Alternative way to handle GTK+ events in Rust

Question

Currently, I manage GTK+ events with Rc and RefCell as shown in the following example:

extern crate gtk;

use std::cell::RefCell;
use std::rc::Rc;

use gtk::{Button, ButtonExt, ContainerExt, Inhibit, Label, WidgetExt, Window, WindowType};
use gtk::Orientation::Vertical;

struct Model {
    count: i32,
}

fn main() {
    gtk::init().unwrap();

    let window = Window::new(WindowType::Toplevel);

    let model = Rc::new(RefCell::new(Model { count: 0 }));

    let vbox = gtk::Box::new(Vertical, 0);
    window.add(&vbox);

    let label = Label::new(Some("0"));
    vbox.add(&label);

    let button = Button::new_with_label("Increment");
    vbox.add(&button);
    window.show_all();

    window.connect_delete_event(|_, _| {
        gtk::main_quit();
        Inhibit(false)
    });

    {
        let model = model.clone();
        button.connect_clicked(move |_| {
            {
                (*model.borrow_mut()).count += 1;
            }
            label.set_text(&format!("{}", (*model.borrow()).count));
        });
    }

    gtk::main();
}

My main issue with this code is the boilerplate needed because of the RefCells.

Also I feel like it is bad practise and can lead to panics (this is not my main issue so don't propose to use a Mutex because in some examples, that can lead to a deadlock).

So I thought I could handle events in a way similar to Elm: with one function receiving signals where the model could be updated. However, I am unable to implement this in Rust. Here is an attempt:

extern crate gtk;

use std::cell::RefCell;
use std::collections::VecDeque;
use std::rc::Rc;

use gtk::{Button, ButtonExt, ContainerExt, Inhibit, Label, WidgetExt, WindowType};
use gtk::Orientation::Vertical;

use Message::Increment;

enum Message {
    Increment,
}

struct Window {
    label: Label,
    model: Model,
    queue: Rc<RefCell<VecDeque<Message>>>,
    view: gtk::Window,
}

impl Window {
    fn new() -> Self {
        let window = gtk::Window::new(WindowType::Toplevel);

        let vbox = gtk::Box::new(Vertical, 0);
        window.add(&vbox);

        let label = Label::new(Some("0"));
        vbox.add(&label);

        let button = Button::new_with_label("Increment");
        vbox.add(&button);
        window.show_all();

        window.connect_delete_event(|_, _| {
            gtk::main_quit();
            Inhibit(false)
        });

        let queue = Rc::new(RefCell::new(VecDeque::new()));

        {
            let queue = queue.clone();
            button.connect_clicked(move |_| {
                (*queue.borrow_mut()).push_back(Increment);
            });
        }

        Window {
            label: label,
            queue: queue,
            model: Model { count: 0 },
            view: window,
        }
    }

    // How to call this method when a message is received?
    fn update(&mut self, message: Message) {
        match message {
            Increment => {
                self.model.count += 1;
                self.label.set_text(&format!("{}", self.model.count));
            },
        }
    }
}

struct Model {
    count: i32,
}

fn main() {
    gtk::init().unwrap();

    let window = Window::new();

    gtk::main();
}

How can I call the update() method when the message queue is updated?

Is it a viable approach?

If not, do you know any alternatives that would provide a solution to this issue?

Perhaps some solution based on the future crate could be used? In this case, how do I manage both main loops (the gtk+ one and the tokio one).

Or a solution using channels?

The mutex deadlocks because GTK+ signals run on the same thread as the one gtk::main() runs on. I wouldn't know a good Rust-specific solution to the problem, but I can give a GTK+-specific one: use idle callbacks to send stuff from another thread to the GTK+ thread. — andlabs
I know about idle_add(), but I don't know how I can call my update() method from it because the object would need to be static (or in a Rc<RefCell<_>> which is what I'm trying to avoid). — antoyo

antoyo antoyo · Accepted Answer · 2016-11-11T04:30:21

I found a solution to this issue here.

Here is what I achieved with my example:

extern crate gtk;

use gtk::{Button, ButtonExt, ContainerExt, Inhibit, Label, WidgetExt, WindowType};
use gtk::Orientation::Vertical;

use Message::Increment;

macro_rules! connect {
    ($source:ident :: $event:ident, $target:ident :: $message:expr) => {
        let target = &mut *$target as *mut _;
        $source.$event(move |_| {
            let target: &mut Window = unsafe { &mut *target };
            target.update($message);
        });
    };
}

enum Message {
    Increment,
}

struct Window {
    label: Label,
    model: Model,
    view: gtk::Window,
}

impl Window {
    fn new() -> Box<Self> {
        let window = gtk::Window::new(WindowType::Toplevel);

        let vbox = gtk::Box::new(Vertical, 0);
        window.add(&vbox);

        let label = Label::new(Some("0"));
        vbox.add(&label);

        let button = Button::new_with_label("Increment");
        vbox.add(&button);
        window.show_all();

        window.connect_delete_event(|_, _| {
            gtk::main_quit();
            Inhibit(false)
        });

        let mut window = Box::new(Window {
            label: label,
            model: Model { count: 0 },
            view: window,
        });

        connect!(button::connect_clicked, window::Increment);

        window
    }

    fn update(&mut self, message: Message) {
        match message {
            Increment => {
                self.model.count += 1;
                self.label.set_text(&format!("{}", self.model.count));
            },
        }
    }
}

struct Model {
    count: i32,
}

fn main() {
    gtk::init().unwrap();

    let _window = Window::new();

    gtk::main();
}

Do you think this use of unsafe can cause segfault? For instance, if the button could outlive the window, the window could be used after being freed, no? Is there a way to build a safe wrapper around this?

On an esthetical note, is there a way to achieve this:

connect!(button::clicked, window::Increment);

Alternative way to handle GTK+ events in Rust

1 Answers