callback - 如何 : Idiomatic Rust for callbacks with gtk (rust-gnome)

Question

我目前正在学习 Rust 并希望用它来开发 GUI基于 GTK+ 的应用程序。我的问题与注册回调有关在这些回调中响应 GTK 事件/信号和变异状态。我有一个有效但不优雅的解决方案，所以我想问一下是否有是一种更简洁、更惯用的解决方案。

我已经将我的代码实现为具有方法实现的结构，其中该结构维护对 GTK 小部件的引用以及其他状态它需要。它构造一个传递给 GtkWidget::connect*为了接收事件的功能，绘制到 Canvas 等。这可能会导致借用检查器出现问题，就像我现在一样解释。我有一些工作但(恕我直言)非理想代码，我将显示。

初始的非工作解决方案:

#![cfg_attr(not(feature = "gtk_3_10"), allow(unused_variables, unused_mut))]

extern crate gtk;
extern crate cairo;

use gtk::traits::*;
use gtk::signal::Inhibit;
use cairo::{Context, RectangleInt};


struct RenderingAPITestWindow {
    window: gtk::Window,
    drawing_area: gtk::DrawingArea,
    width: i32,
    height: i32
}

impl RenderingAPITestWindow {
    fn new(width: i32, height: i32) -> RenderingAPITestWindow {
        let window = gtk::Window::new(gtk::WindowType::TopLevel).unwrap();
        let drawing_area = gtk::DrawingArea::new().unwrap();
        drawing_area.set_size_request(width, height);
        window.set_title("Cairo API test");
        window.add(&drawing_area);

        let instance = RenderingAPITestWindow{window: window,
            drawing_area: drawing_area,
            width: width,
            height: height,
        };

        instance.drawing_area.connect_draw(|widget, cairo_context| {
            instance.on_draw(cairo_context);
            instance.drawing_area.queue_draw();
            Inhibit(true)
        });

        instance.drawing_area.connect_size_allocate(|widget, rect| {
            instance.on_size_allocate(rect);
        });

        instance.window.show_all();

        return instance;
    }

    fn exit_on_close(&self) {
        self.window.connect_delete_event(|_, _| {
            gtk::main_quit();
            Inhibit(true)
        });
    }


    fn on_draw(&mut self, cairo_ctx: Context) {
        cairo_ctx.save();
        cairo_ctx.move_to(50.0, (self.height as f64) * 0.5);
        cairo_ctx.set_font_size(18.0);
        cairo_ctx.show_text("The only curse they could afford to put on a tomb these days was 'Bugger Off'. --PTerry");
        cairo_ctx.restore();
    }

    fn on_size_allocate(&mut self, rect: &RectangleInt) {
        self.width = rect.width as i32;
        self.height = rect.height as i32;
    }
}


fn main() {
    gtk::init().unwrap_or_else(|_| panic!("Failed to initialize GTK."));
    println!("Major: {}, Minor: {}", gtk::get_major_version(), gtk::get_minor_version());

    let window = RenderingAPITestWindow::new(800, 500);
    window.exit_on_close();
    gtk::main();
}

上面的代码无法编译为闭包 RenderingAPITestWindow::new创建并传递给调用 GtkWidget::connect*方法试图借用 instance .这编译器声明闭包可能比其中的函数长寿他们被宣布并且instance由外部函数拥有，因此问题。鉴于 GTK 可能会保留对这些闭包的引用在不确定的时间内，我们需要一种方法生命周期可以在运行时确定，因此我下一次尝试解决这个问题其中RenderingAPITestWindow实例被包裹在 Rc<RefCell<...>> .

