chore: rename api folder to composition

src/composition/use-relation.ts

@@ -0,0 +1,27 @@
+import { inject, computed, onUnmounted } from 'vue';
+
+export type Parent<T = unknown> = null | {
+  children: T[];
+};
+
+export function useParent<T = unknown>(key: string, child: T = {} as T) {
+  const parent = inject<Parent<T>>(key, null);
+
+  if (parent) {
+    const { children } = parent;
+    const index = computed(() => children.indexOf(child));
+
+    children.push(child);
+
+    onUnmounted(() => {
+      children.splice(index.value, 1);
+    });
+
+    return {
+      index,
+      parent,
+    };
+  }
+
+  return {};
+}