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Listin g 20-3. Custom NSView Drawing Method
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- (void)drawRect:(NSRect)rect { NSImage *chalkboardImage = [NSImage imageNamed:@"Chalkboard"]; NSRect imageRect; imageRect.origin = NSMakePoint(0.0,0.0); imageRect.size = [chalkboardImage size]; [chalkboardImage drawInRect:[self bounds] fromRect:imageRect operation:NSCompositeSourceOver fraction:1.0]; }
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Invalidating and Drawing Views
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Drawing in Cocoa is almost identical to that in Swing and similar GUI frameworks. When the contents of a view need to be redrawn, the region it occupies is invalidated. To invalidate an NSView, set needsDisplay to YES or send a -setNeedsDisplayInRect: message. The AppKit framework adds the region, or subregion, of the view to the conglomerate area of the user interface that needs to be redrawn. Eventually, the application framework sends each invalid view object a -drawRect: message. The message includes the specific subregion of the view that requires drawing. Unless your view is very complex, this can be ignored; all drawing is automatically clipped to the invalid region.
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Note The -drawRect: method is only responsible for drawing its content. A view is actually drawn when it
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receives a -display message. This is the high-level message that recursively sends -drawRect: messages to itself and all of its subviews. You typically don t override -display, and you d normally never send -display to a view object. To redraw a view, set its needsDisplay property and let the framework add it to the queue of view objects that need to be updated, and then wait to receive a -drawRect: message.
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CHAPTER 20 MODEL-VIEW-CONTROLLER PATTERN
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Graphics Context
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In Java, the graphics object used by the component to draw itself is passed to the JComponent.paint(Graphics) method as a parameter. In Cocoa, all drawing occurs in the implied context of the global NSGraphicsContext object. The framework will prepare the context for your view before the -drawRect: method is sent. Your view draws in the local coordinate system defined by its bounds property. You should not make any assumptions about the drawing context after your -drawRect: method has returned. The NSGraphics Context defines a number of properties that apply to all drawing commands: clipping region drawing color stroke (pen) color fill color font shadow affine transform Quite unlike the organization of java.awt.Graphics, the methods that set these properties are scattered among the classes that define them. Table 20-2 lists where to find the approximate equivalents of java.awt.Graphics2D properties in the Cocoa framework. Table 20-2. Graphics Context Property Settings
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jav a.awt.Gr aph ic s2D
setClip(int,int,int,int) clipRect(int,int,int,int) setColor(Color) setBackground(Color)
Coc oa F un cti on or Meth od
NSRectClip( ), NSRectClipList( ) -[NSBezierPath addClip] -[NSColor set], -[NSColor setStroke], -[NSColor setFill] None: NSEraseRect( ) always paints with white. Fill a rectangle with NSRectFill( ). -[NSFont set] -[NSShadow set]
setFont(Font)
setTransform(AffineTransform) setComposite(Composite)
-[NSAffineTransform set] -[NSGraphicsContext setCompositingOperation:]
Setting these properties and using them is identical to Java: set the desired properties, and then invoke a drawing command. The drawing command will use the applicable properties of the current graphics context, as demonstrated in Listing 20-4.
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CHAPTER 20 MODEL-VIEW-CONTROLLER PATTERN
Listin g 20-4. Setting and Using Graphics Context Properties
NSBezierPath *path = [[NSColor blueColor] setFill]; [[NSColor greenColor] setStroke]; [path fill]; [path stroke];
// // // //
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The graphics context also includes a loose set of esoteric attributes and rendering hints, much like Graphics2D, that influence scaling, anti-aliasing, color space adjustments, and so on. Some of the properties of Graphics2D are not properties of the graphics context in Cocoa. Instead, they are properties of the definition object. For example, the width and shape of lines drawn by an NSBezierPath are properties of the Bezier path object, not the graphics context. NSGraphicsContext has a compositingOperation property that is used for drawing commands that do not specify an explicit compositing mode as a parameter.
The Graphics Context State Stack
You may need several graphics contexts, configured differently, to accomplish your drawing tasks. It isn t always easy to undo the changes made to any one graphics context, and all subsequent drawing operations will be affected by any changes, so it s easy for the current context to become polluted. This is where the graphics context state stack is handy. The current state of the graphics context can be pushed, preserving all of its properties on a per-thread stack. You can then make whatever changes you want to the graphics context. When the previous state is restored, any changes made since it was saved are discarded. This is particularly useful when setting complex properties, like clipping, shadow, and affine transforms, that only apply to a few drawing commands. Listing 20-5 demonstrates the basic pattern for pushing and restoring a graphics context.
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