CoreGraphics in iOS: how can I save the paths drawn in a CGContext and retrieve them in the next draw call?

Question

EDIT:

What I am trying to achieve is skip the call to firstDraw() the second time that drawRect is called. Reason for this is that in my real code I have got a lot of data points and lines to draw, so I thought thata if I recycle the previously drawn things I could optimize the performance. Is this possible at all with CoreGraphics?

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I'd like to be able to perform multiple drawings on a UIView without having to re-draw what I have already drawn.

The code below will execute two draws at a time distance of 2.4 seconds. After the first call I use CGContextSaveGState to save the state. In the second call I retrieve the current graphic context and then add some lines. However this seem to fail as it appears that the previous context is lost.

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What I get:

Here is what I get at the first draw call:

Here is what I get after the second call:

This instead is what I would like to get instead after the second call:

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Here is the code:

import UIKit

class GraphView: UIView {

    var count : Int = 0

    override func drawRect(rect: CGRect) {
        if ( count == 0){
            firstDraw()
         NSTimer.scheduledTimerWithTimeInterval(2.4, target: self, selector: "setNeedsDisplay", userInfo: nil, repeats: false)
            count++
        }
        else{
            addLinesToDraw()
        }
    }

    func firstDraw(){
        // Drawing code
        let context = UIGraphicsGetCurrentContext()

        CGContextMoveToPoint(context, 50, 50);

        CGContextSetRGBStrokeColor(context, 0, 0, 0, 1.0);
        CGContextSetRGBFillColor(context, 0, 0.0, 0.0, 1.0);
        CGContextAddLineToPoint(context, 60, 60);
        CGContextMoveToPoint(context, 150, 150);
        CGContextAddLineToPoint(context, 110, 90);
        CGContextSetLineWidth(context, 2);
        CGContextStrokePath(context);

        // Draw a Point as a small square
        CGContextSetRGBStrokeColor(context, 0.5, 0.5, 0.5, 1.0);
        CGContextSetRGBFillColor(context, 0.5, 0.5, 0.5, 1.0);
        //NSLog(@"Drawing rect at point [x: %i, y: %i]", xPosition+resolution, pressureIntValue);
        CGContextFillRect(context, CGRectMake(0, 0, 10, 10));
        CGContextAddRect(context, CGRectMake(0, 0, 10, 10));
        CGContextStrokePath(context);

        CGContextSaveGState(context)
    }

    func addLinesToDraw(){
        // Drawing code
        let context = UIGraphicsGetCurrentContext()

        CGContextMoveToPoint(context, 30, 60);

        CGContextSetRGBStrokeColor(context, 0, 0, 0, 1.0);
        CGContextSetRGBFillColor(context, 0, 0.0, 0.0, 1.0);
        CGContextAddLineToPoint(context, 20, 20);
        CGContextMoveToPoint(context, 120, 250);
        CGContextAddLineToPoint(context, 110, 90);
        CGContextSetLineWidth(context, 2);
        CGContextStrokePath(context);

        // Draw a Point as a small square
        CGContextSetRGBStrokeColor(context, 0.5, 0.5, 0.5, 1.0);
        CGContextSetRGBFillColor(context, 0.5, 0.5, 0.5, 1.0);
        //NSLog(@"Drawing rect at point [x: %i, y: %i]", xPosition+resolution, pressureIntValue);
        CGContextFillRect(context, CGRectMake(20, 30, 10, 10));
        CGContextAddRect(context, CGRectMake(20, 30, 10, 10));
        CGContextStrokePath(context);

        CGContextSaveGState(context)

    }
}

Answer 1

Each time you call drawRect: , you have to draw everything from scratch. So, in your case, you want to change your code to something like:

override func drawRect(rect: CGRect) {
    firstDraw()

    if ( count == 0){
     NSTimer.scheduledTimerWithTimeInterval(2.4, target: self, selector: "setNeedsDisplay", userInfo: nil, repeats: false)
        count++
    }
    else{
        addLinesToDraw()
    }
}

If you don't like this solution, you could always do all your drawing in a separate CGContext and blit it to the screen in your drawRect method.

Answer 2

Use UIGraphicsPushContext(context) instead of CGContextSaveGState(context) to make your context the current drawing context.

Difference described here: CGContextSaveGState vs UIGraphicsPushContext