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

Question

i have a struct Screen with its implementation

pub struct Screen {
    stdin:  Stdin,
// types are irrelevant
    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.

Dogbert · Accepted Answer · 2022-04-26 18:36:22Z

2

One pattern I use to handle such a situation is to pass self: &mut Self back into the closure from handle and use that inside the closure. A simplified version of your code:

struct Screen {}

struct Event {}

impl Screen {
    fn handle<F: FnMut(&mut Screen, Event)>(&mut self, mut handler: F) {
        handler(self, Event {})
    }

    fn print(&mut self) {}
}

fn main() {
    let mut screen = Screen {};
    screen.handle(|screen, _event| screen.print());
    screen.handle(|screen, _event| screen.print());
}

answered Apr 26, 2022 at 18:36

Dogbert

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1 Comment

alexzander Over a year ago

yes. that works very well.

cdhowie · Accepted Answer · 2022-04-26 18:36:18Z

1

You can't do what you're trying to do because it violates Rust's rule that you can't mutate a value while something else has access to it.

However, your handle_keys_loop method doesn't even use self which means the &self parameter is redundant. There's no reason to give a function an argument it's not going to use (except when implementing a trait that requires it).

Just remove the argument:

    pub fn handle_keys_loop<F: FnMut(&Event) -> ()>(
        mut handler: F,
    ) {

And call it as Screen::handle_keys_loop(|event| { ... }).

Alternatively, make it a free function, external to Screen entirely, since it doesn't depend on Screen in any way.

answered Apr 26, 2022 at 18:36

cdhowie

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1 Comment

alexzander Over a year ago

true. it doesnt use self. it was, but i made some modifications. i would like to keep the self because maybe i will need it in the future, for example to call some other functions of screen on other branches of the match

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