How would I draw to a mutable in-memory bitmap?

I'm trying to write a simple painting app for iOS as a first non-trivial project. Basically, on every touch event, I need to open a graphics context on the bitmap, draw something over top of where the user left off, and close it.

UIImage is immutable, so it's not exactly suitable for my purposes; I'd have to build a new bitmap and draw the old one into the new one. I ca开发者_运维技巧n't imagine that performing well. Is there any sort of mutable bitmap class in UIKit, or am I going to have to go down to CGImageRef?

If you're willing to venture away from cocoa, I would strongly recommend using OpenGL for this purpose. Apple provide a great sample app (GLPaint) that demonstrates this. Tackling the learning curve of OpenGL will certainly pay off in terms of appearance, performance, and sheer power & flexibility.

However, if you're not up for that then another approach is to create a new CALayer subclass overriding drawInContext:, and store each drawing stroke (path and line properties) there. You can then add each 'strokeLayer' to the drawing view's layer hierarchy, and force a redraw each frame. CGLayers can also be used to increase performance (which is likely to become a big issue - when a user paints a long stroke you will see frame rates drop off very rapidly). In fact you will likely end up using a CGLayer to draw to in any case. Here is a bit of code for a drawRect: method which might help illustrate this approach:

- (void)drawRect:(CGRect)rect {
    // Setup the layer and it's context to use as a drawing buffer.
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGLayerRef drawingBuffer = CGLayerCreateWithContext(context, self.bounds.size, NULL);
    CGContextRef bufferContext = CGLayerGetContext(drawingBuffer);

    // Draw all sublayers into the drawing buffer, and display the buffer.
    [self.layer renderInContext:bufferContext];
    CGContextDrawLayerAtPoint(context, CGPointZero, drawingBuffer);
    CGLayerRelease(drawingBuffer);
}

As far as mutability goes, the most obvious thing to do would be to draw the background colour over the painting strokes. This way an eraser stroke would be exactly the same as a painting stroke, just a different colour.

You mentioned using a bitmap image, and this is really beginning to hint at OpenGL render-to-texture, where a series of point sprites (forming a line) can be drawn onto a mutable texture at very high framerates. I don't want to put a damper on things, but you will inevitably hit a performance bottleneck using Core Graphics / Quartz to do your drawing in this fashion.

I hope this helps.

You don't need to recreate offscreen context every time a new stroke is made. You might accumulate the strokes somewhere (NSMutableArray) and when a certain limit is reached, you would flatten those accumulated strokes by first drawing background to offscreen context and then the strokes you've accumulated on top of it. the resulting offscreen drawing would become a new background, so you can empty the array containing the strokes and start over. that way you take kind of a hybrid approach between storing all the strokes in memory + redrawing them every time and constantly recreating offscreen bitmap.

There's entire chapter (7) in this book http://www.deitel.com/Books/iPhone/iPhoneforProgrammersAnAppDrivenApproach/tabid/3526/Default.aspx devoted to creating a simple painting app. there you can find a link to code examples. the approach taken is storing the strokes in memory, but here are modified versions of MainView.h and .m files that take the approach I've described, !!! BUT PLEASE PAY ATTENTION TO COPYRIGHT NOTES AT THE BOTTOM OF BOTH FILES !!!:

    //  MainView.m
//  View for the frontside of the Painter app.
#import "MainView.h"

const NSUInteger kThreshold = 2;

@implementation MainView

@synthesize color; // generate getters and setters for color
@synthesize lineWidth; // generate getters and setters for lineWidth

CGContextRef CreateBitmapContext(NSUInteger w, NSUInteger h);

void * globalBitmapData = NULL;

// method is called when the view is created in a nib file
- (id)initWithCoder:(NSCoder*)decoder
{
   // if the superclass initializes properly
   if (self = [super initWithCoder:decoder])
   {
      // initialize squiggles and finishedSquiggles
      squiggles = [[NSMutableDictionary alloc] init];
      finishedSquiggles = [[NSMutableArray alloc] init];

