file_picker_jni.cpp

struct JniHelper
{
    JniHelper()
    {
        vm = g_App->activity->vm;
        if (vm->GetEnv((void**)&env, JNI_VERSION_1_6))
        {
            if (!vm->AttachCurrentThread(&env, NULL))
            {
                __android_log_print(ANDROID_LOG_INFO, "JNI-TEST", "attaching thread\n");
                thread_attached = true;
            }
        }
    }

    ~JniHelper()
    {
        if (thread_attached && vm)
        {
            vm->DetachCurrentThread();
            __android_log_print(ANDROID_LOG_INFO, "JNI-TEST", "detaching thread\n");
        }
    }

    JNIEnv* env{};
    JavaVM* vm{};
    bool thread_attached{};
};

// https://developer.android.com/reference/android/content/Intent#ACTION_GET_CONTENT
// https://developer.android.com/reference/android/content/Intent#ACTION_OPEN_DOCUMENT
void file_picker()
{
    JniHelper jni;

    // get all the needed classes
    const auto ActivityClass = jni.env->FindClass("android/app/Activity");
    const auto IntentClass = jni.env->FindClass("android/content/Intent");

    // get some constants
    const auto ACTION_GET_CONTENT_id = jni.env->GetStaticFieldID(IntentClass, "ACTION_GET_CONTENT", "Ljava/lang/String;");
    const auto CATEGORY_OPENABLE_id = jni.env->GetStaticFieldID(IntentClass, "CATEGORY_OPENABLE", "Ljava/lang/String;");
    const auto ACTION_GET_CONTENT = (jstring)jni.env->GetStaticObjectField(IntentClass, ACTION_GET_CONTENT_id);
    const auto CATEGORY_OPENABLE = (jstring)jni.env->GetStaticObjectField(IntentClass, CATEGORY_OPENABLE_id);

    // find constructor and create instance of Intent
    const auto IntentClassConstructor = jni.env->GetMethodID(IntentClass, "<init>", "(Ljava/lang/String;)V");
    const auto IntentObj = jni.env->NewObject(IntentClass, IntentClassConstructor, ACTION_GET_CONTENT);

    // need to set a couple of methods
    const auto addCategory = jni.env->GetMethodID(IntentClass, "addCategory", "(Ljava/lang/String;)Landroid/content/Intent;");
    const auto setType = jni.env->GetMethodID(IntentClass, "setType", "(Ljava/lang/String;)Landroid/content/Intent;");

    // these return itself so you can chain methods together
    jni.env->CallObjectMethod(IntentObj, addCategory, CATEGORY_OPENABLE);
    jni.env->CallObjectMethod(IntentObj, setType, jni.env->NewStringUTF("*/*"));

    // find and call startActivity() (todo: handle result somehow)
    const auto startActivityForResult = jni.env->GetMethodID(ActivityClass, "startActivityForResult", "(Landroid/content/Intent;I)V");
    jni.env->CallVoidMethod(g_App->activity->clazz, startActivityForResult, IntentObj, 0);
}