Trying to understand where and how to generate multiple Commands from 1 Event

[veniamin-ilmer]

I would like to listen to the keyboard CTRL+V event, then depending on what is in the clipboard, evoke multiple different updates.

I am confused how to do this. I do see both Subscription and Command have a batch method, but I am confused how to create several Commands.

Currently inside of the subscription method, I use subscription::events_with, where after I detect the CTRL+V, my 1 Message will be converted into 1 Command<Message>.

So a few questions:

1. Where should I read from the clipboard? Inside of the subscription method or the update method?
2. How do I create multiple Commands?

I see there is a Command::perform method for example, but I am confused about having to pass in a function into it.

Current code for reference:

fn subscription(&self) -> iced::Subscription<Message> {
  iced::subscription::events_with(|event, _status| {
    match event {
      iced::Event::Keyboard(iced::keyboard::Event::KeyReleased{modifiers: iced::keyboard::Modifiers::CTRL, key_code: iced::keyboard::KeyCode::V}) => Some(Message::Paste),
      _ => None,
    }
  }
}

fn update(&mut self, message: Message) -> Command<Message> {
  match message {
    Message::Paste => {
      return iced::clipboard::read::<Message>(|optional_data| {
        let mut commands = vec![];
        if let Some(data) = optional_data {
          for c in data.chars() {
            if c.is_numeric() {
              commands.push(Command::/*what goes here?*/(Message::Digit(c)));
            } else {
              commands.push(Command::/*what goes here?*/(Message::NonDigit(c)));
            }
          }
          return Command::batch(commands);
        }
        Message::Done
      });
    },
    Message::Done => (),
    Message::Digit(c) => { /*handle digit..*/ },
    Message::NonDigit(c) => { /*handle non digit..*/ },
  }
  Command::none()
}

[lufte]

The way I understand it there are no Commands involved here. An iced Command is an asynchronous action that may dispatch a Message as a result or not.

In here, you have a subscription which also may or may not dispatch Messages after each event, so you just need to deal with Messages. As far as I know you can only dispatch one at a time form a subscription, but that should be enough for any use case. For example, let's say you have 2 potential updates that you need to perform on your state depending on what was pasted to the clipboard; you could have 3 Messages to represent all combinations and dispatch only one of them:

1. Message::Thing1WasPasted
2. Message::Thing2WasPasted
3. Message::Things1And2WerePasted

You could also have just one Message with enough internal data for the update method to figure out what it needs to do.

Does that make sense?

[veniamin-ilmer]

Although your solution will work, it would mean that the window would just be updated once with the final result.

I specifically need to be able to update the window multiple times, as this would be in line with the application I am trying to port over.

For huge amounts of data, you should be able to see the GUI make some really quick updates on the screen.

Oh! I just realized... Could the messages come in out of order? If that's the case, can we force a view to be run multiple times during an update?

[lufte]

Ohh, I see. So it's all to achieve a sort of aesthetic animation of "things happening" or does it serve a functional goal? One way I imagine would work is:

1. Dispatch Message1 from your subscription.
2. update processes Message1, runs the first round of updates, and executes a Command which dispatches Message2. Maybe that Command sleeps for a few milliseconds to give the whole animation a perceivable duration.
3. update now processes Message2 and runs the second round of updates. Potentially repeat the whole thing more times to achieve the number of steps you want.

It does leave you with a really complex design only to be able to reproduce an animation. I don't know if Iced has support for real animations though.

can we force a view to be run multiple times during and update?

No I don't think that's possible as it doesn't align with the Elm architecture.

[veniamin-ilmer]

Perhaps you can help me out with something else then. If I try and save any data inside of the clipboard reading:

  Message::Paste => {
    return iced::clipboard::read::<Message>(|optional_data| {
      if let Some(data) = optional_data {
        for c in data.chars() {
          if c.is_numeric() {
            self.c = c;
          }
        }
      }
      Message::Done
    });
  }

I get this error:

error[E0594]: cannot assign to `*self.display`, as it is a captured variable in a `Fn` closure
  --> src\main.rs:97:17
   |
97 |                 self.c = c;
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot assign

error[E0521]: borrowed data escapes outside of method
   --> src\main.rs:93:16
    |
80  |     fn update(&mut self, message: Message) -> Command<Message> {
    |               ---------
    |               |
    |               `self` is a reference that is only valid in the method body
    |               let's call the lifetime of this reference `'1`
...
93  |           return iced::clipboard::read::<Message>(|optional_data| {
    |  ________________^
94  | |           if let Some(data) = optional_data {
95  | |             for c in data.chars() {
96  | |               if c.is_numeric() {
...   |
101 | |           Message::Done
102 | |         });
    | |          ^
    | |          |
    | |__________`self` escapes the method body here
    |            argument requires that `'1` must outlive `'static`

So I am trying to figure out, how am I supposed to store data from the clipboard?

[lufte]

Put all the data inside the message (use any of the Enum variants that can hold data within them) and read that data in the update method. You won't have access to your state during the asynchronous execution of a Command.

[veniamin-ilmer]

I see. Ok, this worked. Thank you for your help.