如何消除这段Rust代码中的重复逻辑？

我有一段可正常运行但存在重复逻辑的Rust GTK代码：

fn append_column(tree: &TreeView, attribute: &str, id: i32) {
    let column = TreeViewColumn::new();
    match attribute {
        "text" => {
            let cell = CellRendererText::new();    
            TreeViewColumnExt::pack_start(&column, &cell, true);
            TreeViewColumnExt::add_attribute(&column, &cell, attribute, id);
        }
        "pixbuf" => {
            let cell = CellRendererPixbuf::new();
            TreeViewColumnExt::pack_start(&column, &cell, true);
            TreeViewColumnExt::add_attribute(&column, &cell, attribute, id);
        }
        _ => { panic!("Unexpected attribute") }
    }
    tree.append_column(&column);
}

我尝试用downcast()简化代码，结果运行报错：

fn append_column(tree: &TreeView, attribute: &str, id: i32) {
    let column = TreeViewColumn::new();
    let cell : CellRenderer;
    match attribute {
        "text" => { cell = CellRendererText::new().downcast::<CellRenderer>().unwrap() }
        "pixbuf" => { cell = CellRendererPixbuf::new().downcast::<CellRenderer>().unwrap() }
        _ => { panic!("Unexpected attribute") }
    }
    TreeViewColumnExt::pack_start(&column, &cell, true);
    TreeViewColumnExt::add_attribute(&column, &cell, attribute, id);
    tree.append_column(&column);
}

错误信息：

error[E0277]: the trait bound `CellRenderer: IsA<CellRendererText>` is not satisfied
   --> src/main.rs:23:52
    |
23  |         "text" => { cell = CellRendererText::new().downcast::<CellRenderer>().unwrap() }
    |                                                    ^^^^^^^^ the trait `IsA<CellRendererText>` is not implemented for `CellRenderer`

请问该怎么消除这段代码里的重复逻辑？

解决方法：利用AsRef<CellRenderer>特性 #

GTK的CellRendererText和CellRendererPixbuf都实现了AsRef<CellRenderer>，不需要用downcast，可以通过提取通用逻辑来消除重复：

方案1：用闭包复用处理逻辑

use gtk::{CellRenderer, CellRendererPixbuf, CellRendererText, TreeView, TreeViewColumn, TreeViewColumnExt};

fn append_column(tree: &TreeView, attribute: &str, id: i32) {
    let column = TreeViewColumn::new();
    
    // 把重复的单元格操作封装成闭包
    let setup_cell = |cell: &impl AsRef<CellRenderer>| {
        TreeViewColumnExt::pack_start(&column, cell.as_ref(), true);
        TreeViewColumnExt::add_attribute(&column, cell.as_ref(), attribute, id);
    };

    match attribute {
        "text" => setup_cell(&CellRendererText::new()),
        "pixbuf" => setup_cell(&CellRendererPixbuf::new()),
        _ => panic!("Unexpected attribute"),
    }

    tree.append_column(&column);
}

方案2：封装单元格创建逻辑

use gtk::{CellRenderer, CellRendererPixbuf, CellRendererText, TreeView, TreeViewColumn, TreeViewColumnExt};

// 单独封装单元格创建函数，返回实现AsRef<CellRenderer>的类型
fn create_cell(attribute: &str) -> Box<dyn AsRef<CellRenderer>> {
    match attribute {
        "text" => Box::new(CellRendererText::new()),
        "pixbuf" => Box::new(CellRendererPixbuf::new()),
        _ => panic!("Unexpected attribute"),
    }
}

fn append_column(tree: &TreeView, attribute: &str, id: i32) {
    let column = TreeViewColumn::new();
    let cell = create_cell(attribute);
    
    TreeViewColumnExt::pack_start(&column, cell.as_ref(), true);
    TreeViewColumnExt::add_attribute(&column, cell.as_ref(), attribute, id);
    
    tree.append_column(&column);
}

原代码报错原因 #

downcast是用于将父类型向下转换为子类型的方法，而你需要的是把子类型（CellRendererText）向上转换为父类型（CellRenderer）——这一步GTK已经通过IsA trait自动支持，直接传递子类型引用即可，完全不需要调用downcast。

内容的提问来源于stack exchange，提问作者DobbyTheElf