rx-angular · BioPhoton · Feb 3, 2022 · Feb 3, 2022 · Feb 3, 2022 · Feb 3, 2022
diff --git a/libs/state/README.md b/libs/state/README.md
@@ -12,6 +12,7 @@ RxState is a lightweight, flexible, strongly typed and tested tool dedicated to
 
 ## Sub Modules
 
+- [**{ }** State (root module)](https://github.com/rx-angular/rx-angular/blob/main/libs/state/README.md)
 - [🧩 Selections](https://github.com/rx-angular/rx-angular/blob/main/libs/state/selections/README.md)
 - [☁ Effects](https://github.com/rx-angular/rx-angular/blob/main/libs/state/effects/README.md)
 

diff --git a/libs/state/selections/docs/Readme.md b/libs/state/selections/docs/Readme.md
@@ -1,10 +1,210 @@
 # Motivation
 
-TBD
+![Selections (1)](https://user-images.githubusercontent.com/10064416/152422745-b3d8e094-d0f0-4810-b1b2-5f81fae25938.png)
 
-* [`Interfaces`](./operators/interfaces.md)
-* [`distinctUntilSomeChanged`](./operators/distinct-until-some-changed.md)
-* [`select`](./operators/select.md)
-* [`selectSlice`](./operators/select-slice.md)
-* [`stateful`](./operators/stateful.md)
+When managing state you want to maintain a core unit of data. 
+This data is then later on distributed to multiple places in your component template (local) or whole app (global). 
+
+We can forward this state to their consumers directly or compute specific derivations (selections) for the core unit.
+
+As an example we could think of the following shape: 
+
+**A list and a list title**
+```typescript
+interface GlobalModel {
+  title: string;
+  list: Array<{ id: number, date: Date }>
+}
+```
+
+This data is consumed in different screens:
+
+**A list of all items sorted by id**
+```typescript
+interface SelectionScreen1 {
+  title: string;
+  sortDirection: 'asc' | 'desc' | 'none';
+  sortedList: Array<{ id: number }>
+}
+```
+
+**A list of items filtered by date**
+```typescript
+interface SelectionScreen2 {
+  title: string;
+  startingDate: Date;
+  filteredList: { id: number }
+}
+```
+
+The 2 rendered lists are a derivation, a modified version of the core set of items.
+One time they are displayed in a sorted order, the other time only filtered subset of the items.
+
+> **Hint:**  
+> Derivations are always redundant information of our core data and therefore should not get stored,
+> but cached in the derivation logic.
+
+![Selections (2)](https://user-images.githubusercontent.com/10064416/152422803-bfd07ab2-0a6f-4521-836e-b71677e11923.png)
+
+As this process contains a lot of gotchas and possible pitfalls in terms of memory usage and performance this small helper library was created.
+
+# Benefits
+
+![Selections (3)](https://user-images.githubusercontent.com/10064416/152422856-a483a06c-84e0-4067-9eaa-f3bb54a0156d.png)
+
+
+- Sophisticated set of helpers for any selection problem
+- Enables lazy rendering
+- Computes only distinct values
+- Shares computed result with multiple subscriber
+- Select distinct sub-sets
+- Select from static values
+- Fully Tested
+- Fully Typed
+
+## Selection owner - Template vs Class
+
+As Observables are cold their resulting stream will only get activated by a subscription.
+This leads to a situations called: "the late subscriber problem" or "the early subscriber problem". (LINK)
+
+![Selections (5)](https://user-images.githubusercontent.com/10064416/152422955-cb89d198-1a69-450b-be84-29dd6c8c4fdb.png)
+
+
+In most cases it's best to go with solving problems on the early subscriber side and be sure we never loose values that should render on the screen.
+
+    ![Selections (4)](https://user-images.githubusercontent.com/10064416/152422883-0b5f6006-7929-4520-b0b2-79eb61e4eb08.png)
+
+
