Merge pull request #62 from hecrj/feature/async-actions

Async actions

14 files changed, 432 insertions, 132 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 8f9769b2..8766d814 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -36,6 +36,9 @@ iced_web = { version = "0.1.0-alpha", path = "web" }
 
 [dev-dependencies]
 env_logger = "0.7"
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+directories = "2.0"
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 wasm-bindgen = "0.2.51"
diff --git a/core/Cargo.toml b/core/Cargo.toml
index a244bcba..f2492345 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -6,3 +6,10 @@ edition = "2018"
 description = "The essential concepts of Iced"
 license = "MIT"
 repository = "https://github.com/hecrj/iced"
+
+[features]
+# Exposes a future-based `Command` type
+command = ["futures"]
+
+[dependencies]
+futures = { version = "0.3", optional = true }
diff --git a/core/src/command.rs b/core/src/command.rs
new file mode 100644
index 00000000..e1c865dd
--- /dev/null
+++ b/core/src/command.rs
@@ -0,0 +1,49 @@
+use futures::future::{BoxFuture, Future, FutureExt};
+
+pub struct Command<T> {
+    futures: Vec<BoxFuture<'static, T>>,
+}
+
+impl<T> Command<T> {
+    pub fn none() -> Self {
+        Self {
+            futures: Vec::new(),
+        }
+    }
+
+    pub fn perform<A>(
+        future: impl Future<Output = T> + 'static + Send,
+        f: impl Fn(T) -> A + 'static + Send,
+    ) -> Command<A> {
+        Command {
+            futures: vec![future.map(f).boxed()],
+        }
+    }
+
+    pub fn batch(commands: impl Iterator<Item = Command<T>>) -> Self {
+        Self {
+            futures: commands.flat_map(|command| command.futures).collect(),
+        }
+    }
+
+    pub fn futures(self) -> Vec<BoxFuture<'static, T>> {
+        self.futures
+    }
+}
+
+impl<T, A> From<A> for Command<T>
+where
+    A: Future<Output = T> + 'static + Send,
+{
+    fn from(future: A) -> Self {
+        Self {
+            futures: vec![future.boxed()],
+        }
+    }
+}
+
+impl<T> std::fmt::Debug for Command<T> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Command").finish()
+    }
+}
diff --git a/core/src/lib.rs b/core/src/lib.rs
index b61f2eae..3816f8a2 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -3,6 +3,8 @@ pub mod widget;
 mod align;
 mod background;
 mod color;
+#[cfg(feature = "command")]
+mod command;
 mod font;
 mod length;
 mod point;
@@ -12,6 +14,8 @@ mod vector;
 pub use align::Align;
 pub use background::Background;
 pub use color::Color;
+#[cfg(feature = "command")]
+pub use command::Command;
 pub use font::Font;
 pub use length::Length;
 pub use point::Point;
diff --git a/core/src/widget/text_input.rs b/core/src/widget/text_input.rs
index 450a7cae..16c67954 100644
--- a/core/src/widget/text_input.rs
+++ b/core/src/widget/text_input.rs
@@ -80,7 +80,7 @@ where
     }
 }
 
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Clone)]
 pub struct State {
     pub is_focused: bool,
     cursor_position: usize,
diff --git a/examples/scroll.rs b/examples/scroll.rs
index 50701879..61ad2a53 100644
--- a/examples/scroll.rs
+++ b/examples/scroll.rs
@@ -1,12 +1,12 @@
 use iced::{
-    button, scrollable, Align, Application, Button, Container, Element, Image,
-    Length, Scrollable, Text,
+    button, scrollable, Align, Application, Button, Command, Container,
+    Element, Image, Length, Scrollable, Text,
 };
 
 pub fn main() {
     env_logger::init();
 
-    Example::default().run()
+    Example::run()
 }
 
 #[derive(Default)]
@@ -25,16 +25,22 @@ pub enum Message {
 impl Application for Example {
     type Message = Message;
 
+    fn new() -> (Example, Command<Message>) {
+        (Example::default(), Command::none())
+    }
+
     fn title(&self) -> String {
         String::from("Scroll - Iced")
     }
 