包装 RenderingAPITestWindow实例编译但在运行时死亡:

#![cfg_attr(not(feature = "gtk_3_10"), allow(unused_variables, unused_mut))]

extern crate gtk;
extern crate cairo;

use std::rc::Rc;
use std::cell::RefCell;
use gtk::traits::*;
use gtk::signal::Inhibit;
use cairo::{Context, RectangleInt};


struct RenderingAPITestWindow {
    window: gtk::Window,
    drawing_area: gtk::DrawingArea,
    width: i32,
    height: i32
}

impl RenderingAPITestWindow {
    fn new(width: i32, height: i32) -> Rc<RefCell<RenderingAPITestWindow>> {
        let window = gtk::Window::new(gtk::WindowType::TopLevel).unwrap();
        let drawing_area = gtk::DrawingArea::new().unwrap();
        drawing_area.set_size_request(width, height);
        window.set_title("Cairo API test");
        window.add(&drawing_area);

        let instance = RenderingAPITestWindow{window: window,
            drawing_area: drawing_area,
            width: width,
            height: height,
        };
        let wrapped_instance = Rc::new(RefCell::new(instance));

        let wrapped_instance_for_draw = wrapped_instance.clone();
        wrapped_instance.borrow().drawing_area.connect_draw(move |widget, cairo_context| {
            wrapped_instance_for_draw.borrow_mut().on_draw(cairo_context);

            wrapped_instance_for_draw.borrow().drawing_area.queue_draw();
            Inhibit(true)
        });

        let wrapped_instance_for_sizealloc = wrapped_instance.clone();
        wrapped_instance.borrow().drawing_area.connect_size_allocate(move |widget, rect| {
            wrapped_instance_for_sizealloc.borrow_mut().on_size_allocate(rect);
        });

        wrapped_instance.borrow().window.show_all();

        return wrapped_instance;
    }

    fn exit_on_close(&self) {
        self.window.connect_delete_event(|_, _| {
            gtk::main_quit();
            Inhibit(true)
        });
    }


    fn on_draw(&mut self, cairo_ctx: Context) {
        cairo_ctx.save();
        cairo_ctx.move_to(50.0, (self.height as f64) * 0.5);
        cairo_ctx.set_font_size(18.0);
        cairo_ctx.show_text("The only curse they could afford to put on a tomb these days was 'Bugger Off'. --PTerry");
        cairo_ctx.restore();
    }

    fn on_size_allocate(&mut self, rect: &RectangleInt) {
        self.width = rect.width as i32;
        self.height = rect.height as i32;
    }
}


fn main() {
    gtk::init().unwrap_or_else(|_| panic!("Failed to initialize GTK."));
    println!("Major: {}, Minor: {}", gtk::get_major_version(), gtk::get_minor_version());

    let wrapped_window = RenderingAPITestWindow::new(800, 500);
    wrapped_window.borrow().exit_on_close();
    gtk::main();
}

上面的解决方案编译但不是特别漂亮:

• RenderingAPITestWindow::new返回一个 Rc<RefCell<RenderingAPITestWindow>>而不是一个 RenderingAPITestWindow
• 访问 RenderingAPITestWindow 的字段和方法很复杂事实上 Rc<RefCell<...>>必须打开；现在需要 wrapped_instance.borrow().some_method(...)而不仅仅是 instance.some_method(...)
• 每个闭包都需要它自己的 wrapped_instance 克隆;尝试使用 wrapped_instance会尝试借用一个对象——包装器而不是 RenderingAPITestWindow这次——也就是属于RenderingAPITestWindow::new和以前一样

虽然上面的编译，它在运行时死了:

thread '<main>' panicked at 'RefCell<T> already borrowed', ../src/libcore/cell.rs:442
An unknown error occurred

这是因为对 window.show_all() 的调用导致 GTK 到初始化小部件层次结构，生成绘图区域小部件收到size-allocate事件。访问窗口调用 show_all()要求 Rc<RefCell<...>>被打开(因此 wrapped_instance.borrow().window.show_all(); ) 和实例借来的。 show_all() 借用结束前返回时，GTK 调用绘图区的size-allocate事件处理程序，导致关闭连接到它(上面的 4 行)以被调用。闭包试图借用对 RenderingAPITestWindow 的可变引用实例( wrapped_instance_for_sizealloc.borrow_mut().on_size_allocate(rect); )为了调用 on_size_allocate方法。这试图借用一个可变引用，而第一个不可变引用仍在范围内。第二次借用会导致运行时 panic 。