      // the starting color is black
      color = [[UIColor alloc] initWithRed:0 green:0 blue:0 alpha:1];
      lineWidth = 5; // default line width

       flattenedImage_ = NULL;
   } // end if

   return self; // return this objeoct
} // end method initWithCoder:

// clears all the drawings
- (void)resetView
{
   [squiggles removeAllObjects]; // clear the dictionary of squiggles
   [finishedSquiggles removeAllObjects]; // clear the array of squiggles
   [self setNeedsDisplay]; // refresh the display
} // end method resetView

// draw the view
- (void)drawRect:(CGRect)rect
{   
   // get the current graphics context
    CGContextRef context = UIGraphicsGetCurrentContext();

    if(flattenedImage_)
    {
        CGContextDrawImage(context, CGRectMake(0,0,CGRectGetWidth(self.bounds), CGRectGetHeight(self.bounds)), flattenedImage_);
    }

   // draw all the finished squiggles

   for (Squiggle *squiggle in finishedSquiggles)
      [self drawSquiggle:squiggle inContext:context];

   // draw all the squiggles currently in progress
   for (NSString *key in squiggles)
   {
      Squiggle *squiggle = [squiggles valueForKey:key]; // get squiggle
      [self drawSquiggle:squiggle inContext:context]; // draw squiggle
   } // end for
} // end method drawRect:

// draws the given squiggle into the given context
- (void)drawSquiggle:(Squiggle *)squiggle inContext:(CGContextRef)context
{
   // set the drawing color to the squiggle's color
   UIColor *squiggleColor = squiggle.strokeColor; // get squiggle's color
   CGColorRef colorRef = [squiggleColor CGColor]; // get the CGColor
   CGContextSetStrokeColorWithColor(context, colorRef);

   // set the line width to the squiggle's line width
   CGContextSetLineWidth(context, squiggle.lineWidth);

   NSMutableArray *points = [squiggle points]; // get points from squiggle

   // retrieve the NSValue object and store the value in firstPoint
   CGPoint firstPoint; // declare a CGPoint
   [[points objectAtIndex:0] getValue:&firstPoint];

   // move to the point
   CGContextMoveToPoint(context, firstPoint.x, firstPoint.y);

    // draw a line from each point to the next in order
   for (int i = 1; i < [points count]; i++)
   {
      NSValue *value = [points objectAtIndex:i]; // get the next value
      CGPoint point; // declare a new point
      [value getValue:&point]; // store the value in point

      // draw a line to the new point
      CGContextAddLineToPoint(context, point.x, point.y);
   } // end for

   CGContextStrokePath(context);   
} // end method drawSquiggle:inContext:

// called whenever the user places a finger on the screen
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
   NSArray *array = [touches allObjects]; // get all the new touches

   // loop through each new touch
   for (UITouch *touch in array)
   {
      // create and configure a new squiggle
      Squiggle *squiggle = [[Squiggle alloc] init];
      [squiggle setStrokeColor:color]; // set squiggle's stroke color
      [squiggle setLineWidth:lineWidth]; // set squiggle's line width

      // add the location of the first touch to the squiggle
      [squiggle addPoint:[touch locationInView:self]];

      // the key for each touch is the value of the pointer
      NSValue *touchValue = [NSValue valueWithPointer:touch];
      NSString *key = [NSString stringWithFormat:@"%@", touchValue];

      // add the new touch to the dictionary under a unique key
      [squiggles setValue:squiggle forKey:key];
      [squiggle release]; // we are done with squiggle so release it
   } // end for
} // end method touchesBegan:withEvent:

// called whenever the user drags a finger on the screen
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
   NSArray *array = [touches allObjects]; // get all the moved touches

   // loop through all the touches
   for (UITouch *touch in array)
   {
      // get the unique key for this touch
      NSValue *touchValue = [NSValue valueWithPointer:touch];

      // fetch the squiggle this touch should be added to using the key
      Squiggle *squiggle = [squiggles valueForKey:
         [NSString stringWithFormat:@"%@", touchValue]];

