@craft-ng/core

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sourceFromEvent ​

Creates a source$ from DOM events or event emitters.

Use it when the event's origin is an emitter or an element you already hold, rather than a target resolved lazily — for that, see fromEventToSource$.

Import ​

typescript
import { sourceFromEvent } from '@craft-ng/core';

Basic Usage ​

typescript
import { state, sourceFromEvent } from '@craft-ng/core';

const button = document.querySelector('button')!;

// Create source from button clicks
const clickSource = sourceFromEvent(
  button,
  'click',
  () => (count: number) => count + 1,
);

const { clickCount } = state('clickCount', 0, {
  sources: [clickSource],
});

API ​

typescript
function sourceFromEvent<T, E extends Event>(
  target: EventTarget,
  eventName: string,
  mapper: (event: E) => (state: T) => T,
): Observable<(state: T) => T>;

Examples ​

Inside a Craft component ​

sourceFromEvent takes an EventTarget you already hold. Inside a component that usually means a document- or window-level target, since the component's own elements are better handled with a plain event prop:

typescript
import { craftComponent, p } from '@craft-ng/component';
import { sourceFromEvent, state } from '@craft-ng/core';

export const KeyCounter = craftComponent(
  'KeyCounter',
  {},
  function* () {
    const keySource = sourceFromEvent(
      document,
      'keydown',
      () => (count: number) => count + 1,
    );

    const keys = yield* state('keys', 0, { sources: [keySource] });

    return { keys };
  },
  ({ keys }) => p(() => `Keys pressed: ${keys()}`),
);

For the component's own elements, use an event prop

button({ click: () => counter.increment() }, 'Click me') is simpler and needs no target. Reach for sourceFromEvent when the event comes from outside the component's own markup, or when several states must react to the same event.

Input Changes ​

typescript
const input = document.querySelector('input')!;

const inputSource = sourceFromEvent(
  input,
  'input',
  (event: Event) => () => (event.target as HTMLInputElement).value,
);

const { inputValue } = state('inputValue', '', {
  sources: [inputSource],
});

Mouse Position ​

typescript
interface Position {
  x: number;
  y: number;
}

const mouseMoveSource = sourceFromEvent(
  document,
  'mousemove',
  (event: MouseEvent) => () => ({
    x: event.clientX,
    y: event.clientY,
  }),
);

const { mousePosition } = state<Position>(
  'mousePosition',
  { x: 0, y: 0 },
  {
    sources: [mouseMoveSource],
  },
);

Keyboard Input ​

typescript
const keySource = sourceFromEvent(
  document,
  'keydown',
  (event: KeyboardEvent) => (keys: string[]) => [...keys, event.key],
);

const { pressedKeys } = state<string[]>('pressedKeys', [], {
  sources: [keySource],
});

Scroll Position ​

typescript
const scrollSource = sourceFromEvent(
  window,
  'scroll',
  () => () => window.scrollY,
);

const { scrollPosition } = state('scrollPosition', 0, {
  sources: [scrollSource],
});

Form Submit ​

typescript
const form = document.querySelector('form')!;

interface FormData {
  name: string;
  email: string;
}

const submitSource = sourceFromEvent(form, 'submit', (event: Event) => {
  event.preventDefault();
  const form = event.target as HTMLFormElement;
  const formData = new FormData(form);
  return () => ({
    name: formData.get('name') as string,
    email: formData.get('email') as string,
  });
});

const { formState } = state<FormData>(
  'formState',
  { name: '', email: '' },
  {
    sources: [submitSource],
  },
);

Window Resize ​

typescript
interface WindowSize {
  width: number;
  height: number;
}

const resizeSource = sourceFromEvent(window, 'resize', () => () => ({
  width: window.innerWidth,
  height: window.innerHeight,
}));

const { windowSize } = state<WindowSize>(
  'windowSize',
  { width: window.innerWidth, height: window.innerHeight },
  {
    sources: [resizeSource],
  },
);

With Operators ​

typescript
import { debounceTime, map } from 'rxjs/operators';

const input = document.querySelector('input')!;

// Debounced input with RxJS operators
const debouncedInputSource = sourceFromEvent(
  input,
  'input',
  (event: Event) => () => (event.target as HTMLInputElement).value,
).pipe(debounceTime(300));

const { searchQuery } = state('searchQuery', '', {
  sources: [debouncedInputSource],
});

Best Practices ​

✅ Cleanup automatically handled - Sources unsubscribe when state is destroyed ✅ Use with throttle/debounce - For high-frequency events ✅ Extract to reusable functions - Create helper functions for common patterns ✅ Type your events - Use specific event types (MouseEvent, KeyboardEvent)

See Also ​