Add swift binding

[Andrey1994]

Use other languages(java, c#, python) as an API reference.

Useful link: https://stackoverflow.com/questions/34669958/swift-how-to-call-a-c-function-loaded-from-a-dylib

[ScottThomasMiller]

I am trying to create a bridging header for Swift 5 and Xcode 12.5.1. Must my Swift code instantiate the BoardShim object, or can it just call prepare_session() and start_stream() directly?

[Andrey1994]

The idea of bindings is to provide high-level API which is easier to use than plain C methods exposed from low-level library. So, we created BoardShim class for each binding. Users are expected to work with BoardShim methods instead calling low-level prepare_session and other methods directly.

Example from Java:(https://github.com/brainflow-dev/brainflow/blob/master/java-package/brainflow/src/main/java/brainflow/BoardShim.java)

    public void prepare_session () throws BrainFlowError
    {
        int ec = instance.prepare_session (board_id, input_json);
        if (ec != ExitCode.STATUS_OK.get_code ())
        {
            throw new BrainFlowError ("Error in prepare_session", ec);
        }
    }

[Andrey1994]

So I would say let's create BoardShim, DataFilter, and other classes like in existing bindings

[Andrey1994]

I dont know swift but why do we need a bridging header at all?(and what is it?)

I think it can look like this(pseudocode inspired from python binding):

class BoardShimDynLibLoader {
    handler = nil;
    static func get_handler() {
        if (handler == nil)
            handler = dlopen("libBoardContoller.dylib", RTLD_NOW)
        return handler;
}

class BoardShim {
    func prepare_session() {
        typealias prepare_session = @convention(c) (CInt, Cchar *) -> CInt
        handler = BoardShimDynLibLoader.get_handler()
        var prepare_session = dlsym(handle, "prepare_session")
        if (prepare_session != nil) {
            let f = unsafeBitCast(sym, prepare_session .self)
            let result = f(board_id, params)
       }
    }    
}

Idea is to create a singleton that will call dlopen only once and find addresses of required functions manually.
Is it worse or better?(I don't know swift best practices)

[ScottThomasMiller]

The bridging header tells Swift how to resolve the object names in your Swift code. Without it you get errors such as, "Cannot find 'prepare_stream' in scope". It's typically a .h file which simply includes other .h files. You have to register it in the Xcode build settings, so that Swift knows where to look.

Your example is similar to this: https://stackoverflow.com/questions/35229149/interacting-with-c-classes-from-swift

I should be able to implement something like the that, because I already have all of BrainFlow's C++ headers.

I am still learning Swift, but I think that your idea is basically a wrapper, and wrappers are very "Swifty".

[ScottThomasMiller]

I managed to call the low-level functions directly, such as prepare_session() and get_eeg_channels(), but so far I am unable to instantiate BoardShim(). I need some help. Everything I've found so far, I can't get working.

[Andrey1994]

You should not call methods from C++ BoardShim class. This class even doesn't exist in low-level API(low-level API is for bindings, high-level API is for users).

Instead, you need to create a new class in Swift and call it BoardShim, inside this class you need to wrap methods from low-level API. Take a look at the cpp-package for reference. https://github.com/brainflow-dev/brainflow/blob/master/cpp-package/src/board_shim.cpp#L104 Here method is_prepared from BoardShim class wraps method is_prepared from low-level API, you need to do the same

[ScottThomasMiller]

OK I think I finally understand: you want me to write BoardShim.swift modeled after board_shim.cpp. That seems completely doable. I will also have to write swift bindings for all of BoardShim's dependencies, such as BrainFlowException, BrainFlowArray, BrainFlowInputParams, etc. And nowhere in all of those bindings will I need to instantiate any C++ classes, correct?

[Andrey1994]

Correct, but probably you dont need BrainFlowArray(its specific for c++ for better perf, you can do the same as in java for example). Also, there are two more classes: DataFilter and MLModule, its the same there.(I can help and do it for some of them)

[Andrey1994]

Also, you need to duplicate enums like BoardIds, etc. All of that is pretty small and simple except main classes(BoardShim and DataFilter)

[ScottThomasMiller]

Here's what I have so far: https://github.com/ScottThomasMiller/ScottsML/tree/main/Swift/BrainFlowBindings

[Andrey1994]

Looks good so far!

[ScottThomasMiller]

Thanks. Swift doesn't like binding its String type to "char *" parameters in its bridging headers, so I'm making them all "const char *" in board_controller.h instead.

Also I think Swift will automatically make BoardIds iterable if I add the CaseIterable protocol to its type definition. I'll try that shortly.