-    fn update(&mut self, message: Message) {
+    fn update(&mut self, message: Message) -> Command<Message> {
         match message {
             Message::AddItem => {
                 self.item_count += 1;
             }
         }
+
+        Command::none()
     }
 
     fn view(&mut self) -> Element<Message> {
diff --git a/examples/todos.rs b/examples/todos.rs
index f921a666..a1dfc5aa 100644
--- a/examples/todos.rs
+++ b/examples/todos.rs
@@ -1,25 +1,36 @@
 use iced::{
     button, scrollable, text::HorizontalAlignment, text_input, Align,
-    Application, Background, Button, Checkbox, Color, Column, Container,
-    Element, Font, Length, Row, Scrollable, Text, TextInput,
+    Application, Background, Button, Checkbox, Color, Column, Command,
+    Container, Element, Font, Length, Row, Scrollable, Text, TextInput,
 };
+use serde::{Deserialize, Serialize};
 
 pub fn main() {
-    Todos::default().run()
+    Todos::run()
+}
+
+#[derive(Debug)]
+enum Todos {
+    Loading,
+    Loaded(State),
 }
 
 #[derive(Debug, Default)]
-struct Todos {
+struct State {
     scroll: scrollable::State,
     input: text_input::State,
     input_value: String,
     filter: Filter,
     tasks: Vec<Task>,
     controls: Controls,
+    dirty: bool,
+    saving: bool,
 }
 
 #[derive(Debug, Clone)]
-pub enum Message {
+enum Message {
+    Loaded(Result<SavedState, LoadError>),
+    Saved(Result<(), SaveError>),
     InputChanged(String),
     CreateTask,
     FilterChanged(Filter),
@@ -29,108 +40,180 @@ pub enum Message {
 impl Application for Todos {
     type Message = Message;
 
+    fn new() -> (Todos, Command<Message>) {
+        (
+            Todos::Loading,
+            Command::perform(SavedState::load(), Message::Loaded),
+        )
+    }
+
     fn title(&self) -> String {
-        String::from("Todos - Iced")
+        let dirty = match self {
+            Todos::Loading => false,
+            Todos::Loaded(state) => state.dirty,
+        };
+
+        format!("Todos{} - Iced", if dirty { "*" } else { "" })
     }
 
-    fn update(&mut self, message: Message) {
-        match message {
-            Message::InputChanged(value) => {
-                self.input_value = value;
-            }
-            Message::CreateTask => {
-                if !self.input_value.is_empty() {
-                    self.tasks.push(Task::new(self.input_value.clone()));
-                    self.input_value.clear();
+    fn update(&mut self, message: Message) -> Command<Message> {
+        match self {
+            Todos::Loading => {
+                match message {
+                    Message::Loaded(Ok(state)) => {
+                        *self = Todos::Loaded(State {
+                            input_value: state.input_value,
+                            filter: state.filter,
+                            tasks: state.tasks,
+                            ..State::default()
+                        });
+                    }
+                    Message::Loaded(Err(_)) => {
+                        *self = Todos::Loaded(State::default());
+                    }
+                    _ => {}
                 }
+
+                Command::none()
             }
-            Message::FilterChanged(filter) => {
-                self.filter = filter;
-            }
-            Message::TaskMessage(i, TaskMessage::Delete) => {
-                self.tasks.remove(i);
-            }
-            Message::TaskMessage(i, task_message) => {
-                if let Some(task) = self.tasks.get_mut(i) {
-                    task.update(task_message);
+            Todos::Loaded(state) => {
+                let mut saved = false;
+
+                match message {
+                    Message::InputChanged(value) => {
+                        state.input_value = value;
+                    }
+                    Message::CreateTask => {
+                        if !state.input_value.is_empty() {
+                            state
+                                .tasks
+                                .push(Task::new(state.input_value.clone()));
+                            state.input_value.clear();
+                        }
+                    }
+                    Message::FilterChanged(filter) => {
+                        state.filter = filter;
+                    }
+                    Message::TaskMessage(i, TaskMessage::Delete) => {
+                        state.tasks.remove(i);
+                    }
+                    Message::TaskMessage(i, task_message) => {
+                        if let Some(task) = state.tasks.get_mut(i) {
+                            task.update(task_message);
+                        }
+                    }
+                    Message::Saved(_) => {
+                        state.saving = false;
+                        saved = true;
+                    }
+                    _ => {}
+                }
+
+                if !saved {
+                    state.dirty = true;
+                }
+
+                if state.dirty && !state.saving {
+                    state.dirty = false;
+                    state.saving = true;
+
+                    Command::perform(
+                        SavedState {
+                            input_value: state.input_value.clone(),
+                            filter: state.filter,
+                            tasks: state.tasks.clone(),
+                        }
+                        .save(),
+                        Message::Saved,
+                    )
+                } else {
+                    Command::none()
                 }
             }
         }
-
-        dbg!(self);
     }
 
