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  "identifier" : "/documentation/AppKit/NSOutlineViewDataSource/outlineView(_:numberOfChildrenOfItem:)",
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# outlineView(_:numberOfChildrenOfItem:)

Returns the number of child items encompassed by a given item.

```
@MainActor optional func outlineView(_ outlineView: NSOutlineView, numberOfChildrenOfItem item: Any?) -> Int
```

## Parameters

`outlineView`

The outline view that sent the message.

`item`

An item in the data source.

## Return Value

The number of child items encompassed by `item`. If `item` is `nil`, this method should return the number of children for the top-level item.

## Discussion

The [`outlineView(_:numberOfChildrenOfItem:)`](/documentation/AppKit/NSOutlineViewDataSource/outlineView(_:numberOfChildrenOfItem:)) method is called very frequently, so it must be efficient.

> Important:
> While this method is marked as `@optional` in the protocol, **you must implement this method if you are not providing the data for the outline view using Cocoa bindings.**
> 
> Do not call ``doc://com.apple.appkit/documentation/AppKit/NSTableView/reloadData()`` from this method.

---

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