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Introduction to Mixxx's Control System

Mixxx's control system is best described as a thread-safe communication channel that allows different threads to have shared access to read and modify values by name.

You can think of the Control system as Mixxx's internal API. Since we have one single API for accomplishing most things in Mixxx, we do not have duplicate logic to support keyboard control, MIDI/HID control, and GUI control. The keyboard, MIDI/HID, and GUI all use the same API to interact with Mixxx's mixing engine and other subsystems.

The first version (present since the very beginnings of Mixxx) of the control system is limited to only double-precision floating point values.

Control Naming

Controls are referred to by a 2-part name called a ConfigKey. ConfigKeys have a group and an item. The group is used to explain the category of the control while the item describes specifically what the control is. By convention, the group of a ConfigKey is wrapped with square brackets. This is because the ConfigKey class is also used to indicate values to store in the Mixxx preferences file which is organized like a Windows INI file.


  • Group: [Microphone] Item: volume
    • This control is holds the value of the microphone's volume.
  • Group: [Sampler1] Item: hotcue_1_activate
    • This control is used to conceptually represent a button that activates the first hotcue of the first sampler. Setting it to a value of 1 indicates that the virtual button is down while setting it to 0 indicates that the virtual button is released.

For a mostly complete list of controls which are intended to use in skins or controller mappings, see the MixxxControls page. For a complete list, which contains also unstable internal controls, start Mixxx from the command line using mixxx --developer. Now you can open the list by the menu: “Developer” → “Developer Tools” here you can also watch and edit the current values.

Creating Controls

To create a new control you must instantiate a ControlObject. Once you've done this, you've created a global control that is accessible by the ConfigKey you name it with.

As an example, let's say we want to create a control that keeps track of whether the microphone is enabled:

ConfigKey micEnabledKey = ConfigKey("[Microphone]", "enabled");
ControlObject* pMicEnabled = new ControlObject(micEnabledKey);
// Sets [Microphone],enabled to 1 (on)
// Sets [Microphone],enabled to 0 (off)

Once you have created a ControlObject, it is inserted into a global registry of controls and any other Mixxx thread can look it up by name using a ControlProxy.

For example, if you wanted to create a GUI widget that looked up whether the microphone was enabled or not, you would do this:

ControlProxy* pMicEnabled = new ControlProxy("[Microphone]", "enabled");
bool mic_enabled = pMicEnabled->toBool();
if (mic_enabled) {
    // draw a red light indicating the mic is enabled
} else {
   // draw a dim off light indicating the mic is disabled

Note that we used a ControlProxy to access the ControlObject we looked up from the control system. This is because we are not the original “owner” of this ControlObject. Using a ControlProxy allows you to safely access other ControlObjects from other places within Mixxx.

To change the microphone-enabled value from elsewhere in Mixxx, it's as simple as this:

// Disable the microphone from elsewhere in Mixxx.

Types of Controls

ControlObject has a variety of subclasses. Each of these subclasses allows you to express constraints on the value stored in the ControlObject. For example, if you wanted to represent a knob that had values between -1 and 1 then you could use a ControlPotmeter and set its min and max value to -1 and 1. This would prevent any other part of Mixxx from setting the control to an invalid value.

Another use of using a subclass of ControlObject is that you can make a control that is non-linear in the values that it produces. For example, a ControlLogpotmeter is a knob/potentiometer whose output follows a logarithmic curve even though the input coming from a MIDI controller of the GUI is linear.


A ControlPushButton is a control that emulates a button. The name is a misnomer since it doesn't only emulate push-buttons – it can also emulate multi-state toggle buttons.

ControlPushButton has the following modes:

  • Push-button – acts like a push button where a value > 0 means it is pressed and a value of 0 means it is not pressed.
  • Toggle button – acts like a multi-state toggle button. Pressing it and releasing it will increment its value by one. Once it increments past the total number of states it has it will wrap around to 0. For a 2-state toggle button this means it will toggle between 0 and 1 every time you press it.
  • Power-Window button – acts like a power-window button on your car. Pressing and holding it makes it act like a push-button (e.g. power window goes down or up only as you hold it) but pressing and releasing it quickly will toggle it (e.g. power window goes down or up automatically).

Relevant Methods:

  • To change the mode, call ControlPushButton::setButtonMode(enum ButtonMode)
  • To change the number of states call ControlPushButton::setStates(int)


A ControlPotmeter represents a knob or potentiometer that has a minimum and maximum value. All modifications to the control are checked and snapped within the specified range.

