callback - HOWTO：使用 gtk (rust-gnome) 回调的惯用 Rust

Question

我目前正在学习 Rust，并希望将其用于使用 GTK+ 开发基于 GUI 的应用程序。我的问题涉及注册回调以响应 GTK 事件/信号和这些回调中的变异状态。我有一个可行但不优雅的解决方案，所以我想问一下是否有更清洁、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，其中结构维护对 GTK 小部件的引用以及它需要的其他状态。它构造了一个传递给 GtkWidget::connect*函数的闭包，以便接收事件、绘制到画布等。这可能会导致借用检查器出现问题，我现在将解释。我将展示一些工作但（恕我直言）不理想的代码。

初始，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();
}

上面的代码编译失败，因为闭包 RenderingAPITestWindow::new被创建并传递给对 GtkWidget::connect*方法的调用尝试借用instance。编译器指出，闭包可能比声明它们的函数寿命更长，并且instance由外部函数拥有，因此存在问题。鉴于 GTK 可能会在未指定的时间内保留对这些闭包的引用，我们需要一种可以在运行时确定生命周期的方法，因此我接下来要解决RenderingAPITestWindow实例被包装在 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();
}

上面的解决方案可以编译，但不是特别漂亮：

RenderingAPITestWindow::new返回一个 Rc<RefCell<RenderingAPITestWindow>>而不是一个 RenderingAPITestWindow
访问字段和方法RenderingAPITestWindow很复杂，因为Rc<RefCell<...>>必须打开；它现在需要 wrapped_instance.borrow().some_method(...)而不仅仅是 instance.some_method(...)
每个闭包都需要它自己的 ; 的克隆wrapped_instance。尝试使用wrapped_instance将尝试借用一个对象 - 包装器而不是RenderingAPITestWindow这次 -RenderingAPITestWindow::new像以前一样拥有

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

这是因为调用window.show_all()导致 GTK 初始化小部件层次结构，导致绘图区域小部件接收size-allocate事件。访问要调用的窗口 show_all()需要Rc<RefCell<...>>打开（因此 wrapped_instance.borrow().window.show_all();）并借用实例。在借用结束show_all()返回之前，GTK 调用绘图区域的size-allocate事件处理程序，这会导致调用与其相连的闭包（上面的 4 行）。闭包尝试借用对RenderingAPITestWindow实例 ( wrapped_instance_for_sizealloc.borrow_mut().on_size_allocate(rect);) 的可变引用以调用该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。第二次借用导致运行时恐慌。

到目前为止，我设法开始工作的工作RenderingAPITestWindow但 - 恕我直言 - 不优雅的解决方案是拆分为两个结构，将由回调修改的可变状态移动到一个单独的结构中。

RenderingAPITestWindow拆分结构的工作但不优雅的解决方案：

#![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，但我想找到一种更好的前进方式；我想问一下是否有人知道更好的方法，因为上述方法使编程过程相当复杂，需要使用Rc<RefCell<...>>和拆分结构来满足 Rust 的借用规则。

score 13 · Accepted Answer

这是我想出的工作版本：

#![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一点：

该实例在多个闭包之间共享不确定的时间。Rc是该场景的正确答案，因为它提供了共享所有权。Rc使用起来非常符合人体工程学；它像任何其他指针类型一样工作。
唯一instance真正发生突变的部分是你的状态。由于您的实例是共享的，因此不能使用标准&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RefCell在您正在变异的状态上使用。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添加#[derive(Clone, Copy)]到RenderingState结构的定义中。既然可以Copy（因为它的所有组件类型都是Copy），您可以使用Cell代替RefCell.

callback - HOWTO：使用 gtk (rust-gnome) 回调的惯用 Rust

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