How do I move a variable out of a closure?

Question

I have a simple GTK app and I need to select a folder and pass it's path to a variable outside of the closure.

let mut path = "".to_owned();

button_open.connect_clicked(move |_| {
    let file_chooser = gtk::FileChooserDialog::new(
        "Open File", None, gtk::FileChooserAction::SelectFolder,
        [("Open", gtk::ResponseType::Ok), ("Cancel", gtk::ResponseType::Cancel)]);
    if file_chooser.run() == gtk::ResponseType::Ok as i32 {
        let filename = file_chooser.get_current_folder().unwrap();
    }

    file_chooser.destroy();
});

How do I assign filename to path? If I just write

path = filename;

I get this error:

src\main.rs:46:13: 46:28 error: cannot assign to captured outer variable in an `Fn` closure
src\main.rs:46             path = filename;

Answer 1

You can't do this for several reasons. Here is the definition of connect_clicked() method:

fn connect_clicked<F: Fn(Button) + 'static>(&self, f: F) -> u64

The closure which this method accepts is Fn and is bounded by 'static. This means, first, that it can't modify its environment (that would be FnMut) and it also can't capture anything by reference (roughly what 'static means). Therefore, there is no way for the closure to modify the path variable like you want.

Given that gtk-rs enforces that GTK can only be used from the main thread, and its widgets are not Sendable, there is no way for the handler to access variables from another thread, so I'm not sure why these closures have 'static bound, and I also see absolutely no reason why they are not FnMut. This seems to be an implementation defect.

Anyway, you can use Rc<RefCell<..>> to create a mutable piece of data:

let path: Rc<RefCell<Option<String>>> = Rc::new(RefCell::new(None));
let captured_path = path.clone();  // clone the Arc to use in closure
...
// inside the closure
*captured_path.borrow_mut() = Some(filename);

It is also possible to use Arc<Mutex<..>>, but I don't think it is necessary because gtk-rs protects against using GTK from any thread except the main one.