+## Advanced derivation architecture
+
+**The problem**
+
+We have the following state sources to manage:
+- the list of products received form global state - `Product[]`
+- the title of the list including it's number of children computen in the component class - `string`
+- the sort direction triggered over a UI element click - `boolean`
+
+A setup of the compoents class based on `RxState` could look like this:
+
+```typescript
+@Component({
+  selector: 'app-problem',
+  template: `
+    <ng-container *rxLet="viewModel$; let vm">
+      <h1>{{vm.title}} - {{vm.sortDirection}}</h1>
+      <ul>
+        <li *ngFor="let item of vm.sortedList">{{item}}</li>
+      </ul>
+    </ng-container>
+    `,
+  providers: [RxState],
+})
+export class ProblemComponent {
+
+  viewModel$: Observable<ViewModel>; // ???
+
+  constructor(private globalState: GlobalState, private state: RxState<Model>) {
+    this.state.connect('title', this.globalState.title$);
+    this.state.connect('products', this.globalState.products$);
+  }
+
+  toggleSort() {
+    this.state.set('sort', ({sort}) => !sort))
+  }
+}
+
+```
+
+In a components template we want to render the the UI for the above explained view model `SelectionScreen1`.
+
+```typescript
+interface SelectionScreen1 {
+  title: string;
+  sortDirection: 'asc' | 'desc' | 'none';
+  sortedList: Array<{ id: number }>
+}
+```
+
+A common implementations looks like this:
+
+
+```typescript
+// template removed for brevity
+export class ProblemComponent {
+
+  private sortedList$ = this.state.select(
+      selectSlice(['sortDirection', 'list']),
+      map(() => {
+         // sort `list` by `sortDirection` to `sortedList` here
+        return sortedList;
+      })
+  );
+
+  viewModel$ = this.state.select(
+    selectSlice(['title', 'sortedList', 'sortDirection'])
+  )
+
+  //                                                    ❌ BAD: modle viewmodel mix up 👇
+  constructor(private globalState: GlobalState, private state: RxState<Model & Pick<ViewModel, 'sortedList'>>) {
+    // ...
+
+    // ❌ BAD: store derived state 👇
+    this.state.connect('sortedList', this.sortedList$);
+  }
+
+  // ...
+}
+
+```
+
+![Selections (6)](https://user-images.githubusercontent.com/10064416/152422999-db8260f0-69e1-4d99-b6ac-b2b1d043b4b7.png)
+
+By removing the sorted list form the state and moving it into the selection  
+we can clean up the state's typing and have a nice separation of which data is owned by the component (model) and which data is owned by the template (view model)
+
+```typescript
+// template removed for brevity
+export class ProblemComponent {
+
+  private sortedSlice$ = this.state.select(
+      selectSlice(['sortDirection', 'list']),
+      map(({list, sortDirection}) => {
+        // sort `list` by `sortDirection` to `sortedList` here
+        return { sortDirection, sortedList };
+      })
+  );
+
+  // ✔ GOOD: Derive view model from model 👇
+  viewModel$ = smosh({ title: this.state.select('title')}, this.sortedSlice$);
+
+  // target API
+  viewModel$ = smosh({ 
+    prop1: 'prop1', // string
+    prop2: prop1$ // Observable<string>
+    }, 
+    slice1$, // Observable<{prop3: 3}>
+    slice2$  // Observable<{prop4: 'four'}>,
+    // durationSelector$ (optional)
+  );
+
+
+  // ✔ GOOD: Derive view model from model 👇
+  viewModel$ = smosh({ 
+      title: this.state.select('title'),
+      ...this.sortedSlice$
+  });
+
+
+
+  constructor(private globalState: GlobalState, private state: RxState<Model>) {
+    // ...
+
+  }
+
+  // ...
+}
+
+```
+
+![Selections (7)](https://user-images.githubusercontent.com/10064416/152423026-d23326c2-97d5-4bd0-9015-f498c3fc0e55.png)