UIView with drawRect / update image performance issue on Retina displays

Question

Shortly speaking:

LooksLike drawRectitself (even empty) leads to significant performance bottlenecks depending on the resolution of the device - a larger screen is worse. Is there a way to speed up the redrawing of view content?

More details:

I am creating a small drawing application on iOS - the user moves his finger on the screen to draw a line.

The idea of ​​this is quite simple - while the user moves his finger touchesMoved, accumulates changes to the on-screen buffer image and invalidates the view in order to combine the off-screen buffer with the contents of the view.

A simple piece of code might look like this:

@interface CanvasView : UIView
...
end;


@implementation CanvasView{
    UIImage *canvasContentImage;
    UIImage *bufferImage
    CGContextRef drawingContext;
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    // prepare drawing to start
    UIGraphicsBeginImageContext(canvasSize);
    drawingContext = UIGraphicsGetCurrentContext();
    ...

}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{
    // draw to bufferImage
    CGContextMoveToPoint(drawingContext, prevPoint.x, prevPoint.y);
    CGContextAddLineToPoint(drawingContext, point.x, point.y);
    CGContextStrokePath(drawingContext);
    ...
    [self setNeedDisplay];
}

-(void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{
    //finish drawing
    UIGraphicsEndImageContext();
    //merge canvasContentImage with bufferImage
    ...
}

-(void)drawRect:(CGRect)rect{
    // draw bufferImage - merge it with current view content

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextDrawImage(context, self.bounds, canvasContentImage.CGImage);
    CGContextDrawImage(context, imageRect, bufferImage.CGImage);
    ...

}

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//    -(void)drawRect:(CGRect)rect{
//        // draw bufferImage - merge it with current view content
//        ...
//    }

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EDIT:

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Answer 1

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• UIImageView
• UIImageView.image - UIView setNeedsDisplay/setNeedsDisplayInRect.
• [self performSelector:@selector(setImage:) withObject:img afterDelay:0]; UIImageView.image = img, UIImageView.

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