test.cpp

class DualSliderWithAttachments : public juce::Slider
{
public:
    DualSliderWithAttachments(juce::Slider::SliderStyle style_, juce::RangedAudioParameter* minpar, juce::RangedAudioParameter* maxpar) :
        minAttach(*minpar, [this](float x) { setMinValue(x, juce::dontSendNotification); }),
        maxAttach(*maxpar, [this](float x) { setMaxValue(x, juce::dontSendNotification); })
    {
        setSliderStyle(style_);
        setRange(minpar->getNormalisableRange().start, maxpar->getNormalisableRange().end);
        minAttach.sendInitialUpdate();
        maxAttach.sendInitialUpdate();
        onDragStart = [this]()
        {
            thumbThatWasDragged = getThumbBeingDragged();
            if (thumbThatWasDragged == 1)
                minAttach.beginGesture();
            else if (thumbThatWasDragged == 2)
                maxAttach.beginGesture();
        };
        onValueChange = [this]()
        {
            if (thumbThatWasDragged == 1)
                minAttach.setValueAsPartOfGesture(getMinValue());
            else if (thumbThatWasDragged == 2)
                maxAttach.setValueAsPartOfGesture(getMaxValue());
        };
        onDragEnd = [this]()
        {
            if (thumbThatWasDragged == 1)
                minAttach.endGesture();
            else if (thumbThatWasDragged == 2)
                maxAttach.endGesture();
        };
    }
private:
    juce::ParameterAttachment minAttach;
    juce::ParameterAttachment maxAttach;
    int thumbThatWasDragged = 0;
};