工作 但是 - 恕我直言 - 我设法得到的不优雅的解决方案到目前为止的工作是拆分RenderingAPITestWindow分为两个结构，由回调修改的可变状态移入单独的结构。

拆分 RenderingAPITestWindow 的可行但不优雅的解决方案结构:

#![cfg_attr(not(feature = "gtk_3_10"), allow(unused_variables, unused_mut))]

extern crate gtk;
extern crate cairo;

use std::rc::Rc;
use std::cell::RefCell;
use gtk::traits::*;
use gtk::signal::Inhibit;
use cairo::{Context, RectangleInt};


struct RenderingAPITestWindowState {
    width: i32,
    height: i32
}

impl RenderingAPITestWindowState {
    fn new(width: i32, height: i32) -> RenderingAPITestWindowState {
        return RenderingAPITestWindowState{width: width, height: height};
    }

    fn on_draw(&mut self, cairo_ctx: Context) {
        cairo_ctx.save();
        cairo_ctx.move_to(50.0, (self.height as f64) * 0.5);
        cairo_ctx.set_font_size(18.0);
        cairo_ctx.show_text("The only curse they could afford to put on a tomb these days was 'Bugger Off'. --PTerry");
        cairo_ctx.restore();
    }

    fn on_size_allocate(&mut self, rect: &RectangleInt) {
        self.width = rect.width as i32;
        self.height = rect.height as i32;
    }
}


struct RenderingAPITestWindow {
    window: gtk::Window,
    drawing_area: gtk::DrawingArea,
    state: Rc<RefCell<RenderingAPITestWindowState>>
}

impl RenderingAPITestWindow {
    fn new(width: i32, height: i32) -> Rc<RefCell<RenderingAPITestWindow>> {
        let window = gtk::Window::new(gtk::WindowType::TopLevel).unwrap();
        let drawing_area = gtk::DrawingArea::new().unwrap();
        drawing_area.set_size_request(width, height);
        window.set_title("Cairo API test");
        window.add(&drawing_area);

        let wrapped_state = Rc::new(RefCell::new(RenderingAPITestWindowState::new(width, height)))
        ;

        let instance = RenderingAPITestWindow{window: window,
            drawing_area: drawing_area,
            state: wrapped_state.clone()
        };
        let wrapped_instance = Rc::new(RefCell::new(instance));

        let wrapped_state_for_draw = wrapped_state.clone();
        let wrapped_instance_for_draw = wrapped_instance.clone();
        wrapped_instance.borrow().drawing_area.connect_draw(move |widget, cairo_context| {
            wrapped_state_for_draw.borrow_mut().on_draw(cairo_context);

            wrapped_instance_for_draw.borrow().drawing_area.queue_draw();
            Inhibit(true)
        });

        let wrapped_state_for_sizealloc = wrapped_state.clone();
        wrapped_instance.borrow().drawing_area.connect_size_allocate(move |widget, rect| {
            wrapped_state_for_sizealloc.borrow_mut().on_size_allocate(rect);
        });

        wrapped_instance.borrow().window.show_all();

        return wrapped_instance;
    }

    fn exit_on_close(&self) {
        self.window.connect_delete_event(|_, _| {
            gtk::main_quit();
            Inhibit(true)
        });
    }
}


fn main() {
    gtk::init().unwrap_or_else(|_| panic!("Failed to initialize GTK."));
    println!("Major: {}, Minor: {}", gtk::get_major_version(), gtk::get_minor_version());

    let wrapped_window = RenderingAPITestWindow::new(800, 500);
    wrapped_window.borrow().exit_on_close();
    gtk::main();
}

