Added getAt function to collections

packages/@react-stately/grid/src/GridCollection.ts

@@ -150,4 +150,9 @@ export class GridCollection<T> implements IGridCollection<T> {
   getItem(key: Key) {
     return this.keyMap.get(key);
   }
+
+  at(idx: number) {
+    const keys = [...this.getKeys()];
+    return this.getItem(keys[idx]);
+  }
 }

packages/@react-stately/list/src/ListCollection.ts

@@ -94,4 +94,9 @@ export class ListCollection<T> implements Collection<Node<T>> {
   getItem(key: Key) {
     return this.keyMap.get(key);
   }
+
+  at(idx: number) {
+    const keys = [...this.getKeys()];
+    return this.getItem(keys[idx]);
+  }
 }

packages/@react-stately/table/src/TableCollection.ts

@@ -272,4 +272,9 @@ export class TableCollection<T> extends GridCollection<T> {
   getItem(key: Key) {
     return this.keyMap.get(key);
   }
+
+  at(idx: number) {
+    const keys = [...this.getKeys()];
+    return this.getItem(keys[idx]);
+  }
 }

packages/@react-stately/tree/src/TreeCollection.ts

@@ -95,4 +95,9 @@ export class TreeCollection<T> implements Collection<Node<T>> {
   getItem(key: Key) {
     return this.keyMap.get(key);
   }
+
+  at(idx: number) {
+    const keys = [...this.getKeys()];
+    return this.getItem(keys[idx]);
+  }
 }

packages/@react-types/shared/src/collections.d.ts

@@ -130,6 +130,9 @@ export interface Collection<T> extends Iterable<T> {
   /** Get an item by its key. */
   getItem(key: Key): T,
 
+  /** Get an item by the index of its key. */
+  at(idx: number): T,
+
   /** Get the key that comes before the given key in the collection. */
   getKeyBefore(key: Key): Key | null,