Write documentation for iced_pure

src/pure.rs

@@ -14,6 +14,84 @@
 //! offers an alternate [`Application`] trait with a completely pure `view`
 //! method.
 //!
+//! # The Elm Architecture, purity, and continuity
+//! As you may know, applications made with `iced` use [The Elm Architecture].
+//!
+//! In a nutshell, this architecture defines the initial state of the application, a way to `view` it, and a way to `update` it after a user interaction. The `update` logic is called after a meaningful user interaction, which in turn updates the state of the application. Then, the `view` logic is executed to redisplay the application.
+//!
+//! Since `view` logic is only run after an `update`, all of the mutations to the application state must only happen in the `update` logic. If the application state changes anywhere else, the `view` logic will not be rerun and, therefore, the previously generated `view` may stay outdated.
+//!
+//! However, the `Application` trait in `iced` defines `view` as:
+//!
+//! ```ignore
+//! pub trait Application {
+//!     fn view(&mut self) -> Element<Self::Message>;
+//! }
+//! ```
+//!
+//! As a consequence, the application state can be mutated in `view` logic. The `view` logic in `iced` is __impure__.
+//!
+//! This impurity is necessary because `iced` puts the burden of widget __continuity__ on its users. In other words, it's up to you to provide `iced` with the internal state of each widget every time `view` is called.
+//!
+//! If we take a look at the classic `counter` example:
+//!
+//! ```ignore
+//! struct Counter {
+//!     value: i32,
+//!     increment_button: button::State,
+//!     decrement_button: button::State,
+//! }
+//!
+//! // ...
+//!
+//! impl Counter {
+//!     pub fn view(&mut self) -> Column<Message> {
+//!         Column::new()
+//!             .push(
+//!                 Button::new(&mut self.increment_button, Text::new("+"))
+//!                     .on_press(Message::IncrementPressed),
+//!             )
+//!             .push(Text::new(self.value.to_string()).size(50))
+//!             .push(
+//!                 Button::new(&mut self.decrement_button, Text::new("-"))
+//!                     .on_press(Message::DecrementPressed),
+//!             )
+//!     }
+//! }
+//! ```
+//!
+//! We can see how we need to keep track of the `button::State` of each `Button` in our `Counter` state and provide a mutable reference to the widgets in our `view` logic. The widgets produced by `view` are __stateful__.
+//!
+//! While this approach forces users to keep track of widget state and causes impurity, I originally chose it because it allows `iced` to directly consume the widget tree produced by `view`. Since there is no internal state decoupled from `view` maintained by the runtime, `iced` does not need to compare (e.g. reconciliate) widget trees in order to ensure continuity.
+//!
+//! # Stateless widgets
+//! As the library matures, the need for some kind of persistent widget data (see #553) between `view` calls becomes more apparent (e.g. incremental rendering, animations, accessibility, etc.).
+//!
+//! If we are going to end up having persistent widget data anyways... There is no reason to have impure, stateful widgets anymore!
+//!
+//! With the help of this module, we can now write a pure `counter` example:
+//!
+//! ```ignore
+//! struct Counter {
+//!     value: i32,
+//! }
+//!
+//! // ...
+//!
+//! impl Counter {
+//!     fn view(&self) -> Column<Message> {
+//!         Column::new()
+//!             .push(Button::new("Increment").on_press(Message::IncrementPressed))
+//!             .push(Text::new(self.value.to_string()).size(50))
+//!             .push(Button::new("Decrement").on_press(Message::DecrementPressed))
+//!     }
+//! }
+//! ```
+//!
+//! Notice how we no longer need to keep track of the `button::State`! The widgets in `iced_pure` do not take any mutable application state in `view`. They are __stateless__ widgets. As a consequence, we do not need mutable access to `self` in `view` anymore. `view` becomes __pure__.
+//!
+//! [The Elm Architecture]: https://guide.elm-lang.org/architecture/
+//!
 //! [the original widgets]: crate::widget
 //! [`button::State`]: crate::widget::button::State
 //! [impure `Application`]: crate::Application