ClassMapper

Anthony Catel edited this page Feb 28, 2017 · 24 revisions

Basic concept

ClassMapper<T> is a class template built in nidium that helps creating mapping between C++ classes and exposed Javascript classes.

It provides a way to map constructor, methods, getter and setter in a 1:1 manner between the two world.

Example

class MyClass : public ClassMapper<MyClass>
{
public:
    static void RegisterObject(JSContext *cx);

    static MyClass *Constructor(JSContext *cx, JS::CallArgs &args,
        JS::HandleObject obj);

    static JSFunctionSpec *ListMethods();
    static JSPropertySpec *ListProperties();

    virtual ~MyClass() {}
    MyClass() {}
protected:
    /*
        Declare a JS method
    */
    NIDIUM_DECL_JSCALL(myMethod);

    /*
        Declare a JS property
    */
    NIDIUM_DECL_JSGETTER(myProperty);
};



void MyClass::RegisterObject(JSContext *cx)
{
    /**
        Expose |MyClass| to the global scope
        Prototype is available through "MyClass.prototype"
    */
    MyClass::ExposeClass<1>(cx, "MyClass");
//                       ^-------,
//                               Minimum number of arguments
//                               for the Constructor
}

/**
    Called during a JS "var obj = new MyClass(param) call"
    Returning nullptr trigger a JS exception
    
    @param args  A list of arguments provided to the contructor
    @param obj   The JSObject corresponding to the instance
*/
MyClass *MyClass::Constructor(JSContext *cx, JS::CallArgs &args,
    JS::HandleObject obj)
{
    /**
        arguments are available in args() like a regular method call.
    */
    return new MyClass();
}

/**
    Called during a JS "obj.myMethod(param)" call
    Returning nullptr trigger a JS exception

    @param args  A list of arguments provided to method call
                 Return value is set via args.rval().setXXX()
*/
bool MyClass::JS_myMethod(JSContext *cx, JS::CallArgs &args)
{

    /*
        Convert the first argument as a string
    */
    JS::RootedString myparam(cx, JS::ToString(cx, args[0]));
    JSAutoByteString cmyparam;

    cmyparam.encodeUtf8(cx, myparam);

    printf("First argument is %s\n", cmyparam.ptr());

    /*
        Set the JavaScript value returned to "myMethod"
    */
    args.rval().setBoolean(true);

    /*
        No Exception
    */
    return true;
}

/**
    Called when accessing "obj.myProperty".

    @param vp  The value of "myProperty" to be returned.
               Set using vp.setXXX()
*/
bool MyClass::JSGetter_myProperty(JSContext *cx, JS::MutableHandleValue vp)
{
    vp.setInt32(42);

    return true;
}

/**
    Optional.
    List all the methods available on MyClass prototype

    CLASSMAPPER_FN arguments :
        1. Must be the name of the current C++ class
        2. The name of the method exposed to the JS
        3. Minimum number of arguments
*/
JSFunctionSpec *MyClass::ListMethods()
{
    static JSFunctionSpec funcs[] = {
        CLASSMAPPER_FN(MyClass, myMethod, 1),
        JS_FS_END
    };

    return funcs;
}

/**
    Optional.
    List all the properties available on MyClass prototype
*/
JSPropertySpec *MyClass::ListProperties()
{
    static JSPropertySpec props[] = {
        CLASSMAPPER_PROP_G(MyClass, myProperty),
        JS_PS_END
    };

    return props;
}

GC Rooting and object lifetime

ClassMapper provides two different ways to handle the lifetime of instantiated objects.

The default behavior is to let the JS garbage collector handles the destruction of the object. That is, when the object become unreachable from the JS code, the JS engine will finalize the underlying JSObject and ClassMapper will delete the C++ instance.

Example of GC controlled lifetime

function foobar()
{
    var x = new MyClass("hello");

    // After foobar() has returned, |x| becomes unreachable
    // and is marked for deletion by the garbage collector.
    // The C++ destructor ~MyClass() is then automatically called.
}

foobar();

However it's also possible for a ClassMapper instance to takeover the ownership of the object and controls its lifetime.
Let's say you have some internal pending operation on your C++ object and you don't want the GC to delete the instance for you.

The only thing you need to do is to call root() on your C++ instance.

Example of C++ controlled lifetime

MyClass *MyClass::Constructor(JSContext *cx, JS::CallArgs &args,
    JS::HandleObject obj)
{
    MyClass *myclass = new MyClass();
    myclass->root(); // Tells the JS engine to keep a
                     // reference to the underlying JSObject.
    return myclass();
}
function foobar()
{
    var x = new MyClass("hello");

    // After foobar() has returned, |x| becomes unreachable to the JavaScript code.
    // However, |x| was internally maked as
    // reachable by calling |myclass->root()| meanings 
    // it's still reachable by C++
}

foobar();

It's now up to the C++ to call either unroot() on the instance or directly delete the object.

Non constructible Class

It's possible that a JS class is not constructible by the end-user but only by the C++.

For instance, imagine a Socket class (which is constructible), where an instance of SocketClientConnection is passed as a callback argument when a connection is established.
In that case, the user doesn't have to create the instance manually.

Example

var socket = new Socket(8080).listen();
socket.onaccept = function(client) {
    // client is an instance of SocketClientConnection
}

It's however possible for the user to change the prototype of SocketClientConnection, so we need to expose this Class to the JavaScript but disallow its constructor.

Example

// Error Uncaught TypeError: Illegal constructor(…)
var test = new SocketClientConnection();
/** Extending the prototype through
    the exposed Class is possible.
*/

SocketClientConnection.prototype.foo = 42;

var socket = new Socket(8080).listen();
socket.onaccept = function(client) {
    console.log(client.foo); // 42
}

In order to create a non constructible Class, the only thing you need is to omit the Constructor method in your C++ class (static T *Constructor()).

You can then create an instance from the C++ using ClassMapper<T>::CreateObject().

Example

MyClass *myclass = new MyClass();
JS::RootedObject jinstance(m_Cx,
    MyClass::CreateObject(m_Cx, myclass));

Relationship between JSObject and C++ instance

Because ClassMapper<T> maps a JS Class to a C++ Class, there is a close relationship between a JSObject and its C++ mapped object.

You can easily retrieve a JSObject from a ClassMapper<T> instance and Vice versa.

Get a C++ object from a JSObject

ClassMapper<T>::GetInstance(JS::HandleObject) returns the mapped C++ object if existing. Otherwise returns nullptr

MyClass *myclass = MyClass::GetInstance(a_js_object);
if (myclass != nullptr) {

}

Get a JSObject from a C++ object

JS::RootedObject my_js_object(cx, myclass->getJSObject());

Custom JSClass flags and properties

A default JSClass * is created per ClassMapper variant (but has the same address for different instance of the same variant).

It's possible to pass over custom flags (.flags field of struct JSClass) via ClassMapper<T>::ExposeClass third argument.

Example

void MyClass::RegisterObject(JSContext *cx)
{
    MyClass::ExposeClass<1>(cx, "MyClass", JSCLASS_HAS_RESERVED_SLOTS(2));
}

Please note that it's not possible to use JSCLASS_HAS_PRIVATE because it's internally used and reserved by ClassMapper.

Replacing the default JSClass entirely.

It's possible to replace the default JSClass by setting the static method static JSClass *GetJSClass(); on your Class.

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