      // get the current and previous touch locations
      CGPoint current = [touch locationInView:self];
      CGPoint previous = [touch previousLocationInView:self];
      [squiggle addPoint:current]; // add the new point to the squiggle

      // Create two points: one with the smaller x and y values and one
      // with the larger. This is used to determine exactly where on the
      // screen needs to be redrawn.
      CGPoint lower, higher;
      lower.x = (previous.x > current.x ? current.x : previous.x);
      lower.y = (previous.y > current.y ? current.y : previous.y);
      higher.x = (previous.x < current.x ? current.x : previous.x);
      higher.y = (previous.y < current.y ? current.y : previous.y);

      // redraw the screen in the required region
      [self setNeedsDisplayInRect:CGRectMake(lower.x-lineWidth,
         lower.y-lineWidth, higher.x - lower.x + lineWidth*2,
         higher.y - lower.y + lineWidth * 2)];
   } // end for
} // end method touchesMoved:withEvent:

// called when the user lifts a finger from the screen
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
   // loop through the touches
   for (UITouch *touch in touches)
   {
      // get the unique key for the touch
      NSValue *touchValue = [NSValue valueWithPointer:touch];
      NSString *key = [NSString stringWithFormat:@"%@", touchValue];

      // retrieve the squiggle for this touch using the key
      Squiggle *squiggle = [squiggles valueForKey:key];

      // remove the squiggle from the dictionary and place it in an array
      // of finished squiggles
      [finishedSquiggles addObject:squiggle]; // add to finishedSquiggles
      [squiggles removeObjectForKey:key]; // remove from squiggles

    if([finishedSquiggles count] > kThreshold)  
    {   
        CGContextRef context = CreateBitmapContext(CGRectGetWidth(self.bounds), CGRectGetHeight(self.bounds));

        if(flattenedImage_)
        {
            CGContextDrawImage(context, CGRectMake(0,0,CGRectGetWidth(self.bounds), CGRectGetHeight(self.bounds)), flattenedImage_);
        }

       for (Squiggle *squiggle in finishedSquiggles)
          [self drawSquiggle:squiggle inContext:context];

        CGImageRef imgRef = CGBitmapContextCreateImage(context);
        CGContextRelease(context);
        if(flattenedImage_ != NULL)
            CFRelease(flattenedImage_);

        flattenedImage_ = imgRef;

        [finishedSquiggles removeAllObjects];
    }
   } // end for   
} // end method touchesEnded:withEvent:

// called when a motion event, such as a shake, ends
- (void)motionEnded:(UIEventSubtype)motion withEvent:(UIEvent *)event
{
   // if a shake event ended
   if (event.subtype == UIEventSubtypeMotionShake)
   {
      // create an alert prompting the user about clearing the painting
      NSString *message = @"Are you sure you want to clear the painting?";
      UIAlertView *alert = [[UIAlertView alloc] initWithTitle:
         @"Clear painting" message:message delegate:self
         cancelButtonTitle:@"Cancel" otherButtonTitles:@"Clear", nil];
      [alert show]; // show the alert
      [alert release]; // release the alert UIAlertView
   } // end if

   // call the superclass's moetionEnded:withEvent: method
   [super motionEnded:motion withEvent:event];
} // end method motionEnded:withEvent:

// clear the painting if the user touched the "Clear" button
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:
   (NSInteger)buttonIndex
{
   // if the user touched the Clear button
   if (buttonIndex == 1)
      [self resetView]; // clear the screen
} // end method alertView:clickedButtonAtIndex:

// determines if this view can become the first responder
- (BOOL)canBecomeFirstResponder
{
   return YES; // this view can be the first responder
} // end method canBecomeFirstResponder

// free MainView's memory
- (void)dealloc
{
   [squiggles release]; // release the squiggles NSMutableDictionary
   [finishedSquiggles release]; // release finishedSquiggles
   [color release]; // release the color UIColor
   [super dealloc];
} // end method dealloc
@end

