Bug 1877307 - RFC: Introduce a Store for UI components

docs/contributing.md

@@ -45,6 +45,7 @@ Before contributing, please review our [Community Participation Guidelines](http
 * [0006 - Determining the set of default search engines]({{ site.baseurl }}/rfc/0006-search-defaults)
 * [0007 - Synchronizing the branching and versioning of Android Components with the Mozilla release trains]({{ site.baseurl }}/rfc/0007-synchronized-releases)
 * [0008 - Adding tab partitions (groups) to BrowserState]({{ site.baseurl }}/rfc/0008-tab-groups)
+* [0012 - Allow customisation of the Store dispatcher]({{ site.baseurl }}/rfc/0012-customizable-store-dispatcher)
 
 ### Presentations
 

docs/rfcs/0012-introduce-ui-store.md

@@ -0,0 +1,177 @@
+---
+layout: page
+title: Introduce a Store for UI components
+permalink: /rfc/0012-introduce-ui-store
+---
+
+* Start date: 2024-01-29
+* RFC PR: [#5353](https://github.com/mozilla-mobile/firefox-android/pull/5353)
+
+## Summary
+
+In most applications of the `Store`, it is preferable to have reducers perform work on the main thread. Having actions reduced immediately at the point of dispatch, simplifies the reasoning a developer would need to go through for most UI-based work that happens on the main thread.
+
+## Motivation
+
+Android embedders use the main thread for UI, user-facing, or gesture handling work. For example, notifying UI components when IO from storage layers have completed, an engine's task that can happen on a separate thread, or global-level state updates for different components to observe.
+
+When components dispatch actions, they are performed on an independant single thread dispatcher in the `Store` to avoid overloading the main thread with heavy work that might be performed during the `reduce` or in a `Middleware`. In practice, these actions have been short and fast so they do not cause overhead (most of these actions have been [data class copying][0]).  In addition, side-effects done in a `Middleware` which can be slow, like I/O,  are put onto separate Dispatchers. The performance optimization to switch to a `Store` thread, requires that components which are always run on the main thread, to ensure synchronisation is now kept between the main thread and the store thread for observers of the `State`.
+
+There are some advantages to this change:
+
+* Simplicity for `Store`s that are meant for UI facing work.
+* Unit testing can now occur on the test framework's thread.
+* Fewer resources needed for context shifting between threads[^1].
+
+For an example of thread simplicity, an `Engine` typically has its own 'engine thread' to perform async work and post/request results to the main thread (these APIs are identified with the `@UiThread` annotation). Once we get the callback for those results, we then need to dispatch an action to the store that will then happen on a `Store` thread. Feature components then observe for state changes and then make UI changes on the main thread. A simplified form of this thread context switching can be seen in the example below:
+
+```kotlin
+// engine thread
+engineView.requestApiResult { result ->
+  // received on the main thread.
+  store.dispatch(UpdateResultAction(result))
+}
+
+// store thread
+fun reduce(state: State, action: Action) {
+  is UpdateResultAction -> {
+    // do things here.
+  }
+}
+
+// store thread
+Middleware {
+  override fun invoke(
+    context: MiddlewareContext<State, Action>,
+    next: (Action) -> Unit,
+    action: Action,
+  ) {
+    // perform side-effects that also happen on the store thread.
+  }
+}
+
+// main thread
+store.flowScoped { flow ->
+  flow.collect {
+    // perform work on the main thread.
+  }
+}
+```
+
+With the changes in this RFC, this switching of threads can be reduced (notable comments marked with 📝):
+
+```kotlin
+// engine thread
+engineView.requestApiResult { result ->
+  // received on the main thread.
+  store.dispatch(UpdateResultAction(result))
+}
+
+// 📝 main thread - now on the same thread, processed immediately.
+fun reduce(state: State, action: Action) {
+  is UpdateResultAction -> {
+    // do things here.
+  }
+}
+
