Вам не нужно воссоздавать закадровый контекст каждый раз, когда делается новый штрих.Вы можете где-то накапливать обводки (NSMutableArray), и когда будет достигнут определенный предел, вы сгладите эти накопленные обводки, сначала рисуя фон в закадровом контексте, а затем обводки, которые вы накопили поверх него.результирующий закадровый рисунок станет новым фоном, поэтому вы можете очистить массив, содержащий штрихи, и начать заново.Таким образом, вы используете гибридный подход между хранением всех штрихов в памяти + перерисовкой их каждый раз и постоянным воссозданием закадрового изображения.
В этой книге есть целая глава (7) http://www.deitel.com/Books/iPhone/iPhoneforProgrammersAnAppDrivenApproach/tabid/3526/Default.aspx, посвященная созданию простого приложения для рисования.там вы можете найти ссылку на примеры кода.выбранный подход хранит штрихи в памяти, но здесь есть модифицированные версии файлов MainView.h и .m, которые используют описанный мной подход !!!НО, ПОЖАЛУЙСТА, ОБРАТИТЕ ВНИМАНИЕ НА ЗАМЕТКИ ОБ АВТОРСКИХ ПРАВАХ НА ДНИ ОБОИХ ФАЙЛОВ !!!:
// 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. *
*************************************************************************/
Таким образом, вы в основном замените исходные версии этих файлов в проекте, чтобы получить желаемое поведение