Lazy loading components

accepted/0000-lazy-loading-components.md

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+# Lazy loading components
+
+## Summary
+
+Allow to lazy load components in runtime and render them as usual dynamic components.
+
+## Motivation
+
+Currently Orchestrator supports components to be provided via DI which limits it to build-time mostly.
+However it is benefitial to be able to load components on demand whenewer they are required to be rendered in the UI to reduce initial bundle size of the application.
+
+## Detailed Explanation
+
+To support lazy loading usecase the following changes have to be done:
+
+1. Allow to register dynamic components by providing a function that returns Promise of an actual component
+2. Adjust `orc-render-item` component to await for a lazy component to be loaded before rendering
+3. Allow to add component registries at runtime via `ComponentLocatorService` service so that custom lazy loading techniques may be implemented
+
+## Rationale and Alternatives
+
+The most simplest version for lazy loading would be to just implement item #3 from the section above and let devs to implement custom lazy loading components, however this will make a simple usecase harder to implment and different projects may have different implementations of the same problem - making them less interoperable between each other which is not ideal.
+
+## Implementation
+
+1. Update type definitions of `ComponentMap` and `ComponentRegistry` to support lazy loading by adding function signature that returns the same type it currently holds:
+
+```ts
+export type LazyComponentLoader<T extends Type<any> = DefaultDynamicComponent> =
+  () => Promise<T>;
+
+export type ComponentOrLoader<T extends Type<any> = DefaultDynamicComponent> =
+  | T
+  | LazyComponentLoader<T>;
+
+export interface ComponentMap<T extends Type<any> = DefaultDynamicComponent> {
+  [k: string]: ComponentOrLoader<T>;
+}
+
+export type ComponentRegistry<T extends Type<any> = DefaultDynamicComponent> =
+  | ComponentOrLoader<T>[]
+  | ComponentMap<T>;
+```
+
+Update `ComponentLocatorService` to handle lazy component signatures:
+
+```ts
+class ComponentLocatorService {
+  resolve<T, C = GetOrchestratorDynamicComponentConfig<T>>(
+    component: string | OrchestratorDynamicComponentType<C>,
+  ): Promise<OrchestratorDynamicComponentType<C>> | undefined;
+}
+```
+
+Method `resolve` MUST perform the following actions:
+
+- Check if the component map has entry for requested component
+  - If NOT found - eeturn `undefined`
+- Check if the component map has lazy loader of the component
+  - If true - execute lazy loader and return it's promise
+  - Once resolved - replace lazy loader in the component map with a loaded component
+  - If rejected - throw an error with details about which component failed to load and original error reason using `ErrorStrategy.handle()` API
+- Othrwise return component from the component map as-is
+
+2. Update `orc-render-item`:
+
+   1. Wait for a component to be resolved from `ComponentLocatorService`
+   2. Resolve config for a loaded component
+   3. Render loaded component
+
+3. Update `ComponentLocatorService` to allow registering new components and expose all loaded/registered components:
+
+```ts
+class ComponentLocatorService {
+  +registerComponent(component: ComponentOrLoader): void;
+  +getComponentNames(): Observable<string[]>;
+  +getLoadedComponents(): Observable<OrchestratorDynamicComponentType[]>;
+}
+```
+
+Method `registerComponent` MUST perform the following actions:
+
+- Extend currently registered component map
+
+We specifically have 2 separate methods for getting components which are Observables so they can update whenever a new component is registered/loaded:
+
+- component names that are registered/loaded `getComponentNames`
+- already loaded components `getLoadedComponents`
+
+## Unresolved Questions and Bikeshedding
+
+Open questions:
+
+- What should be/not be rendered while a component is being loaded?
+  For example we can have optional InjectionToken that can provide a component for rendering.
+- Should we provide a way for custom loader strategy ie. custom service?