iPad SDK : New Graphics Functionality - The Basic Drawing Architecture

For maximum modularity, let's decide that each drawing operation should know how to draw itself. Then all our DudelView class really needs to do is hang onto an array and pass along a draw request whenever it's time to redraw. We'll try to maintain some sort of order here by defining a protocol called Drawable, which contains a single method called draw. Any object that represents a drawing operation should conform to this protocol. Also, there will be times—such as while the user is creating a drawing operation by dragging a finger around the screen—that some temporary drawing will need to be done. The view class won't be responsible for this, however. We'll pass that to our delegate object, the DudelViewController. So, we'll add a method to the DudelViewDelegate method, giving the controller a chance to do some temporary, context-based drawing.

Create a new protocol header file (one of the file types found in the Cocoa Touch Class section of the New File Assistant) in your project, name it Drawable.h, and give it the following content:

//  Drawable.h

@protocol Drawable
- (void)draw;
@end

Now we're ready to flesh out the DudelView class. First, add a few lines to DudelView.h:

//  DudelView.h
#import <UIKit/UIKit.h>

@protocol DudelViewDelegate
- (void)drawTemporary;
@end

@interface DudelView : UIView {
  NSMutableArray *drawables;
  IBOutlet id <DudelViewDelegate> delegate;
}
@property (retain, nonatomic) NSMutableArray *drawables;
@end

Then define the DudelView.m file as follows:

// DudelView.m
#import "DudelView.h"
#import "Drawable.h"

@implementation DudelView
@synthesize drawables;
- (id)initWithFrame:(CGRect)frame {
  if ((self = [super initWithFrame:frame])) {
    drawables = [[NSMutableArray alloc] initWithCapacity:100];
  }
  return self;
}
- (id)initWithCoder:(NSCoder *)aDecoder {
  if ((self = [super initWithCoder:aDecoder])) {
    drawables = [[NSMutableArray alloc] initWithCapacity:100];
  }
  return self;
}
- (void)drawRect:(CGRect)rect {
  for (<Drawable> d in drawables) {
    [d draw];
  }
  [delegate drawTemporary];
}

					  

- (void)dealloc {
  [drawables release];
  [super dealloc];
}
@end

You'll notice that we define two different initialization methods. The first is normally called within code; the second is called when an object is being instantiated from a nib file. In our current implementation, only the nib file version is being used, but we may as well cover the other possibility as well.

Apart from that, this code is quite straightforward. Other objects can directly add drawable display operations to the view's array. Whenever drawRect: is called (as a result of someone, somewhere, calling setNeedsDisplay on the view), the view just calls draw everywhere else.