CGContextRef CreateBitmapContext(NSUInteger w, NSUInteger h)
{
    CGContextRef    context = NULL;

    int             bitmapByteCount;
    int             bitmapBytesPerRow;

    bitmapBytesPerRow   = (w * 4);
    bitmapByteCount     = (bitmapBytesPerRow * h);

    if(globalBitmapData == NULL)
        globalBitmapData = malloc( bitmapByteCount );
    memset(globalBitmapData, 0, sizeof(globalBitmapData));
    if (globalBitmapData == NULL)
    {
        return nil;
    }

    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();

    context = CGBitmapContextCreate (globalBitmapData,w,h,8,bitmapBytesPerRow,
                                     colorspace,kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colorspace);

    return context;
}



/**************************************************************************
 * (C) Copyright 2010 by Deitel & Associates, Inc. All Rights Reserved.   *
 *                                                                        *
 * DISCLAIMER: The authors and publisher of this book have used their     *
 * best efforts in preparing the book. These efforts include the          *
 * development, research, and testing of the theories and programs        *
 * to determine their effectiveness. The authors and publisher make       *
 * no warranty of any kind, expressed or implied, with regard to these    *
 * programs or to the documentation contained in these books. The authors *
 * and publisher shall not be liable in any event for incidental or       *
 * consequential damages in connection with, or arising out of, the       *
 * furnishing, performance, or use of these programs.                     *
 *                                                                        *
 * As a user of the book, Deitel & Associates, Inc. grants you the        *
 * nonexclusive right to copy, distribute, display the code, and create   *
 * derivative apps based on the code for noncommercial purposes only--so  *
 * long as you attribute the code to Deitel & Associates, Inc. and        *
 * reference www.deitel.com/books/iPhoneFP/. If you have any questions,   *
 * or specifically would like to use our code for commercial purposes,    *
 * contact deitel@deitel.com.                                             *
 *************************************************************************/




    // MainView.h
// View for the frontside of the Painter app.
// Implementation in MainView.m
#import <UIKit/UIKit.h>
#import "Squiggle.h"

@interface MainView : UIView
{
   NSMutableDictionary *squiggles; // squiggles in progress
   NSMutableArray *finishedSquiggles; // finished squiggles
   UIColor *color; // the current drawing color
   float lineWidth; // the current drawing line width

    CGImageRef flattenedImage_;
} // end instance variable declaration

// declare color and lineWidth as properties
@property(nonatomic, retain) UIColor *color;
@property float lineWidth;

// draw the given Squiggle into the given graphics context
- (void)drawSquiggle:(Squiggle *)squiggle inContext:(CGContextRef)context; 
- (void)resetView; // clear all squiggles from the view
@end // end interface MainView

/**************************************************************************
 * (C) Copyright 2010 by Deitel & Associates, Inc. All Rights Reserved.   *
 *                                                                        *
 * DISCLAIMER: The authors and publisher of this book have used their     *
 * best efforts in preparing the book. These efforts include the          *
 * development, research, and testing of the theories and programs        *
 * to determine their effectiveness. The authors and publisher make       *
 * no warranty of any kind, expressed or implied, with regard to these    *
 * programs or to the documentation contained in these books. The authors *
 * and publisher shall not be liable in any event for incidental or       *
 * consequential damages in connection with, or arising out of, the       *
 * furnishing, performance, or use of these programs.                     *
 *                                                                        *
 * As a user of the book, Deitel & Associates, Inc. grants you the        *
 * nonexclusive right to copy, distribute, display the code, and create   *
 * derivative apps based on the code for noncommercial purposes only--so  *
 * long as you attribute the code to Deitel & Associates, Inc. and        *
 * reference www.deitel.com/books/iPhoneFP/. If you have any questions,   *
 * or specifically would like to use our code for commercial purposes,    *
 * contact deitel@deitel.com.                                             *
 *************************************************************************/

So you would basically replace the original versions of those files in the project to get desired behavior