# How to add a listener for a parameter variable?

**URL:** <https://forum.juce.com/t/how-to-add-a-listener-for-a-parameter-variable/56085>\
**Category:** Audio Plugins\
**Created:** [May 4, 2023, 9:17pm UTC](https://forum.juce.com/t/how-to-add-a-listener-for-a-parameter-variable/56085 "2023-05-04T21:17:32Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![fredofaure](https://sea2.discourse-cdn.com/flex026/user_avatar/forum.juce.com/fredofaure/32/18205_2.png) [@fredofaure](https://forum.juce.com/u/fredofaure)\
**Post date:** [May 4, 2023, 9:17pm UTC](https://forum.juce.com/t/how-to-add-a-listener-for-a-parameter-variable/56085/1 "2023-05-04T21:17:32Z")

</div>

Hello,  
I am new with JUCE.  
i try to learn how to use parameters in a simple way, but i am bloqued.  
In the processor.h i have:

```auto
	juce::AudioParameterFloat* test; 

```

In the constructor i have:

```auto
	addParameter (test = new juce::AudioParameterFloat ("test","Test", 0.0f, 1.0f, 0.5f));

```

In Processor::processBlock () i put:

```auto
	cout<<"test="<< test->get() <<endl;

```

and i see indeed that the values of test change if the host change them by automation.  
**My question is:** what is the simplest way to be informed that the value test has been changed by the host? (i imagine using a Listerner but i don’t find how to do exactly. I could compare the history of values of test, but i guess this is a bad solution?).  
Thanks,  
Frédéric.

---

<div class="post-metadata">

**Author:** ![railjonrogut](https://sea2.discourse-cdn.com/flex026/user_avatar/forum.juce.com/railjonrogut/32/505_2.png) [@railjonrogut](https://forum.juce.com/u/railjonrogut)\
**Post date:** [May 4, 2023, 11:18pm UTC](https://forum.juce.com/t/how-to-add-a-listener-for-a-parameter-variable/56085/2 "2023-05-04T23:18:16Z")

</div>

[https://docs.juce.com/master/classAudioProcessorParameter\_1\_1Listener.html#a5f2211a053ce9c130d55d160e13a9a66](https://docs.juce.com/master/classAudioProcessorParameter_1_1Listener.html#a5f2211a053ce9c130d55d160e13a9a66)

Rail

---

<div class="post-metadata">

**Author:** ![fredofaure](https://sea2.discourse-cdn.com/flex026/user_avatar/forum.juce.com/fredofaure/32/18205_2.png) [@fredofaure](https://forum.juce.com/u/fredofaure)\
**Post date:** [May 5, 2023, 10:09am UTC](https://forum.juce.com/t/how-to-add-a-listener-for-a-parameter-variable/56085/3 "2023-05-05T10:09:30Z")

</div>

Thanks a lot.  
I am still at the beginner level of using the tutorials to understand JUCE. Thanks for them. I don’t understand enough the whole system to be able to use directly the documentation with all the modules and class list.  
Following your link i did the following modifications of the code, **it works** , and i hope this is the good solution:

1. In the header file processor.h, i added this:

```auto
class Processor : public juce::AudioProcessor
                            #if JucePlugin_Enable_ARA
                             , public juce::AudioProcessorARAExtension
#endif
				 , public juce::AudioProcessorParameter::Listener
{
...
	juce::AudioParameterFloat* test;
   void parameterValueChanged (int parameterIndex, float newValue) override;

	void parameterGestureChanged(int parameterIndex, bool gestureIsStarting) override;
...
}

```

In the

1. In the file processor.cpp, i added this:

```auto
Processor::Processor()
#ifndef JucePlugin_PreferredChannelConfigurations
     : AudioProcessor (BusesProperties()
                     #if ! JucePlugin_IsMidiEffect
                      #if ! JucePlugin_IsSynth
                       .withInput ("Input", juce::AudioChannelSet::stereo(), true)
                      #endif
                       .withOutput ("Output", juce::AudioChannelSet::stereo(), true)
                     #endif
                       )
#endif
{	

	test = new juce::AudioParameterFloat(juce::ParameterID("test", 0), "Test", 0.0f, 1.0f, 0.5f);
	addParameter (test);
	test->addListener(this);
...

}

//==================================
void Processor::parameterValueChanged (int parameterIndex, float newValue) 
{

	cout<<"Processor::parameterValueChanged (), parameterIndex="<< parameterIndex<<" newValue="<< newValue<<endl;
	
}

//=======================
void Processor::parameterGestureChanged(int parameterIndex, bool gestureIsStarting)
{
	cout<<"Processor::parameterGestureChanged() parameterIndex="<<parameterIndex<<" gestureIsStarting="<< gestureIsStarting<< endl;

}

```

---

<div class="post-metadata">

**Author:** ![DKDiveDude](https://sea2.discourse-cdn.com/flex026/user_avatar/forum.juce.com/dkdivedude/32/7020_2.png) [@DKDiveDude](https://forum.juce.com/u/DKDiveDude)\
**Post date:** [May 5, 2023, 6:34pm UTC](https://forum.juce.com/t/how-to-add-a-listener-for-a-parameter-variable/56085/4 "2023-05-05T18:34:13Z")

</div>

This is how I do it’;

```auto
AudioProcessorValueTreeState::ParameterLayout createParameterLayout ()
{
	AudioProcessorValueTreeState::ParameterLayout params;

    params.add (std::make_unique<AudioParameterFloat> (String (module) + 
      "TGDutyCycleID", "TG " + String (module + 1) + " Phase Distortion", 
      10.0f, 90.0f, 50.0f));
}

```

```auto
MutineerAudioProcessor::MutineerAudioProcessor ()
#ifndef JucePlugin_PreferredChannelConfigurations
	: AudioProcessor (BusesProperties ()
#if ! JucePlugin_IsMidiEffect
#if ! JucePlugin_IsSynth
		.withInput ("Input", AudioChannelSet::stereo (), true)
#endif
		.withOutput ("Output", AudioChannelSet::stereo (), true)
#endif
	),
	parameters (*this, nullptr, "Parameters", createParameterLayout ())
#endif

{
for (int module = 0; module < maxModules; module++) // Note maxModules is 8
	{
		// Tone Generators
        synthParams.tgDutyCycleCKV[module] = parameters.getRawParameterValue 
             (String (module) + "TGDutyCycleID");

        parameters.addParameterListener (String (module) + "TGDutyCycleID", this);
    }
}

```

```auto
void MutineerAudioProcessor::parameterChanged (const String& parameterID, 
        float newValue)
{
	ignoreUnused (newValue);

	std::string moduleStr = parameterID.toStdString ();

	int module = std::stoi (moduleStr);

	// Tone Generator
	if (parameterID.contains ("TGDutyCycleID"))
	{
		if (synthParams.tgDutyCycleTypeKV[module] > 14) calculateDutyModulationCycle 
         (module);

		updateVoiceModuleParametersNew (module, 40);
	}
}

```