Expose `iced_pure` through a `pure` feature in `iced`

Besides exposing the `iced_pure` crate, enabling the `pure` feature also
provides pure versions of both the `Application` and `Sandbox` traits!
:tada:

2 files changed, 301 insertions, 0 deletions

diff --git a/src/pure/application.rs b/src/pure/application.rs
new file mode 100644
index 00000000..973af9f7
--- /dev/null
+++ b/src/pure/application.rs
@@ -0,0 +1,182 @@
+use crate::pure::{self, Pure};
+use crate::window;
+use crate::{Color, Command, Executor, Settings, Subscription};
+
+/// A pure version of [`Application`].
+///
+/// Unlike the impure version, the `view` method of this trait takes an
+/// immutable reference to `self` and returns a pure [`Element`].
+///
+/// [`Application`]: crate::Application
+/// [`Element`]: pure::Element
+pub trait Application: Sized {
+    /// The [`Executor`] that will run commands and subscriptions.
+    ///
+    /// The [default executor] can be a good starting point!
+    ///
+    /// [`Executor`]: Self::Executor
+    /// [default executor]: crate::executor::Default
+    type Executor: Executor;
+
+    /// The type of __messages__ your [`Application`] will produce.
+    type Message: std::fmt::Debug + Send;
+
+    /// The data needed to initialize your [`Application`].
+    type Flags;
+
+    /// Initializes the [`Application`] with the flags provided to
+    /// [`run`] as part of the [`Settings`].
+    ///
+    /// Here is where you should return the initial state of your app.
+    ///
+    /// Additionally, you can return a [`Command`] if you need to perform some
+    /// async action in the background on startup. This is useful if you want to
+    /// load state from a file, perform an initial HTTP request, etc.
+    ///
+    /// [`run`]: Self::run
+    fn new(flags: Self::Flags) -> (Self, Command<Self::Message>);
+
+    /// Returns the current title of the [`Application`].
+    ///
+    /// This title can be dynamic! The runtime will automatically update the
+    /// title of your application when necessary.
+    fn title(&self) -> String;
+
+    /// Handles a __message__ and updates the state of the [`Application`].
+    ///
+    /// This is where you define your __update logic__. All the __messages__,
+    /// produced by either user interactions or commands, will be handled by
+    /// this method.
+    ///
+    /// Any [`Command`] returned will be executed immediately in the background.
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message>;
+
+    /// Returns the event [`Subscription`] for the current state of the
+    /// application.
+    ///
+    /// A [`Subscription`] will be kept alive as long as you keep returning it,
+    /// and the __messages__ produced will be handled by
+    /// [`update`](#tymethod.update).
+    ///
+    /// By default, this method returns an empty [`Subscription`].
+    fn subscription(&self) -> Subscription<Self::Message> {
+        Subscription::none()
+    }
+
+    /// Returns the widgets to display in the [`Application`].
+    ///
+    /// These widgets can produce __messages__ based on user interaction.
+    fn view(&self) -> pure::Element<Self::Message>;
+
+    /// Returns the current [`Application`] mode.
+    ///
+    /// The runtime will automatically transition your application if a new mode
+    /// is returned.
+    ///
+    /// Currently, the mode only has an effect in native platforms.
+    ///
+    /// By default, an application will run in windowed mode.
+    fn mode(&self) -> window::Mode {
+        window::Mode::Windowed
+    }
+
+    /// Returns the background color of the [`Application`].
+    ///
+    /// By default, it returns [`Color::WHITE`].
+    fn background_color(&self) -> Color {
+        Color::WHITE
+    }
+
+    /// Returns the scale factor of the [`Application`].
+    ///
+    /// It can be used to dynamically control the size of the UI at runtime
+    /// (i.e. zooming).
+    ///
+    /// For instance, a scale factor of `2.0` will make widgets twice as big,
+    /// while a scale factor of `0.5` will shrink them to half their size.
+    ///
+    /// By default, it returns `1.0`.
+    fn scale_factor(&self) -> f64 {
+        1.0
+    }
+
+    /// Returns whether the [`Application`] should be terminated.
+    ///
+    /// By default, it returns `false`.
+    fn should_exit(&self) -> bool {
+        false
+    }
+
+    /// Runs the [`Application`].
+    ///
+    /// On native platforms, this method will take control of the current thread
+    /// until the [`Application`] exits.
+    ///
+    /// On the web platform, this method __will NOT return__ unless there is an
+    /// [`Error`] during startup.
+    ///
+    /// [`Error`]: crate::Error
+    fn run(settings: Settings<Self::Flags>) -> crate::Result
+    where
+        Self: 'static,
+    {
+        <Instance<Self> as crate::Application>::run(settings)
+    }
+}
+
+struct Instance<A: Application> {
