How would to make this class as a singleton?

Question

I'm trying to make this class as a singleton but without success. How would you do that ? I have seen out there on internet that I should make modifications to the header file and to the cpp file. I've tried only by changing the cpp file... What I need to do is to be able to access the boolean "isRecording" from JNI calls.

Thanks.

OboeAudioRecorder.cpp

#include <jni.h>
#include <string>
#include <oboe/Oboe.h>
#include "OboeAudioRecorder.h"
#include "oboe/samples/debug-utils/logging_macros.h"

class OboeAudioRecorder: public oboe::AudioStreamCallback {
public:
    bool isRecording = true;

    explicit OboeAudioRecorder() { }

    void StartAudioRecorder() {
        oboe::AudioStreamBuilder builder;
        builder.setDirection(oboe::Direction::Input);
        builder.setPerformanceMode(oboe::PerformanceMode::LowLatency);
        builder.setFormat(oboe::AudioFormat::Float);
        builder.setChannelCount(oboe::ChannelCount::Mono);
        builder.setInputPreset(oboe::InputPreset::Unprocessed);
        builder.setSharingMode(oboe::SharingMode::Shared);
        builder.setSampleRate(48000);
        builder.setAudioApi(oboe::AudioApi::OpenSLES);
        //builder.setCallback(this);

        oboe::Result r = builder.openStream(&stream);
        if (r != oboe::Result::OK) {
            return;
        }

        r = stream->requestStart();
        if (r != oboe::Result::OK) {
            return;
        }

        auto a = stream->getState();
        if (a == oboe::StreamState::Started) {

            constexpr int kMillisecondsToRecord = 2;
            const int32_t requestedFrames = (int32_t) (kMillisecondsToRecord * (stream->getSampleRate() / oboe::kMillisPerSecond));
            __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "requestedFrames = %d", requestedFrames);
            float_t mybuffer[requestedFrames];
            constexpr int64_t kTimeoutValue = 3 * oboe::kNanosPerMillisecond;

            int framesRead = 0;
            do {
                auto result = stream->read(mybuffer, requestedFrames, 0);
                if (result != oboe::Result::OK) {
                    break;
                }
                framesRead = result.value();
                __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "framesRead = %d", framesRead);
                if (framesRead > 0) {
                    break;
                }
            } while (framesRead != 0);

            while (isRecording) {
                auto result = stream->read(mybuffer, requestedFrames, kTimeoutValue * 1000);
                if (result == oboe::Result::OK) {
                    auto nbFramesRead = result.value();
                    __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "nbFramesRead = %d", nbFramesRead);
                    for (int i=0; i<nbFramesRead; i++) {
                        __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "nbFramesRead[%d] = %f", i, mybuffer[i]);
                    }
                } else {
                    auto error = convertToText(result.error());
                    __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "error = %s", error);
                }
            }

            stream->requestStop();
            stream->close();
        }
    }

    oboe::DataCallbackResult onAudioReady(oboe::AudioStream *oboeStream, void *audioData, int32_t numFrames) override {
        LOGE("onAudioReady");
    }

private:
    oboe::ManagedStream outStream;
    oboe::AudioStream *stream{};
};

OboeAudioRecorder.h

#ifndef OBOEAUDIORECORDER_OBOEAUDIORECORDER_H
#define OBOEAUDIORECORDER_OBOEAUDIORECORDER_H


#endif //OBOEAUDIORECORDER_OBOEAUDIORECORDER_H

WHat's the point of that empty header? Also, what's preventing your JNI code from having access to an instance of `OboeAudioRecorder`? — Michael, Jul 03 '20 at 15:29
What I need is a singleton instance. My JNI code works fine with a simple instance of OboeAudioRecorder but I would like it to be a singleton in order to access it from 2 separate JNI functions (one which sets the isRecording boolean to true and one which sets the isRecording boolean to false). The empty header has been generated by Android studio automatically. — KotlinIsland, Jul 03 '20 at 15:47
Well, then look at https://stackoverflow.com/questions/1008019/c-singleton-design-pattern Though I still don't understand from your description why a singleton is needed. — Michael, Jul 03 '20 at 16:08
Ok I'll have a look at the link you posted. What I need is only one instance of OboeAudioRecorder, because I must have control over the isRecording boolean of the initially created instance of OboeAudioRecorder. — KotlinIsland, Jul 03 '20 at 16:16

score 2 · Answer 1 · answered Aug 26 '20 at 17:35

An easy way to make a class into a singleton is to add a getInstance() method that uses a static variable in the method.

