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命令模式

问题​

Rust 中命令模式如何实现？典型应用场景是什么？

答案​

命令模式将操作封装为对象，支持撤销/重做、队列化、日志记录。

实现：文本编辑器撤销/重做​

trait Command {
    fn execute(&mut self, doc: &mut String);
    fn undo(&mut self, doc: &mut String);
}

struct InsertText {
    position: usize,
    text: String,
}

impl Command for InsertText {
    fn execute(&mut self, doc: &mut String) {
        doc.insert_str(self.position, &self.text);
    }

    fn undo(&mut self, doc: &mut String) {
        let end = self.position + self.text.len();
        doc.replace_range(self.position..end, "");
    }
}

struct DeleteText {
    position: usize,
    length: usize,
    deleted: String, // 保存被删除的文本用于 undo
}

impl Command for DeleteText {
    fn execute(&mut self, doc: &mut String) {
        self.deleted = doc[self.position..self.position + self.length].to_string();
        doc.replace_range(self.position..self.position + self.length, "");
    }

    fn undo(&mut self, doc: &mut String) {
        doc.insert_str(self.position, &self.deleted);
    }
}

// 命令管理器
struct Editor {
    document: String,
    history: Vec<Box<dyn Command>>,
    redo_stack: Vec<Box<dyn Command>>,
}

impl Editor {
    fn execute(&mut self, mut cmd: Box<dyn Command>) {
        cmd.execute(&mut self.document);
        self.history.push(cmd);
        self.redo_stack.clear();
    }

    fn undo(&mut self) {
        if let Some(mut cmd) = self.history.pop() {
            cmd.undo(&mut self.document);
            self.redo_stack.push(cmd);
        }
    }

    fn redo(&mut self) {
        if let Some(mut cmd) = self.redo_stack.pop() {
            cmd.execute(&mut self.document);
            self.history.push(cmd);
        }
    }
}

常见面试问题​

Q1: 命令模式在哪些场景下有用？​

答案：

场景说明
撤销/重做编辑器、画板、表单
任务队列异步执行、延迟执行
宏命令批量操作
事务日志操作记录与回放

相关链接​