When writing code for a MIDI Control Surface, you'll likely come across the Control_Surface singleton. The program structure usually looks something like this:

#include <Control_Surface.h>
 
// Instantiate MIDI interfaces, ExtIO elements, MIDI elements, etc.
 
void setup() {
    Control_Surface.begin();
}
 
void loop() {
    Control_Surface.loop();
}

The Control_Surface singleton is responsible for the following important tasks:

Initializing all MIDI interfaces, ExtIO elements, MIDI elements, selectors, display elements, etc. This is done by Control_Surface.begin().
Periodically updating all elements, so they can poll digital or analog input pins etc. This happens when you call Control_Surface.loop(), so it is important to call this function frequently and without blocking for a long time.
Routing all MIDI messages sent by MIDI Output Elements to the right MIDI interface. The default interface can be set using setAsDefault() function or using manual routing as explained in the Routing MIDI messages section of the MIDI Tutorial.
Dispatching incoming MIDI messages to the appropriate MIDI Input Elements.
Refreshing all displays and Display Elements.

Control Surface keeps track of elements that inherit from the UpdatableCRTP base class. Specifically, it keeps a list of all ExtendedIOElements, MIDI_Interfaces, MIDI Output Elements, MIDI Input Elements and Display Elements. If you have certain elements that should not be initialized or updated by Control Surface, you can call disable() before calling Control_Surface.begin(). It is then your responsibility to call the begin() and update() functions whenever appropriate. You can also enable these disabled elements again using enable().