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Stremio Core is built on the Elm Architecture, a pattern for building reliable, maintainable applications using a unidirectional data flow.

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Core Concepts

The Elm Architecture has three main components:
  1. Model: The state of your application
  2. Messages: Events that describe state changes
  3. Update: A function that produces new state and effects

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The Msg Type

Messages are the only way to change state in Stremio Core. Every user interaction, API response, or timer event becomes a message: 
pub enum Msg {
    Action(Action),    // User actions
    Internal(Internal), // Internal state changes
    Event(Event),      // Events emitted to the UI
}

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Action Messages

Actions are dispatched by the application UI: 
// src/runtime/msg/action.rs:113
#[derive(Clone, Deserialize, Debug)]
#[serde(tag = "action", content = "args")]
pub enum ActionCtx {
    Authenticate(AuthRequest),
    Logout,
    InstallAddon(Descriptor),
    UpdateSettings(ProfileSettings),
    AddToLibrary(MetaItemPreview),
    // ...
}

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Internal Messages

Internal messages are used for state transitions that don’t come from user actions: 
Internal::CtxAuthResult(auth_request, result)
Internal::ProfileChanged
Internal::ResourceRequestResult(request, result)

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Event Messages

Events are emitted to the UI to notify about important state changes: 
Event::UserAuthenticated { auth_request }
Event::UserLoggedOut { uid }
Event::Error { error, source }

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The Update Trait

The Update trait defines how models respond to messages: 
// src/runtime/update.rs:18
pub trait Update<E: Env> {
    fn update(&mut self, msg: &Msg) -> Effects;
}
Models that need access to the global context use UpdateWithCtx: 
// src/runtime/update.rs:22
pub trait UpdateWithCtx<E: Env> {
    fn update(&mut self, msg: &Msg, ctx: &Ctx) -> Effects;
}

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Update Implementation Example

Here’s how the Ctx model handles authentication: 
// src/models/ctx/ctx.rs:110
impl<E: Env + 'static> Update<E> for Ctx {
    fn update(&mut self, msg: &Msg) -> Effects {
        match msg {
            Msg::Action(Action::Ctx(ActionCtx::Authenticate(auth_request))) => {
                // Set loading state
                self.status = CtxStatus::Loading(auth_request.to_owned());
                // Return effect to perform authentication
                Effects::one(authenticate::<E>(auth_request)).unchanged()
            }
            Msg::Action(Action::Ctx(ActionCtx::Logout)) => {
                // Convert to internal message
                Effects::msg(Msg::Internal(Internal::Logout(false))).unchanged()
            }
            // ...
        }
    }
}

​
Effects

Effects represent asynchronous operations or additional messages to process: 
// src/runtime/effects.rs:14
pub enum Effect {
    Msg(Box<Msg>),              // Immediate message
    Future(EffectFuture),       // Async operation
}

pub enum EffectFuture {
    Concurrent(Future),   // Run in parallel
    Sequential(Future),   // Run in order
}

​
Creating Effects

The Effects builder provides convenient methods: 
// No effects, state changed
Effects::none()

// Single message effect
Effects::msg(Msg::Event(Event::UserLoggedOut { uid }))

// Multiple messages
Effects::msgs(vec![msg1, msg2, msg3])

// Async effect
Effects::future(EffectFuture::Concurrent(fetch_data()))

// Mark state as unchanged
Effects::none().unchanged()

// Combine multiple effects
effect1.join(effect2).join(effect3)

​
The unchanged() Method

Effects track whether the model state changed: 
// src/runtime/effects.rs:81
pub fn unchanged(mut self) -> Self {
    self.has_changed = false;
    self
}
When has_changed is true, the runtime emits a NewState event to notify the UI. Use .unchanged() when effects don’t modify the model: 
// State changed, notify UI
self.status = CtxStatus::Loading(...);
Effects::one(effect)

// State unchanged, don't notify UI  
Effects::one(effect).unchanged()

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Example: CatalogWithFilters Update

Here’s a complete example from the catalog model: 
// src/models/catalog_with_filters.rs:155
impl<E, T> UpdateWithCtx<E> for CatalogWithFilters<T>
where
    E: Env + 'static,
    T: CatalogResourceAdapter + PartialEq,
{
    fn update(&mut self, msg: &Msg, ctx: &Ctx) -> Effects {
        match msg {
            Msg::Action(Action::Load(ActionLoad::CatalogWithFilters(selected))) => {
                // Update selected filters
                let selected_effects =
                    selected_update::<T>(&mut self.selected, &self.selectable, selected);
                
                // Load catalog data
                let catalog_effects = match self.selected.as_ref() {
                    Some(selected) => catalog_update::<E, _>(
                        &mut self.catalog,
                        CatalogPageRequest::First,
                        &selected.request,
                    ),
                    _ => Effects::none().unchanged(),
                };
                
                // Update selectable options
                let selectable_effects = selectable_update(
                    &mut self.selectable,
                    &self.selected,
                    &self.catalog,
                    &ctx.profile,
                );
                
                // Combine all effects
                selected_effects
                    .join(catalog_effects)
                    .join(selectable_effects)
            }
            // Handle other messages...
        }
    }
}

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Benefits of This Pattern

Predictability

All state changes go through update functions, making the data flow easy to trace.

Testability

Update functions are pure (except for effects), making them easy to test.

Composability

Effects can be combined using .join(), making complex updates manageable.

Type Safety

The compiler ensures all messages are handled correctly.

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Next Steps

Models and State

Learn how models manage application state

Effects and Runtime

Understand how effects are executed

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