windowWillResizetoSize

windowWillResize:toSize: is a delegate method in the NSWindowDelegate protocol in AppKit. It informs a window’s delegate that the window is about to be resized and provides an opportunity to adjust or veto the proposed size. The delegate can alter the size that will actually be used for the window as the user drags to resize.

Signature. In Objective-C, the method is declared as - (NSSize)windowWillResize:(NSWindow *)sender toSize:(NSSize)proposedSize;. In Swift, the corresponding signature is func windowWillResize(_ sender: NSWindow, toSize proposedSize: NSSize) -> NSSize. The method receives the window sending the notification and the proposed size that AppKit would apply by default.

Behavior and timing. The method is called during live resizing, when the user drags the window’s edge or corner. The return value determines the final size to apply. Returning the proposedSize leaves the resize unchanged, while returning a modified NSSize constrains or adjusts the resize according to the delegate’s logic.

Common usage. Typical uses include enforcing minimum or maximum window dimensions, restricting the aspect ratio, or snapping sizes to a grid. Implementations can clamp width and height, or compute a new size that preserves content layout or usability constraints.

Notes. The method is optional; you implement it on a class that conforms to NSWindowDelegate. If you need post-resize actions, consider other related delegate methods such as windowDidEndLiveResize or windowDidResize to respond after the resize completes.