[Extending] Obtaining instance ivar values by key?

[STREGAsGate]

Hey,

I'm writing a Swift package for Gravity and I'm having trouble figuring out how to get ivar values by key from an Instance.

I see this commented out [Edit] this was for a debug feature, the ivar names are stored in htable.

gravity/src/shared/gravity_value.h

Line 382 in a135732

//gravity_value_r inames; // ivar names

and I see this function to set values, but the key argument doesn't appear to be used for anything.
[Edit] this function is for expanding the ivar array and getting back it's count.

gravity/src/shared/gravity_value.c

Line 211 in f9e95a4

int16_t gravity_class_add_ivar (gravity_class_t *c, const char *identifier) {

Is it possible to retrieve an ivar value by it's key, or only by index?

[STREGAsGate]

To get an existing instance value:

1. Use gravity_hash_lookup or gravity_hash_lookup_cstring to get a gravity_value_t
2. Cast the gravity_value_t.p to gravity_closure_t
3. Get the index at gravity_closure_t.f.index
4. Use the index to return instance.ivars[index] which is the value for the key.

[Edit] the closure is for getter and setters which will need to be handled separately in addition to the stored retrieval above.

[STREGAsGate]

This seems to be the basic approach to assign a new stored ivar to a class.

let index = gravity_class_add_ivar(gravityClass, name)
let function = gravity_function_new_special(gravity.vm, nil, UInt16(index), nil, nil)
let closure = gravity_closure_new(gravity.vm, function)
gravity_class_bind(gravityClass, name, gravity_value_from_object(closure))

I'm still having an issue where the closure.f pointer gets corrupted or nulled at some point though.

[marcobambini]

Hello @STREGAsGate, can you please describe me what are you trying to do?

[STREGAsGate]

Sure!
I'm working on a cross platform Swift Package that makes using gravity a 1 liner for Swift projects, here.

TLDR Question:
How do I add instance variables to a gravity_class_t using the C API?

Long Question:
I'm trying to allow Swift to describe a Gravity Class.
The documentation shows how to add a function and that's easy enough and I was able to do that no problem.

To add an instance variable, I currently have this from browsing the Gravity source:

let index = gravity_class_add_ivar(gravityClass, name)
let function = gravity_function_new_special(nil, nil, UInt16(index), nil, nil)
let closure = gravity_closure_new(gravity.vm, function)
gravity_class_bind(gravityClass, name, gravity_value_from_object(closure))

However when attempting to retrieve the value from an instance, here, the closure.f is no longer referencing the function.
_{This value retrieval works fine for instances that Gravity creates from a script.}

I wrote a test with a full example of the failure and how I'm using it.

func testHostCreateClientClass() throws {        
    let theThingClass = gravity.createClass(named: "TheThing")
    theThingClass.addInstanceVariable(named: "myVar")
        
    try gravity.compile("extern class TheThing; var theThing = TheThing(); func main() {}")
    try gravity.runMain()
        
    let instance = try gravity.getInstance("theThing")
    
    let gValue = gravity.createValue(randomValue)
    instance.setValue(gValue, named: "myVar")
    
    let result = instance.getValue("myVar")
        
    XCTAssertEqual(result.getInt(), randomValue)
}

[STREGAsGate]

@marcobambini
Fixed.
I thought I was missing a step here, but my issue was from the use of gravity_closure_free in an unrelated location earlier on.

[marcobambini]

Hello @STREGAsGate, good to know that you fixed the issue.

Please take a look at the Gravity <-> ObjC bridge, I think it contains a lot of valuable code to you:
https://github.com/marcobambini/gravity/blob/master/binding/GravityObjC/ObjC/