feat: follow utility

src/utils/follow.ts

@@ -0,0 +1,40 @@
+import {Node} from 'konva/lib/Node';
+import {Center, flipOrigin, Origin} from '../types';
+import {getOriginDelta, isLayoutNode} from '../components/ILayoutNode';
+
+export interface FollowSubscription {
+  dispose(): void;
+  target(newTarget: Node): void;
+}
+
+export function follow(
+  source: Node,
+  target: Node,
+  direction?: Origin,
+): FollowSubscription {
+  direction ??= isLayoutNode(source)
+    ? flipOrigin(source.getOrigin(), Center.Vertical)
+    : Origin.TopLeft;
+
+  const update = () => {
+    const rect = target.getClientRect({relativeTo: target.getLayer()});
+    const offset = getOriginDelta(rect, Origin.TopLeft, direction);
+    source.position({
+      x: rect.x + offset.x,
+      y: rect.y + offset.y,
+    });
+  };
+
+  target.on('absoluteTransformChange', update);
+  update();
+
+  return {
+    dispose: () => target.off('absoluteTransformChange', update),
+    target: (newTarget: Node) => {
+      target.off('absoluteTransformChange', update);
+      target = newTarget;
+      target.on('absoluteTransformChange', update);
+      update();
+    },
+  };
+}

src/utils/index.ts

@@ -1 +1,2 @@
-export * from './pop';
+export * from './pop';
+export * from './follow';