     fn view(&mut self) -> Element<Message> {
-        let Todos {
-            scroll,
-            input,
-            input_value,
-            filter,
-            tasks,
-            controls,
-        } = self;
+        match self {
+            Todos::Loading => loading_message(),
+            Todos::Loaded(State {
+                scroll,
+                input,
+                input_value,
+                filter,
+                tasks,
+                controls,
+                ..
+            }) => {
+                let title = Text::new("todos")
+                    .size(100)
+                    .color([0.5, 0.5, 0.5])
+                    .horizontal_alignment(HorizontalAlignment::Center);
+
+                let input = TextInput::new(
+                    input,
+                    "What needs to be done?",
+                    input_value,
+                    Message::InputChanged,
+                )
+                .padding(15)
+                .size(30)
+                .on_submit(Message::CreateTask);
+
+                let controls = controls.view(&tasks, *filter);
+                let filtered_tasks =
+                    tasks.iter().filter(|task| filter.matches(task));
+
+                let tasks: Element<_> = if filtered_tasks.count() > 0 {
+                    tasks
+                        .iter_mut()
+                        .enumerate()
+                        .filter(|(_, task)| filter.matches(task))
+                        .fold(Column::new().spacing(20), |column, (i, task)| {
+                            column.push(task.view().map(move |message| {
+                                Message::TaskMessage(i, message)
+                            }))
+                        })
+                        .into()
+                } else {
+                    empty_message(match filter {
+                        Filter::All => "You have not created a task yet...",
+                        Filter::Active => "All your tasks are done! :D",
+                        Filter::Completed => {
+                            "You have not completed a task yet..."
+                        }
+                    })
+                };
 
-        let title = Text::new("todos")
-            .size(100)
-            .color([0.5, 0.5, 0.5])
-            .horizontal_alignment(HorizontalAlignment::Center);
+                let content = Column::new()
+                    .max_width(800)
+                    .spacing(20)
+                    .push(title)
+                    .push(input)
+                    .push(controls)
+                    .push(tasks);
 
-        let input = TextInput::new(
-            input,
-            "What needs to be done?",
-            input_value,
-            Message::InputChanged,
-        )
-        .padding(15)
-        .size(30)
-        .on_submit(Message::CreateTask);
-
-        let controls = controls.view(&tasks, *filter);
-        let filtered_tasks = tasks.iter().filter(|task| filter.matches(task));
-
-        let tasks: Element<_> =
-            if filtered_tasks.count() > 0 {
-                tasks
-                    .iter_mut()
-                    .enumerate()
-                    .filter(|(_, task)| filter.matches(task))
-                    .fold(Column::new().spacing(20), |column, (i, task)| {
-                        column.push(task.view().map(move |message| {
-                            Message::TaskMessage(i, message)
-                        }))
-                    })
+                Scrollable::new(scroll)
+                    .padding(40)
+                    .push(
+                        Container::new(content).width(Length::Fill).center_x(),
+                    )
                     .into()
-            } else {
-                empty_message(match filter {
-                    Filter::All => "You have not created a task yet...",
-                    Filter::Active => "All your tasks are done! :D",
-                    Filter::Completed => "You have not completed a task yet...",
-                })
-            };
-
-        let content = Column::new()
-            .max_width(800)
-            .spacing(20)
-            .push(title)
-            .push(input)
-            .push(controls)
-            .push(tasks);
-
-        Scrollable::new(scroll)
-            .padding(40)
-            .push(Container::new(content).width(Length::Fill).center_x())
-            .into()
+            }
+        }
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 struct Task {
     description: String,
     completed: bool,
+
+    #[serde(skip)]
     state: TaskState,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub enum TaskState {
     Idle {
         edit_button: button::State,
@@ -141,6 +224,14 @@ pub enum TaskState {
     },
 }
 
+impl Default for TaskState {
+    fn default() -> Self {
+        TaskState::Idle {
+            edit_button: button::State::new(),
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
 pub enum TaskMessage {
     Completed(bool),
@@ -249,7 +340,7 @@ impl Task {
     }
 }
 