All ControlPotmeters have a set of additional controls that are automatically created for them. Assuming a ConfigKey of [Group],item, the controls that are created are:

  • [Group],item_up – Increment (coarse adjustent, default 10% of range)
  • [Group],item_down – Decrement (coarse adjustent, default 10% of range)
  • [Group],item_up_small – Increment by a small amount (fine adjustent, default 1% of range)
  • [Group],item_down_small – Decrement by a small amount (fine adjustent default 1% of range)
  • [Group],item_set_default – Sets the value to default (the default default is the half-way point
  • [Group],item_set_zero – Sets the value to 0
  • [Group],item_set_one – Sets the value to 1
  • [Group],item_set_minus_one – Sets the value to -1
  • [Group],item_toggle – Toggle between 0 and 1
  • [Group],item_minus_toggle – Toggle between 0 and -1




Read-only Controls

A control can be marked read-only by using ControlObject's setReadOnly method.

This method intercepts calls to the set method of the ControlObject and any associated ControlProxys and prints a warning message that the control is read-only. As the owner, to set the control's value, you must use the ControlObject's forceSet method. Only the owner of the control (see above) should update it this way – do not bypass this by using ControlObject::getControl.

ControlObject track_loaded(ConfigKey(group, "track_loaded"));
// Sets on the ControlObject are ignored.
assert(track_loaded.get() == 0);
// Sets from proxies are also ignored.
ControlProxy proxy(group, "track_loaded");
assert(track_loaded.get() == 0);
// As the owner, the only way to change the control's value is via forceSet.
assert(track_loaded.get() == 1);

Automatically reacting to control changes

As different parts of Mixxx change controls that you create, you will likely want to react to those changes. Similarly, if you are relying on certain controls created elsewhere in Mixxx, you may want to automatically take action when one of them changes.

If you are the creator of a control, i.e. you are using the raw ControlObject and not a ControlProxy then you can listen to changes in your control by connecting to the ControlObject's valueChanged(double) signal.

ControlObject* pMicEnabled = new ControlObject(ConfigKey("[Microphone]", "enabled"));
connect(pMicEnabled, SIGNAL(valueChanged(double)), 
        this, SLOT(slotMicrophoneEnabledRequest(double)));
// In your slot, handle the changes to the [Microphone],enabled control
void MicrophoneManager::slotMicrophoneEnabledRequest(double v) {
  // The microphone's enabled value has changed
  bool mic_enabled = v > 0.0;
  if (mic_enabled) {
    // some other part of Mixxx is requesting to enable the microphone, do that
  }  else {
    // some other part of Mixxx is requesting to disable the microphone, do that

ControlObject actually has two value-change signals valueChanged(double) and valueChangedFromEngine(double). These two signals should really be named valueChangedByProxy(double) and valueChangedByControlObject(double), respectively. The first signal is fired when a proxy (ControlObjectThread, etc.) was used to change the control while the second is fired when the ControlObject itself is used to change the value by its set(double) method. The reason for having two signals is to prevent infinite loops of signal processing. If you create the ControlObject and then call set(1.0) on it but you also connect to its valueChanged(double) signal, then you will get a call for the valueChanged(double) signal which you might want to react to by in turn changing the enabled ControlObject. If you do this and get a callback for the value you changed then you will go into an infinite loop of setting and reacting to your ControlObject. For this reason, there are two different types of value-changed signals.

Going back to the microphone-enabled GUI widget example, if you are using a ControlProxy, you can listen to changes in the control by doing the following:

ControlObjectThread* pMicEnabled = new ControlProxy("[Microphone]", "enabled", this);
pMicEnabled->connectValueChanged(SLOT(slotMicrophoneEnabledChanged(double v)));
// In your slot, handle the changes to the [Microphone],enabled control
void MicrophoneWidget::slotMicrophoneEnabledChanged(double v) {
  // The microphone's enabled value has changed
  bool mic_enabled = v > 0.0;
  if (mic_enabled) {
    // redraw the widget with a red light to indicate the microphone is enabled
  }  else {
    // redraw the widget with a dark light to indicate the microphone is not enabled

Deleting Controls

Be sure you delete all ControlObjects after they are not used anymore to avoid memory leaks. In general once you create a control you should not delete it until Mixxx shuts down. Since creating a control is like setting up an API call.

Translations of this page:
developer_guide_control.txt · Last modified: 2017/04/15 17:51 by gramanas