---
title: "outlineView(_:draggingSession:willBeginAt:forItems:)"
framework: appkit
role: symbol
role_heading: Instance Method
path: "appkit/nsoutlineviewdatasource/outlineview(_:draggingsession:willbeginat:foritems:)"
---

# outlineView(_:draggingSession:willBeginAt:forItems:)

Implement this method know when the given dragging session is about to begin and potentially modify the dragging session.

## Declaration

```swift
@MainActor optional func outlineView(_ outlineView: NSOutlineView, draggingSession session: NSDraggingSession, willBeginAt screenPoint: NSPoint, forItems draggedItems: [Any])
```

## Parameters

- `outlineView`: The outline view in which the drag is about to begin.
- `session`: The dragging session that is about to begin.
- `screenPoint`: The point onscreen at which the drag is to begin.
- `draggedItems`: A array of items to be dragged, excluding items for which doc://com.apple.appkit/documentation/AppKit/NSOutlineViewDataSource/outlineView(_:pasteboardWriterForItem:) returns nil.

## Discussion

Discussion The draggedItems array directly matches the pasteboard writer array used to begin the dragging session with the NSView method beginDraggingSession(with:event:source:). Hence, the order is deterministic, and can be used in outlineView(_:acceptDrop:item:childIndex:) when enumerating the NSDraggingInfo protocol’s pasteboard classes.

## See Also

### Instance Methods

- [outlineView(_:acceptDrop:item:childIndex:)](appkit/nsoutlineviewdatasource/outlineview(_:acceptdrop:item:childindex:).md)
- [outlineView(_:child:ofItem:)](appkit/nsoutlineviewdatasource/outlineview(_:child:ofitem:).md)
- [outlineView(_:draggingSession:endedAt:operation:)](appkit/nsoutlineviewdatasource/outlineview(_:draggingsession:endedat:operation:).md)
- [outlineView(_:isItemExpandable:)](appkit/nsoutlineviewdatasource/outlineview(_:isitemexpandable:).md)
- [outlineView(_:itemForPersistentObject:)](appkit/nsoutlineviewdatasource/outlineview(_:itemforpersistentobject:).md)
- [outlineView(_:namesOfPromisedFilesDroppedAtDestination:forDraggedItems:)](appkit/nsoutlineviewdatasource/outlineview(_:namesofpromisedfilesdroppedatdestination:fordraggeditems:).md)
- [outlineView(_:numberOfChildrenOfItem:)](appkit/nsoutlineviewdatasource/outlineview(_:numberofchildrenofitem:).md)
- [outlineView(_:objectValueFor:byItem:)](appkit/nsoutlineviewdatasource/outlineview(_:objectvaluefor:byitem:).md)
- [outlineView(_:pasteboardWriterForItem:)](appkit/nsoutlineviewdatasource/outlineview(_:pasteboardwriterforitem:).md)
- [outlineView(_:persistentObjectForItem:)](appkit/nsoutlineviewdatasource/outlineview(_:persistentobjectforitem:).md)
- [outlineView(_:setObjectValue:for:byItem:)](appkit/nsoutlineviewdatasource/outlineview(_:setobjectvalue:for:byitem:).md)
- [outlineView(_:sortDescriptorsDidChange:)](appkit/nsoutlineviewdatasource/outlineview(_:sortdescriptorsdidchange:).md)
- [outlineView(_:updateDraggingItemsForDrag:)](appkit/nsoutlineviewdatasource/outlineview(_:updatedraggingitemsfordrag:).md)
- [outlineView(_:validateDrop:proposedItem:proposedChildIndex:)](appkit/nsoutlineviewdatasource/outlineview(_:validatedrop:proposeditem:proposedchildindex:).md)
- [outlineView(_:writeItems:to:)](appkit/nsoutlineviewdatasource/outlineview(_:writeitems:to:).md)