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Clone)]
 pub struct Controls {
     all_button: button::State,
     active_button: button::State,
@@ -318,7 +409,7 @@ impl Controls {
     }
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 pub enum Filter {
     All,
     Active,
@@ -341,6 +432,18 @@ impl Filter {
     }
 }
 
+fn loading_message() -> Element<'static, Message> {
+    Container::new(
+        Text::new("Loading...")
+            .horizontal_alignment(HorizontalAlignment::Center)
+            .size(50),
+    )
+    .width(Length::Fill)
+    .height(Length::Fill)
+    .center_y()
+    .into()
+}
+
 fn empty_message(message: &str) -> Element<'static, Message> {
     Container::new(
         Text::new(message)
@@ -375,3 +478,80 @@ fn edit_icon() -> Text {
 fn delete_icon() -> Text {
     icon('\u{F1F8}')
 }
+
+// Persistence
+#[derive(Debug, Clone, Serialize, Deserialize)]
+struct SavedState {
+    input_value: String,
+    filter: Filter,
+    tasks: Vec<Task>,
+}
+
+#[derive(Debug, Clone)]
+enum LoadError {
+    FileError,
+    FormatError,
+}
+
+#[derive(Debug, Clone)]
+enum SaveError {
+    DirectoryError,
+    FileError,
+    WriteError,
+    FormatError,
+}
+
+impl SavedState {
+    fn path() -> std::path::PathBuf {
+        let mut path = if let Some(project_dirs) =
+            directories::ProjectDirs::from("rs", "Iced", "Todos")
+        {
+            project_dirs.data_dir().into()
+        } else {
+            std::env::current_dir()
+                .expect("The current directory is not accessible")
+        };
+
+        path.push("todos.json");
+
+        path
+    }
+
+    async fn load() -> Result<SavedState, LoadError> {
+        use std::io::Read;
+
+        let mut contents = String::new();
+
+        let mut file = std::fs::File::open(Self::path())
+            .map_err(|_| LoadError::FileError)?;
+
+        file.read_to_string(&mut contents)
+            .map_err(|_| LoadError::FileError)?;
+
+        serde_json::from_str(&contents).map_err(|_| LoadError::FormatError)
+    }
+
+    async fn save(self) -> Result<(), SaveError> {
+        use std::io::Write;
+
+        let json = serde_json::to_string_pretty(&self)
+            .map_err(|_| SaveError::FormatError)?;
+
+        let path = Self::path();
+        let dir = path.parent().ok_or(SaveError::DirectoryError)?;
+
+        std::fs::create_dir_all(dir).map_err(|_| SaveError::DirectoryError)?;
+
+        let mut file =
+            std::fs::File::create(path).map_err(|_| SaveError::FileError)?;
+
+        file.write_all(json.as_bytes())
+            .map_err(|_| SaveError::WriteError)?;
+
+        // This is a simple way to save at most once every couple seconds
+        // We will be able to get rid of it once we implement event subscriptions
+        std::thread::sleep(std::time::Duration::from_secs(2));
+
+        Ok(())
+    }
+}
diff --git a/examples/tour.rs b/examples/tour.rs
index 34ad0a34..6d7a080f 100644
--- a/examples/tour.rs
+++ b/examples/tour.rs
@@ -1,13 +1,14 @@
 use iced::{
     button, scrollable, slider, text::HorizontalAlignment, text_input,
-    Application, Background, Button, Checkbox, Color, Column, Container,
-    Element, Image, Length, Radio, Row, Scrollable, Slider, Text, TextInput,
+    Application, Background, Button, Checkbox, Color, Column, Command,
+    Container, Element, Image, Length, Radio, Row, Scrollable, Slider, Text,
+    TextInput,
 };
 
 pub fn main() {
     env_logger::init();
 
-    Tour::new().run()
+    Tour::run()
 }
 
 pub struct Tour {
@@ -18,26 +19,27 @@ pub struct Tour {
     debug: bool,
 }
 
-impl Tour {
-    pub fn new() -> Tour {
-        Tour {
-            steps: Steps::new(),
-            scroll: scrollable::State::new(),
-            back_button: button::State::new(),
-            next_button: button::State::new(),
-            debug: true,
-        }
-    }
-}
-
 impl Application for Tour {
     type Message = Message;
 
