Cannot pass lambda expression as Button's on_click() function

[georgjz]

I'm not sure if this is a bug or just me being dense (relearning C++, last time I touched it was pre-C++11), but it seems I can't pass a lambda as Button's on_click() argument:

    Component exit_button = Button( "Exit", screen.ExitLoopClosure() );
    Component exit_button = Button( "Exit", [&screen]{ screen.ExitLoopClosure(); } );

The first example will work, while the latter won't. I've looked into FTXUI/src/ftxui/component/button.cpp and I can't see a reason why it shouldn't be able to call a lambda expression for on_click() (again, relearning C++ currently, could be wrong).

Would it some cases make code easier to write I could pass lambdas directly.

[ArthurSonzogni]

Hello,

ScreenInteractive::ExitLoopClosure() returns the closure. It does not execute it.

So you should use

    Component exit_button = Button( "Exit", screen.ExitLoopClosure());

or:

    Component exit_button = Button( "Exit", [&screen] { screen.ExitLoopClosure()(); });

[ArthurSonzogni]

To avoid people being confused, it might worth to expose:

void ScreenInteractive::ExitLoop() {
    quit_ = true;
    event_sender_.reset();
}

And rewrite ScreenInteractive::ExitLoopClosure() as:

ScreenInteractive::Callback ScreenInteractive::ExitLoopClosure() {
  return [this] { Exit(); }
}

I would be happy to add this if you would like to make a PR.

[georgjz]

Thanks for the clarification! I didn't think to look at ExitLoopClosure(), but that makes a lot of sense. I'll consider a PR this weekend when I have more times.

[kcbit]

Hello,
I having problem with screen_.ExitLoopClosure();
in the main

  auto screen = ftxui::ScreenInteractive::FitComponent();
    Cgui cgui(screen);
    cgui.showdialoge();
    std::cout << "Delete keys " << cgui.selected_item << "\n";

and in Cgui

Cgui::Cgui(ftxui::ScreenInteractive &screen) : selected_index(0), screen_(screen) {
}

void Cgui::showdialoge() {
    std::vector<std::string> key = {
            "DKLK4PM.TEST.WPG",
            "DKLK4XAC.WEST",
            "DKLK4PM.TEST.WEST",
    };
    auto button_del = ftxui::Button("Keys", [&key, this] {
        Selected_key(key[selected_index]);
    });
    screen_.Loop(ftxui::Container::Vertical({
           ftxui::Dropdown(&key, &selected_index),
           button_del,
   }));
}
void Cgui::Selected_key(std::string &key) {
    selected_item = key;
    screen_.ExitLoopClosure();
    // want to return to main, do not work, why?
}

appreciate any help.
kcbit

[ArthurSonzogni]

@kcbit
We were saying above:

screen_.ExitLoopClosure();

returns a function. You need to call it.

screen_.ExitLoopClosure()();

[kcbit]

I am very grateful for your help.
kcbit.