+// 📝 main thread - now on the same thread, processed immediately.
+Middleware {
+  override fun invoke(
+    context: MiddlewareContext<State, Action>,
+    next: (Action) -> Unit,
+    action: Action,
+  ) {
+    // 📝 perform side-effects that now happen on the main thread.
+  }
+}
+
+// main thread
+store.flowScoped { flow ->
+  flow.collect {
+      // perform work on the main thread.
+    }
+  }
+}
+```
+
+Additionally, from [performance investigations already done][2], we know that Fenix creates over a hundred threads within a few seconds of startup. Reducing the number of threads for Stores that do not have a strong requirement to run on a separate thread will lower the applications memory footprint.
+
+## Guide-level explanation
+
+Extending the existing `Store` class to use the `Dispatchers.Main.immediate` will ensure that UI stores will stay on the same UI thread and have that work done immediately. Using a distinct class named `UiStore` also makes it clear to the developer that this is work that will be done on the UI thread and its implications will be made a bit more clear when it's used.
+
+```kotlin
+@MainThread
+open class UiStore<S : State, A : Action>(
+  initialState: S,
+  reducer: Reducer<S, A>,
+  middleware: List<Middleware<S, A>> = emptyList(),
+) : Store<S, A>(
+  initialState,
+  reducer,
+  middleware,
+  UiStoreDispatcher(),
+)
+
+open class Store<S : State, A : Action> internal constructor(
+  initialState: S,
+  reducer: Reducer<S, A>,
+  middleware: List<Middleware<S, A>>,
+  dispatcher: StoreDispatcher,
+) {
+  constructor(
+    initialState: S,
+    reducer: Reducer<S, A>,
+    middleware: List<Middleware<S, A>> = emptyList(),
+    threadNamePrefix: String? = null,
+  ) : this(
+    initialState = initialState,
+    reducer = reducer,
+    middleware = middleware,
+    dispatcher = DefaultStoreDispatcher(threadNamePrefix),
+  )
+}
+
+interface StoreDispatcher {
+  val dispatcher: CoroutineDispatcher
+  val scope: CoroutineScope
+  val coroutineContext: CoroutineContext
+
+  // Each Store has it's own `assertOnThread` because in the Thread owner is different in both context.
+  fun assertOnThread()
+}
+```
+
+Applications can use this similar to any other store then. An "AppStore" example below can switch :
+
+```kotlin
+// changing the one line below from `UiStore` to `Store` gives the developer the ability to switch existing Stores between the different Store types.
+class AppStore(
+  initialState: AppState = AppState(),
+) : UiStore<AppState, AppAction>(
+  initialState = initialState,
+  reducer = AppStoreReducer::reduce,
+)
+```
+
+## Drawbacks
+
+* Mistakenly doing work on the main thread - we could end up performing large amounts of work on the main thread unintentionally if we are not careful. This could be because of a large number of small tasks, a single large task, a blocking task, or a combination. As the developer is choosing to use a `UiStore`, they will be expected to ensure that heavy work they do, as is with mobile UI development done today, is not done on the main thread.
+
+## Rationale and alternatives
+
+Not introducing this new Store type would not change current development where the developer needs to ensure understanding that dispatched actions will be processed at a later time.
+
+## Future work
+
+We have opportunities to iterate from here and consider if/how we want to pass a CoroutineScope in. This can be part of future RFC proposals however.
+
+## Unresolved questions
+
+* While performance gains are not an explicit intent, there is a theoretical advantage, but not one we will pursue as part of this RFC. How much would we save, if any?
+* Some additional changes need to be done to allow the `Store` to override the default `StoreThreadFactory` that will allow assertions against a thread (`MainThread`) not created by the `StoreThreadFactory` itself. This should be possible, but will this add to additional complexity?
+
+[0]: https://kotlinlang.org/docs/data-classes.html#copying
+[^1]: https://github.com/mozilla-mobile/android-components/issues/9424
+[2]: https://github.com/mozilla-mobile/android-components/issues/9424#issue-787013588
+