// execute in ROOT with
// .x roller.C
//
void roller() {
    TCanvas *c = new TCanvas();
    c->Draw("");
    //c->Update();
    c->AddExec("dynamic","mouse()");
}

void mouse() {
    int evno = 0;
    gPad->GetCanvas()->FeedbackMode(kTRUE);
    evno = gPad->GetEvent();
    cout << evno << endl;
}