refactor(web): extract allocation multi-drag session

apps/web/src/hooks/timelineAllocationMultiDragSession.ts

@@ -0,0 +1,75 @@
+type MutableCurrent<T> = {
+  current: T;
+};
+
+type AttachDocumentMouseDrag = (
+  documentTarget: Document,
+  onMove: (event: MouseEvent) => void,
+  onUp: (event: MouseEvent) => void,
+) => () => void;
+
+type MultiDragStateLike<TMode extends string = string> = {
+  selectedAllocationIds: string[];
+  multiDragDaysDelta: number;
+  isMultiDragging: boolean;
+  multiDragMode: TMode;
+};
+
+type BeginAllocationMultiDragSessionParams<TState extends MultiDragStateLike, TMode extends string> = {
+  startMouseX: number;
+  dragMode: TMode;
+  documentTarget: Document;
+  cleanupRef: MutableCurrent<(() => void) | null>;
+  stateRef: MutableCurrent<TState>;
+  setState: (state: TState) => void;
+  startState: (state: TState, dragMode: TMode) => TState;
+  updateState: (state: TState, daysDelta: number) => TState | null;
+  finalizeState: (state: TState) => TState;
+  toDaysDelta: (deltaX: number) => number;
+  onComplete: (daysDelta: number, finalState: TState) => void;
+  attachDrag: AttachDocumentMouseDrag;
+};
+
+export function beginAllocationMultiDragSession<TState extends MultiDragStateLike, TMode extends string>({
+  startMouseX,
+  dragMode,
+  documentTarget,
+  cleanupRef,
+  stateRef,
+  setState,
+  startState,
+  updateState,
+  finalizeState,
+  toDaysDelta,
+  onComplete,
+  attachDrag,
+}: BeginAllocationMultiDragSessionParams<TState, TMode>) {
+  const initialState = startState(stateRef.current, dragMode);
+  stateRef.current = initialState;
+  setState(initialState);
+  cleanupRef.current?.();
+
+  function handleMove(event: MouseEvent) {
+    const updated = updateState(stateRef.current, toDaysDelta(event.clientX - startMouseX));
+    if (!updated) return;
+
+    stateRef.current = updated;
+    setState(updated);
+  }
+
+  function handleUp(event: MouseEvent) {
+    cleanupRef.current?.();
+    cleanupRef.current = null;
+
+    const finalState = finalizeState(stateRef.current);
+    stateRef.current = finalState;
+    setState(finalState);
+
+    const finalDelta = toDaysDelta(event.clientX - startMouseX);
+    if (finalDelta !== 0) {
+      onComplete(finalDelta, finalState);
+    }
+  }
+
+  cleanupRef.current = attachDrag(documentTarget, handleMove, handleUp);
+}