Sharing variable in two closures

Question

I'm listening to user input in an gtk-rs input element. input.connect_changed triggers when the input changes and input.connect_activate triggers when Enter is pressed.

use gtk::prelude::*; use gtk::{Application, ApplicationWindow}; use std::process::{Command, Output}; fn main() { let app = Application::builder() .application_id("com.jwestall.ui-demo") .build(); app.connect_activate(build_ui); app.run(); } fn run_command(command: &str) -> Output { Command::new("sh") .arg("-c") .arg(command) .output() .unwrap_or_else(|_| panic!("failed to execute {}'", command)) } fn build_ui(app: &Application) { let input = gtk::Entry::builder() .placeholder_text("input") .margin_top(12) .margin_bottom(12) .margin_start(12) .margin_end(12) .build(); let window = ApplicationWindow::builder() .application(app) .title("gtk-app") .child(&input) .build(); window.show_all(); input.connect_changed(|entry| { let input_text = entry.text(); let command = format!("xdotool search --onlyvisible --name {}", input_text); let window_id_output = run_command(&command); if window_id_output.status.success() { println!( "stdout: {}", String::from_utf8_lossy(&window_id_output.stdout) ); } else { println!( "sterr: {}", String::from_utf8_lossy(&window_id_output.stderr) ); } }); input.connect_activate(move |entry| { let input_text = entry.text(); // // `xdotool windowactivate` doesn't produce any output let command = format!("xdotool windowactivate {}", window_id_output); let window_activate_output = run_command(&command); println!("window_activate: {}", window_activate_output); window.hide(); window.close(); }); }

I want to set window_id_output in input.connect_changed, then use it in input.connect_activate (in the xdotool windowactivate {} command).

How can I use window_id_output this way in these two closures?

Rust Playground

If I remember correctly, GTK runs all event handlers in the main thread, so you don't need any locking. The only requirments on your closures is that they are 'static – you don't need Send or Sync. Wrapping window_id_output in Rc<RefCell<…>> should be all you need. — Sven Marnach
– Sven Marnach, Commented Feb 7, 2023 at 15:49
@SvenMarnach Could you give me an example? I tried using Rc<RefCell<...>>: play.rust-lang.org/…. But it's producing compiling errors like: move occurs because window_id_output_rc` has type Rc<RefCell<Option│ <Output>>>, which does not implement the Copy trait`. — wyc
– wyc, Commented Feb 7, 2023 at 16:09
I'd suggest you read up on Rc and RefCell in the Rust book. — Sven Marnach
– Sven Marnach, Commented Feb 7, 2023 at 16:30

Szigeti Péter · Accepted Answer · 2023-02-07 16:24:58Z

As Sven Marnach said, you can use Rc<RefCell<..>> to move data between closures.

The simplest example is probably this one, probably how the gtk event loop works anyways:

use std::rc::Rc; use std::cell::RefCell; fn main() { let a = Rc::new(RefCell::new(0)); let a_ref = Rc::clone(&a); let closure_1 = move || { let mut a = a_ref.borrow_mut(); *a += 1; println!("closure_1: {}", &a); }; let a_ref = Rc::clone(&a); let closure_2 = move || { let a = a_ref.borrow(); println!("closure_2: {}", &a); }; for _ in 1..10 { closure_1(); closure_2(); } }

For your specific case, see a reduced example below (based on your code):

use std::cell::RefCell; use std::rc::Rc; use gtk::prelude::*; use gtk::{Application, ApplicationWindow}; fn main() { let app = Application::builder() .application_id("com.jwestall.ui-demo") .build(); app.connect_activate(build_ui); app.run(); } fn process(s: &str) -> String { format!("you entered '{}'", s) } fn build_ui(app: &Application) { let input = gtk::Entry::builder() .placeholder_text("input") .margin_top(12) .margin_bottom(12) .margin_start(12) .margin_end(12) .build(); let window = ApplicationWindow::builder() .application(app) .title("gtk-app") .child(&input) .build(); window.show_all(); let shared_var = Rc::new(RefCell::new(String::new())); let shared_var_ref = Rc::clone(&shared_var); input.connect_changed(move |entry| { let input_text = entry.text(); let mut shared = shared_var_ref.borrow_mut(); *shared = process(&input_text); }); let shared_var_ref = Rc::clone(&shared_var); input.connect_activate(move |_entry| { let shared = shared_var_ref.borrow(); println!("{}", shared); window.hide(); window.close(); }); }

Thanks, it worked! I'm new to Rc and RefCell. I'll have to read more about them.

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