NSViews

NSViews are the fundamental building blocks for user interfaces in AppKit, the macOS UI framework. An NSView represents a rectangular region within a window, participates in a view hierarchy, and is responsible for drawing, handling user input, performing layout, and managing subviews. Views are arranged in a superview–subview relationship and ultimately belong to a window’s content view. By default, AppKit uses a bottom-left coordinate origin, but subclasses can override isFlipped to adopt a top-left origin for compatibility with other toolkits.

Rendering and drawing are performed by subclasses that override drawRect:. The system marks regions as dirty and calls drawRect: to render content. A view can request redraws with setNeedsDisplay:. Many views can be backed by Core Animation layers, enabling efficient compositing and animations. Layer-backed rendering is enabled by setting wantsLayer to YES and assigning a CALayer to view.layer, which also allows properties such as shadows and corner radius.

Event handling is a key role of NSView. Views receive mouse and keyboard input through methods such as mouseDown:, mouseDragged:, mouseUp:, rightMouseDown:, scrollWheel:, keyDown:, and flagsChanged:. Hit testing via hitTest: determines which view should receive an event. A view can become part of the window’s responder chain by taking the first responder status.

Auto Layout integration is supported. NSView participates in constraints, and the translatesAutoresizingMaskIntoConstraints property controls whether the view’s autoresizing mask is converted into constraints. Layout is managed through layout and layoutSubtreeIfNeeded. Accessibility is supported via the NSAccessibility protocol, enabling assistive technologies to interact with views.