Views and Drawing
Nearly all drawing in Cocoa is done inside views. Views are objects that represent a visual portion of a window. Each view object is responsible for displaying some visual content and responding to user events in its visible area. A view may also be responsible for one or more subviews.
The NSView class is the base class for all view-related objects. Cocoa defines several types of views for displaying standard content, including text views, split views, tab views, ruler views, and so on. Cocoa controls are also based on the NSView class and implement interface elements such as buttons, scrollers, tables, and text fields.
In addition to the standard views and controls, you can also create your own custom views. You create custom views in cases where the behavior you are looking for is not provided by any of the standard views. Cocoa notifies your view that it needs to draw itself by sending your view a drawRect: message. Your implementation of the drawRect: method is where all of your drawing code goes.
Note: Although you can also subclass the standard views and controls to implement custom behavior, it is recommended that you try to use a delegate object whenever possible instead. If you do subclass a standard control, avoid changing the appearance of that control. Doing so goes against the guidance in Apple Human Interface Guidelines.
By default, window updates occur only in response to user actions. This means that your view’s drawRect: method is called only when something about your view has changed. For example, Cocoa calls the method when a scrolling action causes a previously hidden part of your view to be exposed. Cocoa also calls it in response to requests from your own code. If the information displayed by your custom view changes, you must tell Cocoa explicitly that you want the appropriate parts of your view updated. You do so by invalidating parts of your view’s visible area. Cocoa collects the invalidated regions together and generates appropriate drawRect: messages to redraw the content.
Although there are numerous ways to draw, a basic drawRect: method has the following structure:
- (void)drawRect:(NSRect)rect |
{ |
// Draw your content |
} |
That's it! By the time your drawRect: method is called, Cocoa has already locked the drawing focus on your view, saved the graphics state, adjusted the current transform matrix to your view's origin, and adjusted the clipping rectangle to your view's frame. All you have to do is draw your content.
In reality, your drawRect: method is often much more complicated. Your own method might use several other objects and methods to handle the actual drawing. You also might need to save and restore the graphics state one or more times. Because this single method is used for all of your view's drawing, it also has to handle several different situations. For example, you might want to do more precise drawing during printing or use heavily optimized code during a live resizing operation. The options are numerous and covered in more detail in subsequent chapters.
For additional information about views and live resizing, see View Programming Guide for Cocoa. For more information about printing in Cocoa, see Customizing a View's Drawing for Printing in Printing Programming Topics for Cocoa.
Last updated: 2007-10-31
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