message: Implement task addition in `todos` example
diff: use iced::{
  - text::HorizontalAlignment, text_input, Align, Application, Color, Column,
  - Element, Justify, Length, Text, TextInput,
  + scrollable, text::HorizontalAlignment, text_input, Align, Application,
  + Checkbox, Color, Column, Element, Length, Scrollable, Text, TextInput,
  };
  pub fn main() {
  Todos::default().run()
  }
  -#[derive(Default)]
  +#[derive(Debug, Default)]
  struct Todos {
  + scroll: scrollable::State,
  input: text_input::State,
  input_value: String,
  + tasks: Vec<Task>,
  +}
  +
  +#[derive(Debug)]
  +struct Task {
  + description: String,
  + completed: bool,
  +}
  +
  +impl Task {
  + fn new(description: String) -> Self {
  + Task {
  + description,
  + completed: false,
  + }
  + }
  +
  + fn view(&mut self) -> Element<bool> {
  + Checkbox::new(self.completed, &self.description, |checked| checked)
  + .into()
  + }
  }
  #[derive(Debug, Clone)]
  pub enum Message {
  InputChanged(String),
  CreateTask,
  + TaskChanged(usize, bool),
  }
  impl Application for Todos {
  @@ -27,9 +50,21 @@ impl Application for Todos {
  Message::InputChanged(value) => {
  self.input_value = value;
  }
  - Message::CreateTask => {}
  + Message::CreateTask => {
  + if !self.input_value.is_empty() {
  + self.tasks.push(Task::new(self.input_value.clone()));
  + self.input_value = String::new();
  }
  }
  + Message::TaskChanged(i, completed) => {
  + if let Some(task) = self.tasks.get_mut(i) {
  + task.completed = completed;
  + }
  + }
  + }
  +
  + dbg!(self);
  + }
  fn view(&mut self) -> Element<Message> {
  let title = Text::new("todos")
  @@ -47,15 +82,27 @@ impl Application for Todos {
  .size(30)
  .on_submit(Message::CreateTask);
  - Column::new()
  + let tasks = self.tasks.iter_mut().enumerate().fold(
  + Column::new().spacing(20),
  + |column, (i, task)| {
  + column.push(
  + task.view()
  + .map(move |state| Message::TaskChanged(i, state)),
  + )
  + },
  + );
  +
  + let content = Column::new()
  .max_width(Length::Units(800))
  - .height(Length::Fill)
  .align_self(Align::Center)
  - .justify_content(Justify::Center)
  .spacing(20)
  - .padding(20)
  .push(title)
  .push(input)
  + .push(tasks);
  +
  + Scrollable::new(&mut self.scroll)
  + .padding(40)
  + .push(content)
  .into()
  }
  }
  