!!!FEATURE: Centralize error handling

packages/framework-observable-react/README.md

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+# @neos-project/framework-observable-react
+
+> React bindings for @neos-project/framework-observable
+
+This package provides a set of React [hooks](https://react.dev/reference/react/hooks) to let components interact with `Observable`s.
+
+## API
+
+### `useLatestValueFrom`
+
+```typescript
+// Without default value:
+function useLatestValueFrom<V>(observable$: Observable<V>): null | V;
+
+// With default value:
+function useLatestValueFrom<V, D>(
+  observable$: Observable<V>,
+  defaultValue: D
+): D | V;
+```
+
+`useLatestValueFrom` is a way to bind a react component the latest value emitted from an `Observable`.
+
+#### Parameters
+
+| Name                      | Description                                                                                    |
+| ------------------------- | ---------------------------------------------------------------------------------------------- |
+| `observable$`             | The `Observable` to subscribe to                                                               |
+| `defaultValue` (optional) | The value to default for when `observable$` hasn't emitted any values yet (defaults to `null`) |
+
+#### Return Value
+
+This hook returns the latest value from the provided `observable$`. If no value has been emitted from the observable yet, it returns `defaultValue` which itself defaults to `null`.
+
+#### Example
+
+This component will display the amount of seconds that have passed since it was first mounted:
+
+```typescript
+const clock$ = createObservable((next) => {
+  let i = 1;
+  const interval = setInterval(() => {
+    next(i++);
+  }, 1000);
+
+  return () => clearInterval(interval);
+});
+
+const MyComponent = () => {
+  const seconds = useLatestValueFrom(clock$, 0);
+
+  return <pre>{seconds} seconds passed</pre>;
+};
+```
+
+You can combine this with `React.useMemo`, if you wish to create an ad-hoc observable:
+
+```typescript
+const MyComponent = (props) => {
+  const beats = useLatestValueFrom(
+    React.useMemo(
+      () =>
+        createObservable((next) => {
+          let i = 1;
+          const interval = setInterval(() => {
+            next(i++);
+          }, props.millisecondsPerBeat);
+
+          return () => clearInterval(interval);
+        }),
+      [props.millisecondsPerBeat]
+    ),
+    0
+  );
+
+  return <pre>{beats} beats passed</pre>;
+};
+```
+
+### `useLatestState`
+
+```typescript
+function useLatestState<V>(state$: State<V>): V;
+```
+
+`useLatestState` subscribes to a given state observable and keeps track of its latest value.
+
+#### Parameters
+
+| Name     | Description                             |
+| -------- | --------------------------------------- |
+| `state$` | The `State` observable to keep track of |
+
+#### Return Value
+
+This hook returns the latest value from the given `State` observable. Initially it contains the current value of the `State` at the moment the component was first mounted.
+
+#### Example
+
+```typescript
+const count$ = createState(0);
+
+const MyComponent = () => {
+  const count = useLatestState(count$);
+  const handleInc = React.useCallback(() => {
+    count$.update((count) => count + 1);
+  }, []);
+  const handleDec = React.useCallback(() => {
+    count$.update((count) => count - 1);
+  }, []);
+
+  return (
+    <div>
+      <pre>Count {count}</pre>
+      <button onClick={handleInc}>+</button>
+      <button onClick={handleDec}>-</button>
+    </div>
+  );
+};
+```

packages/framework-observable-react/package.json

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+{
+  "name": "@neos-project/framework-observable-react",
+  "version": "",
+  "description": "React bindings for @neos-project/framework-observable",
+  "private": true,
+  "main": "./src/index.ts",
+  "dependencies": {
+    "@neos-project/framework-observable": "workspace:*",
+    "react": "^16.12.0"
+  },
+  "license": "GNU GPLv3"
+}

packages/framework-observable-react/src/index.ts

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+/*
+ * This file is part of the Neos.Neos.Ui package.
+ *
+ * (c) Contributors of the Neos Project - www.neos.io
+ *
+ * This package is Open Source Software. For the full copyright and license
+ * information, please view the LICENSE file which was distributed with this
+ * source code.
+ */
+export {useLatestState} from './useLatestState';
+export {useLatestValueFrom} from './useLatestValueFrom';

packages/framework-observable-react/src/useLatestState.ts

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+/*
+ * This file is part of the Neos.Neos.Ui package.
+ *
+ * (c) Contributors of the Neos Project - www.neos.io
+ *
+ * This package is Open Source Software. For the full copyright and license
+ * information, please view the LICENSE file which was distributed with this
+ * source code.
+ */
+import type {State} from '@neos-project/framework-observable';
+
+import {useLatestValueFrom} from './useLatestValueFrom';
+
+export function useLatestState<V>(state$: State<V>) {
+    return useLatestValueFrom(state$, state$.current);
+}

packages/framework-observable-react/src/useLatestValueFrom.ts

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+/*
+ * This file is part of the Neos.Neos.Ui package.
+ *
+ * (c) Contributors of the Neos Project - www.neos.io
+ *
+ * This package is Open Source Software. For the full copyright and license
+ * information, please view the LICENSE file which was distributed with this
+ * source code.
+ */
+import React from 'react';
+
+import type {Observable} from '@neos-project/framework-observable';
+
+export function useLatestValueFrom<V>(observable$: Observable<V>): null | V;
+export function useLatestValueFrom<V, D>(
+    observable$: Observable<V>,
+    defaultValue: D
+): D | V;
+
+export function useLatestValueFrom<V, D>(
+    observable$: Observable<V>,
+    defaultValue?: D
+) {
+    const [value, setValue] = React.useState<null | D | V>(
+        defaultValue ?? null
+    );
+
+    React.useEffect(() => {
+        const subscription = observable$.subscribe({
+            next: (incomingValue) => {
+                if (incomingValue !== value) {
+                    setValue(incomingValue);
+                }
+            }
+        });
+
+        return () => subscription.unsubscribe();
+    }, [observable$]);
+
+    return value;
+}