+    application: A,
+    state: pure::State<A::Message, crate::Renderer>,
+}
+
+impl<A> crate::Application for Instance<A>
+where
+    A: Application,
+    A::Message: 'static,
+{
+    type Executor = A::Executor;
+    type Message = A::Message;
+    type Flags = A::Flags;
+
+    fn new(flags: Self::Flags) -> (Self, Command<Self::Message>) {
+        let (application, command) = A::new(flags);
+
+        (
+            Instance {
+                application,
+                state: pure::State::new(),
+            },
+            command,
+        )
+    }
+
+    fn title(&self) -> String {
+        A::title(&self.application)
+    }
+
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message> {
+        A::update(&mut self.application, message)
+    }
+
+    fn view(&mut self) -> crate::Element<'_, Self::Message> {
+        let content = A::view(&self.application);
+
+        Pure::new(&mut self.state, content).into()
+    }
+
+    fn mode(&self) -> window::Mode {
+        A::mode(&self.application)
+    }
+
+    fn background_color(&self) -> Color {
+        A::background_color(&self.application)
+    }
+
+    fn scale_factor(&self) -> f64 {
+        A::scale_factor(&self.application)
+    }
+
+    fn should_exit(&self) -> bool {
+        A::should_exit(&self.application)
+    }
+}
diff --git a/src/pure/sandbox.rs b/src/pure/sandbox.rs
new file mode 100644
index 00000000..def90b6b
--- /dev/null
+++ b/src/pure/sandbox.rs
@@ -0,0 +1,119 @@
+use crate::pure;
+use crate::{Color, Command, Error, Settings, Subscription};
+
+/// A pure version of [`Sandbox`].
+///
+/// Unlike the impure version, the `view` method of this trait takes an
+/// immutable reference to `self` and returns a pure [`Element`].
+///
+/// [`Sandbox`]: crate::Sandbox
+/// [`Element`]: pure::Element
+pub trait Sandbox {
+    /// The type of __messages__ your [`Sandbox`] will produce.
+    type Message: std::fmt::Debug + Send;
+
+    /// Initializes the [`Sandbox`].
+    ///
+    /// Here is where you should return the initial state of your app.
+    fn new() -> Self;
+
+    /// Returns the current title of the [`Sandbox`].
+    ///
+    /// This title can be dynamic! The runtime will automatically update the
+    /// title of your application when necessary.
+    fn title(&self) -> String;
+
+    /// Handles a __message__ and updates the state of the [`Sandbox`].
+    ///
+    /// This is where you define your __update logic__. All the __messages__,
+    /// produced by user interactions, will be handled by this method.
+    fn update(&mut self, message: Self::Message);
+
+    /// Returns the widgets to display in the [`Sandbox`].
+    ///
+    /// These widgets can produce __messages__ based on user interaction.
+    fn view(&self) -> pure::Element<Self::Message>;
+
+    /// Returns the background color of the [`Sandbox`].
+    ///
+    /// By default, it returns [`Color::WHITE`].
+    fn background_color(&self) -> Color {
+        Color::WHITE
+    }
+
+    /// Returns the scale factor of the [`Sandbox`].
+    ///
+    /// It can be used to dynamically control the size of the UI at runtime
+    /// (i.e. zooming).
+    ///
+    /// For instance, a scale factor of `2.0` will make widgets twice as big,
+    /// while a scale factor of `0.5` will shrink them to half their size.
+    ///
+    /// By default, it returns `1.0`.
+    fn scale_factor(&self) -> f64 {
+        1.0
+    }
+
+    /// Returns whether the [`Sandbox`] should be terminated.
+    ///
+    /// By default, it returns `false`.
+    fn should_exit(&self) -> bool {
+        false
+    }
+
+    /// Runs the [`Sandbox`].
+    ///
+    /// On native platforms, this method will take control of the current thread
+    /// and __will NOT return__.
+    ///
+    /// It should probably be that last thing you call in your `main` function.
+    fn run(settings: Settings<()>) -> Result<(), Error>
+    where
+        Self: 'static + Sized,
+    {
+        <Self as pure::Application>::run(settings)
+    }
+}
+
+impl<T> pure::Application for T
+where
+    T: Sandbox,
+{
+    type Executor = iced_futures::backend::null::Executor;
+    type Flags = ();
+    type Message = T::Message;
+
+    fn new(_flags: ()) -> (Self, Command<T::Message>) {
+        (T::new(), Command::none())
+    }
+
+    fn title(&self) -> String {
+        T::title(self)
+    }
+
+    fn update(&mut self, message: T::Message) -> Command<T::Message> {
+        T::update(self, message);
+
+        Command::none()
+    }
+
+    fn subscription(&self) -> Subscription<T::Message> {
+        Subscription::none()
+    }
+
+    fn view(&self) -> pure::Element<T::Message> {
+        T::view(self)
+    }
+
+    fn background_color(&self) -> Color {
+        T::background_color(self)
+    }
+
+    fn scale_factor(&self) -> f64 {
+        T::scale_factor(self)
+    }
+
+    fn should_exit(&self) -> bool {
+        T::should_exit(self)
+    }
+}