static OboeAudioRecorder &getInstance() {
    static EngineOpenSLES sInstance;
    return sInstance;
}

You can also prevent other instances from being created by making the constructors private and by deleting the copy operators:

private:
    // Make this a safe Singleton
    OboeAudioRecorder()= default;
    ~OboeAudioRecorder()= default;
public:
    OboeAudioRecorder(const OboeAudioRecorder&)= delete;
    OboeAudioRecorder& operator=(const OboeAudioRecorder&)= delete;

score 0 · Accepted Answer · answered Jul 03 '20 at 19:30

I've been able to make this class a singleton, thanks to the following link (in french) : https://h-deb.clg.qc.ca/Sujets/Divers--cplusplus/CPP--Singletons.html

Here is my class with the singleton pattern (notice the last line that is declared out of the "class" scope) :

#include <jni.h>
#include <string>
#include <oboe/Oboe.h>
#include "OboeAudioRecorder.h"
#include "oboe/samples/debug-utils/logging_macros.h"

class OboeAudioRecorder: public oboe::AudioStreamCallback {

private:
    oboe::ManagedStream outStream;
    oboe::AudioStream *stream{};

    oboe::DataCallbackResult
    onAudioReady(oboe::AudioStream *oboeStream, void *audioData, int32_t numFrames) override {
        LOGE("onAudioReady");
    }

    static OboeAudioRecorder *singleton;
    explicit OboeAudioRecorder() = default;

public:
    static OboeAudioRecorder *get() {
        if (!singleton)
            singleton = new OboeAudioRecorder();
        return singleton;
    }

    bool isRecording = true;

    void StartAudioRecorder() {
        oboe::AudioStreamBuilder builder;
        builder.setDirection(oboe::Direction::Input);
        builder.setPerformanceMode(oboe::PerformanceMode::LowLatency);
        //builder.setFormat(oboe::AudioFormat::Float);
        builder.setFormat(oboe::AudioFormat::I16);
        builder.setChannelCount(oboe::ChannelCount::Mono);
        builder.setInputPreset(oboe::InputPreset::Unprocessed);
        builder.setSharingMode(oboe::SharingMode::Shared);
        builder.setSampleRate(48000);
        builder.setAudioApi(oboe::AudioApi::OpenSLES);
        //builder.setCallback(this);

        oboe::Result r = builder.openStream(&stream);
        if (r != oboe::Result::OK) {
            return;
        }

        r = stream->requestStart();
        if (r != oboe::Result::OK) {
            return;
        }

        auto a = stream->getState();
        if (a == oboe::StreamState::Started) {

            constexpr int kMillisecondsToRecord = 2;
            auto requestedFrames = (int32_t) (kMillisecondsToRecord *
                                                       (stream->getSampleRate() /
                                                        oboe::kMillisPerSecond));
            __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "requestedFrames = %d",
                                requestedFrames);
            //const int32_t requestedFrames = 65536;
            //int16_t mybuffer[requestedFrames];
            int16_t mybuffer[requestedFrames];
            constexpr int64_t kTimeoutValue = 3 * oboe::kNanosPerMillisecond;

            int framesRead = 0;
            do {
                auto result = stream->read(mybuffer, requestedFrames, 0);
                if (result != oboe::Result::OK) {
                    break;
                }
                framesRead = result.value();
                __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "framesRead = %d",
                                    framesRead);
                if (framesRead > 0) {
                    break;
                }
            } while (framesRead != 0);

            while (isRecording) {
                auto result = stream->read(mybuffer, requestedFrames, kTimeoutValue * 1000);
                if (result == oboe::Result::OK) {
                    auto nbFramesRead = result.value();
                    __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "nbFramesRead = %d",
                                        nbFramesRead);
                    for (int i = 0; i < nbFramesRead; i++) {
                        __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder",
                                            "nbFramesRead[%d] = %d", i, mybuffer[i]);
                    }
                } else {
                    auto error = convertToText(result.error());
                    __android_log_print(ANDROID_LOG_INFO, "OboeAudioRecorder", "error = %s", error);
                }
            }

            stream->requestStop();
            stream->close();
        }
    }


};

OboeAudioRecorder *OboeAudioRecorder::singleton = nullptr;

How would to make this class as a singleton?

2 Answers2