packages/framework-observable/README.md

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+# @neos-project/framework-observable
+
+> Observable pattern implementation for the Neos UI
+
+> [!NOTE]
+> This package implements a pattern for which there is a WICG proposal:
+> https://github.com/WICG/observable
+>
+> It is therefore likely that future versions of this package will use the web-native `Observable` primitive under the hood.
+
+## API
+
+### Observables
+
+An `Observable` represents a sequence of values that can be *observed* from the outside. This is a powerful abstraction that allows to encapsule all kinds of value streams like:
+
+- (DOM) Events
+- Timeouts & Intervals
+- Async operations & Promises
+- Websockets
+- etc.
+
+An `Observable` can be created using the `createObservable` function like this:
+
+```typescript
+const numbers$ = createObservable((next) => {
+    next(1);
+    next(2);
+    next(3);
+});
+```
+
+> [!NOTE]
+> Suffixing variable names with `$` is a common naming convention to signify that a variable represents an observable.
+
+Here, the `numbers$` observable represents the sequence of the numbers 1, 2 and 3. This observable can be subscribed to thusly:
+
+```typescript
+numbers$.subscribe((value) => {
+  console.log(value);
+});
+```
+
+Because the `numbers$` observable emits its values immediately, the above subscription will immediately log:
+```
+1
+2
+3
+```
+
+An additional subscription would also immediately receive all 3 values. By default, oberservables are *lazy* and *single-cast*. This means, values are generated exclusively for each subscription, and the generation starts exactly when a subscriber is registered.
+
+The usefulness of observables becomes more apparent when we introduce some asynchrony:
+```typescript
+const timedNumbers$ = createObservable((next) => {
+  let i = 1;
+  const interval = setInterval(() => {
+    next(i++);
+  }, 2000);
+
+  return () => clearInterval(interval);
+});
+```
+
+This `timedNumbers$` observable will emit a new value every two seconds. This time, the callback used to facilitate the observable returns a function:
+```typescript
+// ..
+return () => clearInterval(interval);
+// ..
+```
+
+This function will be called when a subscription is cancelled. This is a way for observables to clean up after themselves.
+
+If we now subscribe to `timedNumbers$` like this:
+```typescript
+const subscription = timedNumbers$.subscribe((value) => {
+  console.log(value);
+});
+```
+
+The following values will be logged to the console:
+```
+1 (After 2 seconds)
+2 (After 4 seconds)
+3 (After 6 seconds)
+4 (After 8 seconds)
+...
+```
+
+This will go on forever, unless we call the `unsubscribe` on our `subscription` which has been the return value we've saved from `timedNumber$.subscribe(...)`. When we call `unsubscribe`, the cleanup function of the `timedNumbers$` observable will be called and so the interval will be cleared:
+```typescript
+subscription.unsubscribe();
+```
+
+That's all there is to it. With this small set of tools, `Observable`s can be used to encapsule all kinds of synchronous or asynchronous value streams.
+
+They can be created from a Promise:
+```typescript
+async function someLongRunningOperation() {
+  // ...
+}
+
+const fromPromise$ = createObservable((next) => {
+  someLongRunningOperation().then(next);
+});
+```
+
+Or DOM events:
+```typescript
+const clicks$ = createObservable((next) => {
+  const button = document.querySelector('button');
+  button.addEventListener('click', next);
+  return () => button.removeEventListener('click', next);
+});
+```
+
+And there are many, many more possibilities.
+
+### State
+
+A `State` is a special `Observable` that can track a value over time. `State`s can be created using the `createState` function like this:
+
+```typescript
+const count$ = createState(0);
+```
+
+The `count$` state is now set to `0`. Unlike regular observables, a `State` instance can be queried for its current value:
+```typescript
+console.log(count$.current); // output: 0
+```
+
+Each `State` instance has an `update` method that can be used to push new values to the state observable. It takes a callback that receives the current value as its first paramater and returns the new value:
+
+```typescript
+count$.update((value) => value + 1);
+
+console.log(count$.current); // output: 1
+```
+
+When a new subscriber is registered to a `State` instance, that subscriber immediately receives the current value:
+```typescript
+const count$ = createState(0);
+count$.update((value) => value + 1); // nothing is logged, nobody has subscribed yet
+count$.update((value) => value + 1); // nothing is logged, nobody has subscribed yet
+count$.update((value) => value + 1); // nothing is logged, nobody has subscribed yet
+
+count$.subscribe((value) => console.log(value)); // immediately logs: 3
+
+count$.update((value) => value + 1); // logs: 4
+```
+
+Unlike regular `Observable`s, `State`s are multi-cast. This means that all subscribers receive updates at the same time, and every subscriber only receives updates that are published after the subscription has been registered.

packages/framework-observable/package.json

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+{
+  "name": "@neos-project/framework-observable",
+  "version": "",
+  "description": "Observable pattern implementation for the Neos UI",
+  "private": true,
+  "main": "./src/index.ts",
+  "license": "GNU GPLv3"
+}