Question

I have a very large UIView approx 3000×3000 in size. In this large view I do some free form drawing with stylus or finger on an iPad. Here is my code for my touches began and moved methods.

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{    

        Line_Point * to_be_added = [[Line_Point alloc] init];
        to_be_added.line_color = [UIColor colorWithCGColor: current_color.CGColor];
        to_be_added.line_pattern = [NSString stringWithString: current_line_pattern]; 

        UITouch *touch = [[event touchesForView:self] anyObject];

        CGPoint location = [touch locationInView:self];
        to_be_added.point = [NSValue valueWithCGPoint:(location)];  

        if (self.points == nil)
        {
            NSMutableArray *newPoints = [[NSMutableArray alloc] init];
            self.points = newPoints;
            [newPoints release];
        }

        [self.points addObject:to_be_added];           



}

Here is my touches moved method which seems to be causing the issue because it isnt getting called enough because the app calls setNeedsDisplay

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{

        Line_Point * to_be_added = [[Line_Point alloc] init];
        to_be_added.line_color = [UIColor colorWithCGColor: current_color.CGColor];
        to_be_added.line_pattern = [NSString stringWithString: current_line_pattern];

        UITouch * touch = [[event touchesForView:self] anyObject];
        CGPoint location = [touch locationInView:self];
        to_be_added.point = [NSValue valueWithCGPoint:(location)];

        [self.points addObject:to_be_added];           


        [self setNeedsLayout]; 

}

Now here is my drawRect method which displays the drawing

-(void)drawRect:(CGRect)rect
{
 if (self.points.count == 0)        
    {

    }
    else 
    {    
        Line_Point * first = [self.points objectAtIndex:0];
        CGContextRef context2 = UIGraphicsGetCurrentContext();
        CGContextSetStrokeColorWithColor(context2, first.line_color.CGColor);
        if ([first.line_pattern isEqualToString:@"normal"]) 
        {
            CGContextSetLineWidth(context2, 4.0);
            CGContextSetLineDash(context2, 0, NULL, 0);
        }
        else if([first.line_pattern isEqualToString:@"thick"])            
        {
            CGContextSetLineWidth(context2, 6.0);
            CGContextSetLineDash(context2, 0, NULL, 0);
        }
        else if([first.line_pattern isEqualToString:@"dotted"])            
        {
            CGFloat Pattern[] = {5, 5, 5, 5};
            CGContextSetLineDash(context2, 0, Pattern, 4);
        }
        else if([first.line_pattern isEqualToString:@"super_dotted"])            
        {
            CGFloat Pattern[] = {1, 2, 1, 2}; 
            CGContextSetLineDash(context2, 0, Pattern, 4); 
        }           
        CGPoint firstPoint2 = [first.point CGPointValue];
        CGContextBeginPath(context2);
        CGContextMoveToPoint(context2, firstPoint2.x, firstPoint2.y);

        int i2 = 1;
        while (i2 < self.points.count)
        {
            Line_Point * nextPoint = [self.points objectAtIndex:i2]; 

            if (nextPoint.point.CGPointValue.x < 0 && nextPoint.point.CGPointValue.y < 0)
            {
                CGContextDrawPath(context2, kCGPathStroke);

                if (i2 < (self.points.count-1))
                {

                    CGContextBeginPath(context2);
                    Line_Point * nextPoint2 = [self.points objectAtIndex:i2+1];                
                    CGContextMoveToPoint(context2, nextPoint2.point.CGPointValue.x, nextPoint2.point.CGPointValue.y);
                    CGContextSetStrokeColorWithColor(context2, nextPoint2.line_color.CGColor);
                    if ([nextPoint2.line_pattern isEqualToString:@"normal"]) 
                    {
                        CGContextSetLineWidth(context2, 4.0);
                        CGContextSetLineDash(context2, 0, NULL, 0);
                    }
                    else if([nextPoint2.line_pattern isEqualToString:@"thick"])            
                    {
                        CGContextSetLineWidth(context2, 6.0);
                        CGContextSetLineDash(context2, 0, NULL, 0);
                    }
                    else if([nextPoint2.line_pattern isEqualToString:@"dotted"])            
                    {
                        CGFloat Pattern[] = {5, 5, 5, 5};
                        CGContextSetLineDash(context2, 0, Pattern, 4);  
                    }
                    else if([nextPoint2.line_pattern isEqualToString:@"super_dotted"])            
                    {
                        CGFloat Pattern[] = {1, 2, 1, 2}; 
                        CGContextSetLineDash(context2, 0, Pattern, 4);
                    }        
                    i2 = i2 + 2;

                }
                else
                    i2++;
            }
            else
            {
                CGContextAddLineToPoint(context2, nextPoint.point.CGPointValue.x, nextPoint.point.CGPointValue.y);
                i2++;
            }
        }

        CGContextDrawPath(context2, kCGPathStroke);
    }

The app lags incredibly when drawing in the free form mode. I believe it is because the view is too large and setNeedsDisplay is reloading the entire view every time the touches moved method is called. Is there a way to speed up the setNeedsDisplay method so the free form drawing does not lag on the iPad. Can i just target the context where the drawing is occurring not reload the whole thing? When I adjust the size to about 1000×1000 it does not have the lag problem.

Thanks for any help on the matter.

EDIT TRYED USING
-SetNeedsDisplayinRect

The UIView is wrapped inside a UISCrollview and here is the code.

   -(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
   {

    Line_Point * to_be_added = [[Line_Point alloc] init];
    to_be_added.line_color = [UIColor colorWithCGColor: current_color.CGColor];
    to_be_added.line_pattern = [NSString stringWithString: current_line_pattern];

    UITouch * touch = [[event touchesForView:self] anyObject];
    CGPoint location = [touch locationInView:self];
    to_be_added.point = [NSValue valueWithCGPoint:(location)];

    [self.points addObject:to_be_added];           


          CGRect visibleRect;
        visibleRect.origin = big_view.scrollable_view.contentOffset;
        visibleRect.size = CGSizeMake(950, 500);            
        [self setNeedsDisplayInRect:visibleRect];  

 }

This method of approach does not help either.

Thanks again if you can help.

Answer 1

you can use

- (void)setNeedsDisplayInRect:(CGRect)invalidRect

instead of -setNeedsDisplay

another solution (if there is too many points and the app lags because of drawing them) – not to render all points every time when -drawInRect is called. You can, for example, at every 10-th call of it save image from current context and then display only image.

Update:

try adding

+(Class)layerClass
{
return [CATiledLayer class];
}

to your view in scrollView