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PODraw rectangle using Core Graphics with live preview
 

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  1. PODraw rectangle using Core Graphics with live preview
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    <p>I'm creating a simple drawing application and would like some help with drawing rectangles based on the user's touch events. I'm able to draw a rectangle using Core Graphics from the point in touchesBegan to the point in touchesEnded. This rectangle is then displayed once the touchesEnded event is fired. </p> <p>However, I would like to be able to do this in a 'live' fashion. Meaning, the rectangle is updated and displayed as the user drags their finger. Is this possible? Any help would be greatly appreciated, thank you!</p> <p><strong>UPDATE:</strong></p> <p>I was able to get this to work by using the following code. It works perfectly fine for small images, however it get's very slow for large images. I realize my solution is hugely inefficient and was wondering if anyone could suggest a better solution. Thanks.</p> <pre><code> - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event { UITouch *touch = [touches anyObject]; _firstPoint = [touch locationInView:self.view]; _firstPoint.y -= 40; _lastPoint = _firstPoint; } - (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event { UITouch *touch = [touches anyObject]; CGPoint currentPoint = [touch locationInView:self.view]; currentPoint.y -= 40; [self drawSquareFrom:_firstPoint to:currentPoint]; _lastPoint = currentPoint; } - (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event { UITouch *touch = [touches anyObject]; CGPoint currentPoint = [touch locationInView:self.view]; currentPoint.y -= 40; [self drawSquareFrom:_firstPoint to:currentPoint]; [_modifiedImage release]; _modifiedImage = [[UIImage alloc] initWithCGImage:_imageView.image.CGImage]; } - (void)drawSquareFrom:(CGPoint)firstPoint to:(CGPoint)lastPoint { _imageView.image = _modifiedImage; CGFloat width = lastPoint.x - firstPoint.x; CGFloat height = lastPoint.y - firstPoint.y; _path = [UIBezierPath bezierPathWithRect:CGRectMake(firstPoint.x, firstPoint.y, width, height)]; UIGraphicsBeginImageContext( _originalImage.size ); [_imageView.image drawInRect:CGRectMake(0, 0, _originalImage.size.width, _originalImage.size.height)]; _path.lineWidth = 10; [[UIColor redColor] setStroke]; [[UIColor clearColor] setFill]; [_path fill]; [_path stroke]; _imageView.image = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); } </code></pre>
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    1. COInstead of drawing into a bitmap image, why don't you create images that are compositions of CALayer objects? (i.e. do Illustrator vs Photoshop) In this case, you could easily add a CAShapeLayer with a rectangular path to your document temporarily.. commit it to your document when the user finishes dragging.
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    2. COIn any case you could preview your rectangle using a CAShapeLayer with a rectangular path over your current image in your document view.
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