[Swig-user] Interfacing frontend with async C library

[Ustulation <ust...@gm...>]

Hey, I have successfully got to invoking async C code i wrote from Java.

This is how the C header looks like:

*// native C header: will call the given function pointer from a diffent
thread (async)
void backend_with_string(void *ctx, void(*o_cb)(void *ctx, const char
*result));
*

For this I manually created my jni interface source file:

* // jni source code
static void backend_with_string_jni_cb(void *user_data, const char *result)
{
    JniContextAsync *ctx = (JniContextAsync*)user_data;

    JNIEnv *new_env;
    (*ctx->vm)->AttachCurrentThread(ctx->vm, (void**)&new_env, 0);
    jstring java_result = (*new_env)->NewStringUTF(new_env, result);
    (*new_env)->ExceptionClear(new_env);
    (*new_env)->CallVoidMethod(new_env, ctx->obj, ctx->cb_method_id,
java_result);
    (*new_env)->DeleteGlobalRef(new_env, ctx->obj);
    (*ctx->vm)->DetachCurrentThread(ctx->vm);

    free(ctx);
}

JNIEXPORT void JNICALL Java_Frontend_BackendWithStringJNI(
        JNIEnv *env,
        jobject obj,
        jstring cb_name
) {
    const char *cb_name_cstr = (*env)->GetStringUTFChars(env, cb_name, 0);

    jclass obj_class = (*env)->GetObjectClass(env, obj);
    jmethodID cb_method_id = (*env)->GetMethodID(
            env,
            obj_class,
            cb_name_cstr,
            "(Ljava/lang/String;)V");

    (*env)->ReleaseStringUTFChars(env, cb_name, cb_name_cstr);

    // Assume this is just a C struct with 3 member variables as below
    JniContextAsync *jni_ctx =
(JniContextAsync*)malloc(sizeof(JniContextAsync));
    (*env)->GetJavaVM(env, &jni_ctx->vm);
    jni_ctx->obj = (*env)->NewGlobalRef(env, obj);
    jni_ctx->cb_method_id = cb_method_id;

    backend_with_string(jni_ctx, backend_with_string_async_jni_cb);
}*


and finally the Java source file:

*// java Frontend.java source file
class Frontend {
    private native void BackendWithStringJNI(String cb_name);
    public void backend_with_string_cb(String result) {
        System.out.println("- Got in the Java callback: " + result);
    }
    public void backend_with_string() {
        BackendWithStringJNI("backend_with_string_cb");
    }

    public static void main(String args[]) {
        Frontend obj = new Frontend();
        obj.backend_with_string();
        System.out.println("- Control returned back in Java");
    }

    static {
        System.loadLibrary("frontend");
    }
}*


I think it's a pretty standard paradigm where the async C backend takes a
void* as and argument which it will pass back to the callback on as-is basis
for the caller to retain their context. The callback is called in C thread
(not Java main thread) so the jni intermediary callback I defined above does
all that standard *AttachCurrentThread()* etc. Then finally 
*backend_with_string_cb* in Java is called. This is of-course called in C
thread so somehow the result needs to be posted in the main Java thread, but
that's impl detail for later (and doable with mutexes. condition vars etc -
I'm not a Java dev so just taking a guess here).

I have Frontend code in Java, Objective-C, NodeJS so far which i need to
cater to. Was just wondering if SWIG could help me here ?

I did get SWIG to work with normal blocking API function calls but no clue
how to get async-callbacks like above to work. Can you guys please help me
here or provide me pointers as to if this is possible with SWIG and how
people go about it ?

Thanks



--
Sent from: http://swig.10945.n7.nabble.com/swig-user-f3.html