If necessary, schedules a new frame by calling
ui.window.scheduleFrame.
After this is called, the engine will (eventually) call handleBeginFrame. (This call might be delayed, e.g. if the device's screen is turned off it will typically be delayed until the screen is on and the application is visible.)
Source
void scheduleFrame() {
if (_hasScheduledFrame)
return;
ui.window.scheduleFrame();
_hasScheduledFrame = true;
}