3 title: Introduce a Store for UI components
4 permalink: /rfc/0012-introduce-ui-store
7 * Start date: 2024-01-29
8 * RFC PR: [#5353](https://github.com/mozilla-mobile/firefox-android/pull/5353)
12 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.
16 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.
18 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`.
20 There are some advantages to this change:
22 * Simplicity for `Store`s that are meant for UI facing work.
23 * Unit testing can now occur on the test framework's thread.
24 * Fewer resources needed for context shifting between threads[^1].
26 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:
30 engineView.requestApiResult { result ->
31 // received on the main thread.
32 store.dispatch(UpdateResultAction(result))
36 fun reduce(state: State, action: Action) {
37 is UpdateResultAction -> {
45 context: MiddlewareContext<State, Action>,
46 next: (Action) -> Unit,
49 // perform side-effects that also happen on the store thread.
54 store.flowScoped { flow ->
56 // perform work on the main thread.
61 With the changes in this RFC, this switching of threads can be reduced (notable comments marked with 📝):
65 engineView.requestApiResult { result ->
66 // received on the main thread.
67 store.dispatch(UpdateResultAction(result))
70 // 📝 main thread - now on the same thread, processed immediately.
71 fun reduce(state: State, action: Action) {
72 is UpdateResultAction -> {
77 // 📝 main thread - now on the same thread, processed immediately.
80 context: MiddlewareContext<State, Action>,
81 next: (Action) -> Unit,
84 // 📝 perform side-effects that now happen on the main thread.
89 store.flowScoped { flow ->
91 // perform work on the main thread.
97 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.
99 ## Guide-level explanation
101 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.
105 open class UiStore<S : State, A : Action>(
107 reducer: Reducer<S, A>,
108 middleware: List<Middleware<S, A>> = emptyList(),
116 open class Store<S : State, A : Action> internal constructor(
118 reducer: Reducer<S, A>,
119 middleware: List<Middleware<S, A>>,
120 dispatcher: StoreDispatcher,
124 reducer: Reducer<S, A>,
125 middleware: List<Middleware<S, A>> = emptyList(),
126 threadNamePrefix: String? = null,
128 initialState = initialState,
130 middleware = middleware,
131 dispatcher = DefaultStoreDispatcher(threadNamePrefix),
135 interface StoreDispatcher {
136 val dispatcher: CoroutineDispatcher
137 val scope: CoroutineScope
138 val coroutineContext: CoroutineContext
140 // Each Store has it's own `assertOnThread` because in the Thread owner is different in both context.
145 Applications can use this similar to any other store then. An "AppStore" example below can switch :
148 // changing the one line below from `UiStore` to `Store` gives the developer the ability to switch existing Stores between the different Store types.
150 initialState: AppState = AppState(),
151 ) : UiStore<AppState, AppAction>(
152 initialState = initialState,
153 reducer = AppStoreReducer::reduce,
159 * 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.
161 ## Rationale and alternatives
163 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.
167 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.
169 ## Unresolved questions
171 * 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?
172 * 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?
174 [0]: https://kotlinlang.org/docs/data-classes.html#copying
175 [^1]: https://github.com/mozilla-mobile/android-components/issues/9424
176 [2]: https://github.com/mozilla-mobile/android-components/issues/9424#issue-787013588