虽然上面的代码可以按要求工作，但我想找到一个更好的方法为了前进；我想问问是否有人知道更好的方法以上使编程过程变得相当复杂，需要使用 Rc<RefCell<...>>并拆分结构以满足 Rust 的借用规则。

Answer 1

这是我想出的一个工作版本:

#![cfg_attr(not(feature = "gtk_3_10"), allow(unused_variables, unused_mut))]

extern crate gtk;
extern crate cairo;

use std::rc::Rc;
use std::cell::RefCell;
use gtk::traits::*;
use gtk::signal::Inhibit;
use cairo::{Context, RectangleInt};


struct RenderingAPITestWindow {
    window: gtk::Window,
    drawing_area: gtk::DrawingArea,
    state: RefCell<RenderingState>,
}

struct RenderingState {
    width: i32,
    height: i32,
}

impl RenderingAPITestWindow {
    fn new(width: i32, height: i32) -> Rc<RenderingAPITestWindow> {
        let window = gtk::Window::new(gtk::WindowType::TopLevel).unwrap();
        let drawing_area = gtk::DrawingArea::new().unwrap();
        drawing_area.set_size_request(width, height);
        window.set_title("Cairo API test");
        window.add(&drawing_area);

        let instance = Rc::new(RenderingAPITestWindow {
            window: window,
            drawing_area: drawing_area,
            state: RefCell::new(RenderingState {
                width: width,
                height: height,
            }),
        });

        {
            let instance2 = instance.clone();
            instance.drawing_area.connect_draw(move |widget, cairo_context| {
                instance2.state.borrow().on_draw(cairo_context);
                instance2.drawing_area.queue_draw();
                Inhibit(true)
            });
        }
        {
            let instance2 = instance.clone();
            instance.drawing_area.connect_size_allocate(move |widget, rect| {
                instance2.state.borrow_mut().on_size_allocate(rect);
            });
        }
        instance.window.show_all();
        instance
    }

    fn exit_on_close(&self) {
        self.window.connect_delete_event(|_, _| {
            gtk::main_quit();
            Inhibit(true)
        });
    }
}

impl RenderingState {
    fn on_draw(&self, cairo_ctx: Context) {
        cairo_ctx.save();
        cairo_ctx.move_to(50.0, (self.height as f64) * 0.5);
        cairo_ctx.set_font_size(18.0);
        cairo_ctx.show_text("The only curse they could afford to put on a tomb these days was 'Bugger Off'. --PTerry");
        cairo_ctx.restore();
    }

    fn on_size_allocate(&mut self, rect: &RectangleInt) {
        self.width = rect.width as i32;
        self.height = rect.height as i32;
    }
}

fn main() {
    gtk::init().unwrap_or_else(|_| panic!("Failed to initialize GTK."));
    println!("Major: {}, Minor: {}", gtk::get_major_version(), gtk::get_minor_version());

    let window = RenderingAPITestWindow::new(800, 500);
    window.exit_on_close();
    gtk::main();
}

我通过一些观察得出了这个结论:

• 实例正在多个闭包中共享，持续时间不确定。 Rc 是该场景的正确答案，因为它提供共享所有权。 Rc 使用起来非常符合人体工学；它像任何其他指针类型一样工作。
• instance 中唯一实际发生变化的部分是您的状态。由于您的实例正在共享，因此无法使用标准 &mut 指针可变地借用它。因此，您必须使用内部可变性。这就是 RefCell 提供的。但请注意，您只需要在要改变的状态上使用 RefCell。所以这仍然将状态分离到一个单独的结构中，但它在 IMO 中运行良好。
• 对此代码的可能修改是将 #[derive(Clone, Copy)] 添加到 RenderingState 结构的定义中。因为它可以是Copy(因为它所有的组件类型都是Copy)，所以你可以使用Cell而不是 RefCell。