myApi_template->NewInstance();
myApi_instance->SetInternalField(0, v8::External::New(this));

// Create a new execution environment containing the built-in
// functions
v8::Handle<v8::Context> context = v8::Context::New(NULL, global_template);

execution_context_ = v8::Persistent<v8::Context>::New(context);

// Create a top level object named 'myApi' in the global object
context->Global()->Set(v8::String::New("myApi"), myApi_instance);

...
[/CODE]

Stack trace:
npmyplugin.dll!v8::internal::Array::length() Line 1375 + 0xa bytes C++
npmyplugin.dll!v8::internal::FixedArray::get(int index=0x00000004)
Line 1157 + 0xe bytes C++
npmyplugin.dll!v8::internal::Context::global() Line 238 + 0xa bytes C++
npmyplugin.dll!v8::internal::Context::builtins() Line 37 + 0x8 bytes C++
npmyplugin.dll!v8::internal::Top::builtins() Line 267 + 0xb bytes C++
npmyplugin.dll!v8::internal::Execution::InstantiateObject(v8::internal::Handle<v8::internal::ObjectTemplateInfo>
data={...}, bool * exc=0x05e4f3ff) Line 473 + 0x13 bytes C++
npmyplugin.dll!v8::ObjectTemplate::NewInstance() Line 2360 + 0x20 bytes C++
> npmyplugin.dll!apu::MyClass::Init() Line 96 C++
...

[Alex Iskander]

I believe you need to be in a Context Scope before you can call
NewInstance.

Try something like this instead:

// Create a new execution environment containing the built-in
// functions
v8::Handle<v8::Context> context = v8::Context::New(NULL,
global_template);

execution_context_ = v8::Persistent<v8::Context>::New(context);

// Create a top level object named 'myApi' in the global object
context->Global()->Set(v8::String::New("myApi"), myApi_instance);

/* ADD THIS LINE TO ENTER CONTEXT SCOPE: */
v8::Context::Scope context_scope(execution_context);

/* NOW CREATE THE INSTANCE */

// *** myApi_template->NewInstance crashes
v8::Local<v8::Object> myApi_instance = myApi_template->NewInstance();
myApi_instance->SetInternalField(0, v8::External::New(this));

Alex

[Eric Ayers]

Thank you Alex. That was the problem!

[bmbsage]

ericzundel  what was your final code?

I'm having the same issue..?