如何在使用下拉列表小部件时返回函数值

Question

我在Ubuntu上。当我点击下拉列表时，我需要返回一个函数，但我不知道如何做到这一点。例如：

...
dropdownList.set_active_text("Choose");
dropdownList.signal_changed().connect(sigc::mem_fun(*this, &usb_boot::showing));
std::cout << "var = " << var << std::endl;
...
void usb_boot::showing(){
Gtk::MessageDialog dialogue(*this, dropdownList.get_active_text());
dialog.set_secondary_text("Choose list");
dialog.run();
std::cout << "You choose :n" << dropdownList.get_active_row_number() << " " << dropdownList.get_active_text() << std::endl;
add return here ?

如何将dropdownList.get_active_text()和dropdownList.get_active_row_number()返回到变量？

Answer 1

如果我正确理解您的问题，您希望在处理程序(即showing(的范围之外重用从处理程序中获得的一些值。为此，我建议使用lambda表达式并通过引用变量来捕获。例如：

#include <iostream>
#include <gtkmm.h>
int main(int argc, char **argv)
{
auto app = Gtk::Application::create(argc, argv, "so.question.q63850579");

Gtk::Window window;
Gtk::ComboBoxText combo;
combo.append("option 1");
combo.append("option 2");
combo.append("option 3");
// These are your variables, which you want to set to the values the user will choose. They are defined
// outise the handler:
std::string choice;
int row;
// Here you set the handler. The variables 'choice', 'row' and 'combo' are all passed by reference to
// the handler (notice the '&'):
combo.signal_changed().connect([&choice, &row, &combo](){
choice = combo.get_active_text();
row = combo.get_active_row_number(); 
// Inside the scope of the handler, we can see the variable content changing everytime the user
// changes a value:
std::cout << "Your current selection is item #" << row << ", which is: " << choice << std::endl;
});
window.add(combo);
window.show_all();
int returnCode = app->run(window);
// When the window closes, the variables are read again, but this time from outside the handler's
// scope. This is possible because they were references:
std::cout << "Your final selection was item #" << row << ", which is: " << choice << std::endl;
return returnCode;
}

由于使用sigc::mem_fun，另一个选项是将内容保存在表示this所指向对象的类的成员变量中。我个人更喜欢lambda表达式，因为这些值可能与类无关。