CERN ROOT and Keyboard Event Handling in TCanvas

As part of a longstanding quest, I have always wished to possess the capability to use single-keypress events on a TCanvas or a TBrowser in CERN ROOT to trigger actions.
Handling a single keystroke, a la getch() of Borland C++ vintage, is straightforward enough, by using a construct like while(gPad->WaitPrimitive()); peppered within the macro/program, which waits for a keypress in the active macro. If one wants the keypress while a particular TCanvas was active, that was straightfroward too, as while(canvas->WaitPrimitive());
What used to elude my understanding was how to parse the return value of gPad->GetEvent() to recognize keystrokes in the first place, and secondly, what would allow me to get the result of the keypress.
My very crafty coworker over at https://github.com/jmattspartacus told me the wonderful (but very cryptic) trick a few days back: type-casting to char the results of gPad->GetEventX() gives the ascii value of keypress events! This is a little infuriating, since GetEventsX() is used to track mouse X positions in events! Wth, ROOT?
It turns out, one can also cast to short the results from gPad->GetEvent() to filter out keypresses by searching for the number 24. (Which is 42 flipped, makes sense.) gPad->GetEventX() and gPad->GetEventY() both store the same number, which when cast to char return the ascii value of the key pressed.
All in all, the following macro if run with the usual
```
root -l -x -q track_keypress_in_canvas.C
```
should bring up a TCanvas with a histogram. Once you hover over the canvas, you can cycle through ROOT’s default colors by pressing ‘n’ and ‘p’. The title to the canvas will show the current color index.
Fun times! This use-case of TCanvas::AddExec will make a lot of basic user-interfacing a breeze to work with. And to think, this issue needed needless reverse engineering! Hope the people over at CERN ROOT advertise this, instead of hiding it away.


int cc=0;
TH1F h1("h1","h1",800,-400,400);

void myexec()
{
   // get event information
   short event = gPad->GetEvent();
   int px    = gPad->GetEventX();
   int py    = gPad->GetEventY();

   // some magic to get the coordinates...
   /*double xd = gPad->AbsPixeltoX(px);
   double yd = gPad->AbsPixeltoY(py);
   float x = gPad->PadtoX(xd);
   float y = gPad->PadtoY(yd);*/

   //if (event==1) { // left mouse button click, if needed
      //return;
   //}
   if(event==24 && (((char)px =='n' || (char)px=='p'))) { //24 is keypress, px, and py are assigned characters
       //std::cout << (int)event << " " << (char)px << " " << (char)py << std::endl;
       if((char)px=='n') cc++;
       else cc--;
       h1.SetTitle(Form("Color:%d",cc));
       h1.SetFillColorAlpha(cc,0.4);
       gPad->Modified();
       gPad->Update();
       return;
   }
}


void track_keypress_in_canvas()
{
   gStyle->SetOptStat(0);
   h1.GetXaxis()->SetRangeUser(-5,10);
   TF1 f1("gauss",Form("%f*TMath::Exp(-(x-%f)*(x-%f)/(2*2.*2.))",1000.,2.,2.),2.-5,2.+5);
   h1.FillRandom("gauss",1000,nullptr);
   h1.SetLineWidth(2.0);
   h1.GetXaxis()->SetTitle("Hover mouse, press 'n' or 'p' to cycle colors");
   h1.Draw();
   gPad->Modified();
   gPad->Update();

   // add exec
   gPad->AddExec("myexec","myexec()");
}

Embedding CERN ROOT classes in Lua5.1 via tolua++

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