Use an object inside a closure which is passed to a method of that object

[alexzander]

i have a struct Screen with its implementation

pub struct Screen {
    stdin:  Stdin,
    stdout: MouseStdout,
}

impl Screen {
   // ...
    pub fn handle_keys_loop<F: FnMut(&Event) -> ()>(
        &self,
        mut handler: F,
    ) {
        let stdin = stdin();
        for e in stdin.events() {
            let e = e.unwrap();
            match e {
                Event::Key(Key::Ctrl('c')) => break,
                _ => {
                    handler(&e);
                },
            }

        }
    }
}

and usage (which is wrong and know it)

    let mut screen = Screen::new();
    screen.init_screen();

    screen.handle_keys_loop(|event| {
        match event {
            Event::Key(Key::Char('a')) => {
                screen.println("hello there",15, 1, true);
            },
            _ => {}
        }
    });

    screen.end_screen();

the error is

error[E0502]: cannot borrow `screen` as mutable because it is also borrowed as immutable
  --> src/bin/terminal.rs:74:29
   |
74 |       screen.handle_keys_loop(|event| {
   |       -      ---------------- ^^^^^^^ mutable borrow occurs here
   |       |      |
   |  _____|      immutable borrow later used by call
   | |
75 | |         match event {
76 | |             Event::Key(Key::Char('a')) => {
77 | |                 println!("{} {} {} {}", "do something with a", 15, 1, true);
78 | |                 // tried to borrow as mutable
79 | |                 screen.println("hello there",15, 1, true);
   | |                 ------ second borrow occurs due to use of `screen` in closure
...  |
82 | |         }
83 | |     });
   | |______- immutable borrow occurs here

and if i make self mut inside handle_keys_loop to get rid of cannot borrow screen as mutable because it is also borrowed as immutable

    pub fn handle_keys_loop<F: FnMut(&Event) -> ()>(
        + &mut self,
        - &self
....

i get this error

error[E0499]: cannot borrow `screen` as mutable more than once at a time
  --> src/bin/terminal.rs:74:29
   |
74 |       screen.handle_keys_loop(|event| {
   |       -      ---------------- ^^^^^^^ second mutable borrow occurs here
   |       |      |
   |  _____|      first borrow later used by call
   | |
75 | |         match event {
76 | |             Event::Key(Key::Char('a')) => {
77 | |                 screen.println("hello there",15, 1, true);
   | |                 ------ second borrow occurs due to use of `screen` in closure
...  |
80 | |         }
81 | |     });
   | |______- first mutable borrow occurs here

what im trying to do: use the method handle_keys_loop of screen and pass screen inside the closure, which is passed to handle_keys_loop. basically, to use screen inside of screen.

how do i achieve that ?

some people told me to use RefCell, but that didnt work out very well, i got BorrowError.

i will use any workaround to just use screen inside the closure which is passed to screen's method.

thanks in advance.

[quinedot]

You might be able to pass &mut self to the closure as well.

It's hard to tell from your example, as where you're not using self at all. If you don't need it, you could just make handle_keys_loop an associated function that doesn't take &self (or &mut self).

[alexzander]

yes, passing screen to closure is a solution.

    screen.handle_keys_loop(|event, screen| {
        match event {
            Event::Key(Key::Char('a')) => {
                screen.println("hello there",15, 1, true);
            },
            _ => {}
        }
    });

works like a charm