+    fn new() -> (Tour, Command<Message>) {
+        (
+            Tour {
+                steps: Steps::new(),
+                scroll: scrollable::State::new(),
+                back_button: button::State::new(),
+                next_button: button::State::new(),
+                debug: true,
+            },
+            Command::none(),
+        )
+    }
+
     fn title(&self) -> String {
         format!("{} - Iced", self.steps.title())
     }
 
-    fn update(&mut self, event: Message) {
+    fn update(&mut self, event: Message) -> Command<Message> {
         match event {
             Message::BackPressed => {
                 self.steps.go_back();
@@ -49,6 +51,8 @@ impl Application for Tour {
                 self.steps.update(step_msg, &mut self.debug);
             }
         }
+
+        Command::none()
     }
 
     fn view(&mut self) -> Element<Message> {
diff --git a/native/Cargo.toml b/native/Cargo.toml
index 38db1610..5dc3ae1a 100644
--- a/native/Cargo.toml
+++ b/native/Cargo.toml
@@ -8,6 +8,6 @@ license = "MIT"
 repository = "https://github.com/hecrj/iced"
 
 [dependencies]
-iced_core = { version = "0.1.0-alpha", path = "../core" }
+iced_core = { version = "0.1.0-alpha", path = "../core", features = ["command"] }
 twox-hash = "1.5"
 raw-window-handle = "0.3"
diff --git a/native/src/lib.rs b/native/src/lib.rs
index 7e55064c..bd03ddcd 100644
--- a/native/src/lib.rs
+++ b/native/src/lib.rs
@@ -216,7 +216,7 @@ mod size;
 mod user_interface;
 
 pub use iced_core::{
-    Align, Background, Color, Font, Length, Point, Rectangle, Vector,
+    Align, Background, Color, Command, Font, Length, Point, Rectangle, Vector,
 };
 
 pub use element::Element;
diff --git a/src/lib.rs b/src/lib.rs
index 52b0ff8c..945af421 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -4,21 +4,23 @@ mod platform;
 
 pub use platform::*;
 
-pub trait Application {
-    type Message: std::fmt::Debug;
+pub trait Application: Sized {
+    type Message: std::fmt::Debug + Send;
+
+    fn new() -> (Self, Command<Self::Message>);
 
     fn title(&self) -> String;
 
-    fn update(&mut self, message: Self::Message);
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message>;
 
     fn view(&mut self) -> Element<Self::Message>;
 
-    fn run(self)
+    fn run()
     where
         Self: 'static + Sized,
     {
         #[cfg(not(target_arch = "wasm32"))]
-        iced_winit::Application::run(Instance(self));
+        <Instance<Self> as iced_winit::Application>::run();
 
         #[cfg(target_arch = "wasm32")]
         iced_web::Application::run(Instance(self));
@@ -35,12 +37,18 @@ where
     type Renderer = Renderer;
     type Message = A::Message;
 
+    fn new() -> (Self, Command<A::Message>) {
+        let (app, command) = A::new();
+
+        (Instance(app), command)
+    }
+
     fn title(&self) -> String {
         self.0.title()
     }
 
-    fn update(&mut self, message: Self::Message) {
-        self.0.update(message);
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message> {
+        self.0.update(message)
     }
 
     fn view(&mut self) -> Element<Self::Message> {
diff --git a/src/winit.rs b/src/winit.rs
index d35a339f..c869a269 100644
--- a/src/winit.rs
+++ b/src/winit.rs
@@ -2,8 +2,8 @@ pub use iced_wgpu::{Primitive, Renderer};
 
 pub use iced_winit::{
     button, scrollable, slider, text, text_input, winit, Align, Background,
-    Checkbox, Color, Font, Image, Length, Radio, Scrollable, Slider, Text,
-    TextInput,
+    Checkbox, Color, Command, Font, Image, Length, Radio, Scrollable, Slider,
+    Text, TextInput,
 };
 
 pub type Element<'a, Message> = iced_winit::Element<'a, Message, Renderer>;
diff --git a/winit/Cargo.toml b/winit/Cargo.toml
index 2831ba2f..2a33255d 100644
--- a/winit/Cargo.toml
+++ b/winit/Cargo.toml
@@ -13,4 +13,5 @@ debug = []
 [dependencies]
 iced_native = { version = "0.1.0-alpha", path = "../native" }
 winit = { version = "0.20.0-alpha3", git = "https://github.com/rust-windowing/winit", rev = "709808eb4e69044705fcb214bcc30556db761405"}
+futures = { version = "0.3", features = ["thread-pool"] }
 log = "0.4"
diff --git a/winit/src/application.rs b/winit/src/application.rs
index 331bafa0..bf41d0c8 100644
--- a/winit/src/application.rs
+++ b/winit/src/application.rs
@@ -2,24 +2,26 @@ use crate::{
     conversion,
     input::{keyboard, mouse},
     renderer::{Target, Windowed},
-    Cache, Container, Debug, Element, Event, Length, MouseCursor,
+    Cache, Command, Container, Debug, Element, Event, Length, MouseCursor,
     UserInterface,
 };
 
