ngspice may be compiled as a shared library. This allows adding ngspice to an application that then gains control over the simulator. The shared module offers an interface that exports functions controlling the simulator and callback functions for feedback.

So you may send an input `file' with a netlist to ngspice, start the simulation in a separate thread, read back simulation data at each time point, stop the simulator depending on some condition, alter device or model parameters and then resume the simulation.

Shared ngspice does not have any user interface. The calling process is responsible for this. It may offer a graphical user interface, add plotting capability or any other interactive element. You may develop and optimize these user interface elements without a need to alter the ngspice source code itself, using a console application or GUIs like gtk, Delphi, Qt or others.