# splitView(_:effectiveRect:forDrawnRect:ofDividerAt:) Allows the delegate to modify the rectangle where mouse clicks initiate divider dragging. ``` @MainActor optional func splitView(_ splitView: NSSplitView, effectiveRect proposedEffectiveRect: NSRect, forDrawnRect drawnRect: NSRect, ofDividerAt dividerIndex: Int) -> NSRect ``` ## Parameters `splitView` The split view that sends the message. `proposedEffectiveRect` The proposed rectangle where mouse clicks initiate divider dragging. The rectangle uses the coordinate system that `splitView` defines. `drawnRect` The frame of the divider in the coordinate system that `splitView` defines. `dividerIndex` The index of the divider. ## Return Value A rectangle, in the coordinate system that `splitView` defines, where mouse clicks initiate divider dragging. ## Discussion A split view with thick dividers proposes the drawn frame as the effective frame. A split view with thin dividers proposes an effective frame that’s a little larger than the drawn frame to make it easier for the user to grab the divider. If a split view has no delegate, or if its delegate doesn’t respond to this message, the split view behaves as if it has a delegate that returns `proposedEffectiveRect` when it receives this message. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)