Learning Cocoa

NSView

All objects that inherit from NSView can render themselves on the screen. To be displayed, an NSView must be placed in an NSWindow. NSViews draw themselves by making calls to the Core Graphics (Quartz) programming interface.

Graphically, an NSView can be regarded as a framed canvas. The frame locates the NSView in its superview, defines its size, and clips drawing to its edges, while the canvas defines the NSView’s own internal coordinate system and hosts the actual drawing. The frame can be moved around, resized, and rotated in the superview, so that the NSView’s image moves with it. Similarly, the canvas can be shifted, stretched, and rotated, so that the drawn image moves within the frame.

NSView represents a context within which drawing can take place. This context has three components:

• A rectangular frame within a window to which drawing is clipped

• A coordinate system

• The current graphics state

NSView draws itself as an indirect result of receiving the display message (or a variant of display); this message is sent explicitly or through conditions that cause automatic display. The display message leads to the invocation of an NSView’s drawRect: method and the drawRect: methods of all subviews of that NSView. The drawRect: method should contain all code needed to redraw the NSView completely.

An NSView object can be automatically displayed when:

• Users scroll it (assuming it supports scrolling)

• Users resize or expose the NSView’s window

• The window receives a display message ...