-pub trait Application {
+pub trait Application: Sized {
     type Renderer: Windowed;
 
-    type Message: std::fmt::Debug;
+    type Message: std::fmt::Debug + Send;
+
+    fn new() -> (Self, Command<Self::Message>);
 
     fn title(&self) -> String;
 
-    fn update(&mut self, message: Self::Message);
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message>;
 
     fn view(&mut self) -> Element<Self::Message, Self::Renderer>;
 
-    fn run(mut self)
+    fn run()
     where
-        Self: 'static + Sized,
+        Self: 'static,
     {
         use winit::{
             event::{self, WindowEvent},
@@ -28,10 +30,18 @@ pub trait Application {
         };
 
         let mut debug = Debug::new();
-        let mut title = self.title();
 
         debug.startup_started();
-        let event_loop = EventLoop::new();
+        let event_loop = EventLoop::with_user_event();
+        let proxy = event_loop.create_proxy();
+        let mut thread_pool =
+            futures::executor::ThreadPool::new().expect("Create thread pool");
+        let mut external_messages = Vec::new();
+
+        let (mut application, init_command) = Self::new();
+        spawn(init_command, &mut thread_pool, &proxy);
+
+        let mut title = application.title();
 
         // TODO: Ask for window settings and configure this properly
         let window = WindowBuilder::new()
@@ -59,7 +69,7 @@ pub trait Application {
 
         debug.layout_started();
         let user_interface = UserInterface::build(
-            document(&mut self, size, &mut debug),
+            document(&mut application, size, &mut debug),
             Cache::default(),
             &mut renderer,
         );
@@ -85,15 +95,16 @@ pub trait Application {
                 // handled.
                 debug.layout_started();
                 let mut user_interface = UserInterface::build(
-                    document(&mut self, size, &mut debug),
+                    document(&mut application, size, &mut debug),
                     cache.take().unwrap(),
                     &mut renderer,
                 );
                 debug.layout_finished();
 
                 debug.event_processing_started();
-                let messages =
+                let mut messages =
                     user_interface.update(&renderer, events.drain(..));
+                messages.extend(external_messages.drain(..));
                 debug.event_processing_finished();
 
                 if messages.is_empty() {
@@ -113,12 +124,14 @@ pub trait Application {
                         debug.log_message(&message);
 
                         debug.update_started();
-                        self.update(message);
+                        let command = application.update(message);
+
+                        spawn(command, &mut thread_pool, &proxy);
                         debug.update_finished();
                     }
 
                     // Update window title
-                    let new_title = self.title();
+                    let new_title = application.title();
 
                     if title != new_title {
                         window.set_title(&new_title);
@@ -128,7 +141,7 @@ pub trait Application {
 
                     debug.layout_started();
                     let user_interface = UserInterface::build(
-                        document(&mut self, size, &mut debug),
+                        document(&mut application, size, &mut debug),
                         temp_cache,
                         &mut renderer,
                     );
@@ -143,6 +156,9 @@ pub trait Application {
 
                 window.request_redraw();
             }
+            event::Event::UserEvent(message) => {
+                external_messages.push(message);
+            }
             event::Event::RedrawRequested(_) => {
                 debug.render_started();
 
@@ -288,3 +304,25 @@ where
         .height(Length::Units(size.height.round() as u16))
         .into()
 }
+
+fn spawn<Message: Send>(
+    command: Command<Message>,
+    thread_pool: &mut futures::executor::ThreadPool,
+    proxy: &winit::event_loop::EventLoopProxy<Message>,
+) {
+    use futures::FutureExt;
+
+    let futures = command.futures();
+
+    for future in futures {
+        let proxy = proxy.clone();
+
+        let future = future.map(move |message| {
+            proxy
+                .send_event(message)
+                .expect("Send command result to event loop");
+        });
+
+        thread_pool.spawn_ok(future);
+    }
+}