The core of the Wolfram Language's unique power for building interfaces is in its extremely flexible high-level symbolic paradigm. But the Wolfram Language also allows you to build interfaces at a lower level, more familiar to traditional user interface programmers.

EventHandler — explicitly handle many low-level interface events

CellEventActions  ▪  NotebookEventActions  ▪  FrontEndEventActions

GestureHandler — handle gestures on a touch device

MousePosition — find the current mouse position in many coordinate systems

MouseAppearance — change the appearance of the mouse cursor

TouchPosition — find current touch positions

ControllerState — find the current state of any control in an external controller device

CurrentValue — test and set hundreds of user interface properties

AbsoluteCurrentValue  ▪  Clock  ▪  ControlActive  ▪  $ControlActiveSetting  ▪  $SynchronousEvaluation

Dynamic — define dynamic content

DynamicModule — scope dynamic interface variables

CreateDocument  ▪  CellPrint  ▪  NotebookWrite  ▪  AttachCell  ▪  EmitSound

FrontEndExecute — programmatically execute built-in Wolfram System interface commands