Command を Rust で

Command は、振る舞いに関するデザインパターンの一つで、リクエストや簡単な操作をオブジェクトに変換します。

変換により、コマンドの遅延実行や遠隔実行を可能にしたり、コマンドの履歴の保存を可能にしたりできます。

In Rust, a command instance should NOT hold a permanent reference to global context, instead the latter should be passed from top to down as a mutable parameter of the "execute" method:

fn execute(&mut self, app: &mut cursive::Cursive) -> bool;

Text Editor: Commands and Undo

Key points:

Each button runs a separate command.
Because a command is represented as an object, it can be pushed into a history array in order to be undone later.
TUI is created with cursive crate.

command.rs: Command Inteface

mod copy;
mod cut;
mod paste;

pub use copy::CopyCommand;
pub use cut::CutCommand;
pub use paste::PasteCommand;

/// Declares a method for executing (and undoing) a command.
///
/// Each command receives an application context to access
/// visual components (e.g. edit view) and a clipboard.
pub trait Command {
    fn execute(&mut self, app: &mut cursive::Cursive) -> bool;
    fn undo(&mut self, app: &mut cursive::Cursive);
}

command/copy.rs: Copy Command

use cursive::{views::EditView, Cursive};

use super::Command;
use crate::AppContext;

#[derive(Default)]
pub struct CopyCommand;

impl Command for CopyCommand {
    fn execute(&mut self, app: &mut Cursive) -> bool {
        let editor = app.find_name::<EditView>("Editor").unwrap();
        let mut context = app.take_user_data::<AppContext>().unwrap();

        context.clipboard = editor.get_content().to_string();

        app.set_user_data(context);
        false
    }

    fn undo(&mut self, _: &mut Cursive) {}
}

command/cut.rs: Cut Command

use cursive::{views::EditView, Cursive};

use super::Command;
use crate::AppContext;

#[derive(Default)]
pub struct CutCommand {
    backup: String,
}

impl Command for CutCommand {
    fn execute(&mut self, app: &mut Cursive) -> bool {
        let mut editor = app.find_name::<EditView>("Editor").unwrap();

        app.with_user_data(|context: &mut AppContext| {
            self.backup = editor.get_content().to_string();
            context.clipboard = self.backup.clone();
            editor.set_content("".to_string());
        });

        true
    }

    fn undo(&mut self, app: &mut Cursive) {
        let mut editor = app.find_name::<EditView>("Editor").unwrap();
        editor.set_content(&self.backup);
    }
}

command/paste.rs: Paste Command

use cursive::{views::EditView, Cursive};

use super::Command;
use crate::AppContext;

#[derive(Default)]
pub struct PasteCommand {
    backup: String,
}

impl Command for PasteCommand {
    fn execute(&mut self, app: &mut Cursive) -> bool {
        let mut editor = app.find_name::<EditView>("Editor").unwrap();

        app.with_user_data(|context: &mut AppContext| {
            self.backup = editor.get_content().to_string();
            editor.set_content(context.clipboard.clone());
        });

        true
    }

    fn undo(&mut self, app: &mut Cursive) {
        let mut editor = app.find_name::<EditView>("Editor").unwrap();
        editor.set_content(&self.backup);
    }
}

main.rs: Client code

mod command;

use cursive::{
    traits::Nameable,
    views::{Dialog, EditView},
    Cursive,
};

use command::{Command, CopyCommand, CutCommand, PasteCommand};

/// An application context to be passed into visual component callbacks.
/// It contains a clipboard and a history of commands to be undone.
#[derive(Default)]
struct AppContext {
    clipboard: String,
    history: Vec<Box<dyn Command>>,
}

fn main() {
    let mut app = cursive::default();

    app.set_user_data(AppContext::default());
    app.add_layer(
        Dialog::around(EditView::default().with_name("Editor"))
            .title("Type and use buttons")
            .button("Copy", |s| execute(s, CopyCommand))
            .button("Cut", |s| execute(s, CutCommand::default()))
            .button("Paste", |s| execute(s, PasteCommand::default()))
            .button("Undo", undo)
            .button("Quit", |s| s.quit()),
    );

    app.run();
}

/// Executes a command and then pushes it to a history array.
fn execute(app: &mut Cursive, mut command: impl Command + 'static) {
    if command.execute(app) {
        app.with_user_data(|context: &mut AppContext| {
            context.history.push(Box::new(command));
        });
    }
}

/// Pops the last command and executes an undo action.
fn undo(app: &mut Cursive) {
    let mut context = app.take_user_data::<AppContext>().unwrap();
    if let Some(mut command) = context.history.pop() {
        command.undo(app)
    }
    app.set_user_data(context);
}