Render a line graph on Apple Watch using watchOS 2

Question

I am trying to render a line/step graph on Apple Watch using watchOS 2. Unlike iOS 9, watchOS 2 doesn't support Quartz. It only supports Core Graphics. I tried writing some code to draw a line graph but I am getting an error "CGContextRestoreGState: invalid context 0x0. This is a serious error. This application, or a library it uses, is using an invalid context and is thereby contributing to an overall degradation of system stability and reliability. This notice is a courtesy: please fix this problem. It will become a fatal error in an upcoming update."

Following is the piece of code I used:

import WatchKit
import Foundation
import UIKit

class InterfaceController: WKInterfaceController{
    override func awakeWithContext(context: AnyObject?) {
        super.awakeWithContext(context)
        let path = UIBezierPath()
        let startPoint =  CGPointMake(0.0, 0.0)
        path.moveToPoint(startPoint)
        let nextPoint = CGPointMake(20.0, 20.0)
        path.addLineToPoint(nextPoint)
        path.lineWidth = 1.0
        UIColor.whiteColor().setStroke()
        path.stroke()
    }

    override func willActivate() {
        super.willActivate()

    }

    override func didDeactivate() {
        super.didDeactivate()
    }
}

My end result should be something like Stocks app present on Apple Watch. Wwhenever user clicks on particular stock, he will be able to view/visualize the statistics of that stock. Can anybody please help me in achieving this.

Answer 1

The context here are not CGContext, thus you have the issue. I do not think you can use Core Graphics directly on Apple Watch.

Answer 2

Graph render is done by the help of bezierPath and which is converted to image to attached to Apple Watch

//Swift-3 Xcode-8.1

import UIKit class InterfaceController: WKInterfaceController {

@IBOutlet var graphImage: WKInterfaceImage!
override func awake(withContext context: Any?) {
    super.awake(withContext: context)

    // Configure interface objects here.
}
override func willActivate() {

    super.willActivate()
    let path = createBeizePath()

    //Change graph to image
    let image:UIImage = UIImage.shapeImageWithBezierPath(bezierPath: path, fillColor: .red, strokeColor: .black)
    graphImage.setImage(uiimage)
}

//Draw graph here
func createBeizePath() -> UIBezierPath
{
    let path = UIBezierPath()
    //Rectangle path Trace
    path.move(to: CGPoint(x: 20, y: 100) )
    path.addLine(to: CGPoint(x: 50 , y: 100))
    path.addLine(to: CGPoint(x: 50, y: 150))
    path.addLine(to: CGPoint(x: 20, y: 150))
    return path
  }
}

extension UIImage {
        class func shapeImageWithBezierPath(bezierPath: UIBezierPath, fillColor: UIColor?, strokeColor: UIColor?, strokeWidth: CGFloat = 0.0) -> UIImage! {
            bezierPath.apply(CGAffineTransform(translationX: -bezierPath.bounds.origin.x, y: -bezierPath.bounds.origin.y ) )
            let size = CGSize(width: 100    , height: 100)
            UIGraphicsBeginImageContext(size)
            let context = UIGraphicsGetCurrentContext()
            var image = UIImage()
            if let context  = context {
                context.saveGState()
                context.addPath(bezierPath.cgPath)
                if strokeColor != nil {
                    strokeColor!.setStroke()
                    context.setLineWidth(strokeWidth)
                } else { UIColor.clear.setStroke() }
                fillColor?.setFill()
                context.drawPath(using: .fillStroke)
                 image = UIGraphicsGetImageFromCurrentImageContext()!
                context.restoreGState()
                UIGraphicsEndImageContext()
            }
            return image
        }
    }

Answer 3

Here's the example project that shows how to do dynamic graphs on WatchKit 2.0

https://github.com/vlm/ExampleWatchGraph

The gist of it is in the GraphPainter.swift file, where CoreGraphics is used to draw into an off-screen buffer (as @shu223 suggested), then display this buffer in a WKInterfaceImage.

Animated Watch Interface GIF.

Answer 4

Here's code in watchOS 3

  // Create a graphics context
        let size = CGSize(width:self.contentFrame.size.width, height:100)


        UIGraphicsBeginImageContext(size)
        let context = UIGraphicsGetCurrentContext()

        // Setup for the path appearance
        context!.setStrokeColor(UIColor.white.cgColor)
        context!.setLineWidth(4.0)

        // Draw lines
        context!.beginPath ();

        context?.move(to: CGPoint())
        context?.addLine(to: CGPoint(x: 100, y: 100))
        context?.addLine(to: CGPoint(x: 0, y: 100))
        context?.addLine(to: CGPoint(x: 100, y: 0))

        context!.strokePath();

        // Convert to UIImage
        let cgimage = context!.makeImage();
        let uiimage = UIImage(cgImage: cgimage!)

        // End the graphics context
        UIGraphicsEndImageContext()

        self.graphGroup.setBackgroundImage(uiimage)

Answer 5

I succeeded to render lines with following steps:

• Create a bitmap-based graphics context and makes it the current context using UIGraphicsBeginImageContext.
• Draw into the context.
• Extract CGImageRef from the context and convert it to UIImage object.
• Show the image on WKInterfaceGroup or WKInterfaceImage.

Code:

// Create a graphics context
let size = CGSizeMake(100, 100)
UIGraphicsBeginImageContext(size)
let context = UIGraphicsGetCurrentContext()

// Setup for the path appearance
CGContextSetStrokeColorWithColor(context, UIColor.whiteColor().CGColor)
CGContextSetLineWidth(context, 4.0)

// Draw lines
CGContextBeginPath (context);
CGContextMoveToPoint(context, 0, 0);
CGContextAddLineToPoint(context, 100, 100);
CGContextMoveToPoint(context, 0, 100);
CGContextAddLineToPoint(context, 100, 0);
CGContextStrokePath(context);

// Convert to UIImage
let cgimage = CGBitmapContextCreateImage(context);
let uiimage = UIImage(CGImage: cgimage!)

// End the graphics context
UIGraphicsEndImageContext()

// Show on WKInterfaceImage
image.setImage(uiimage)

image is WKInterfaceImage property. It works for me.

Also I can draw using UIBezierPath on watchOS as follow:

// Create a graphics context
let size = CGSizeMake(100, 100)
UIGraphicsBeginImageContext(size)
let context = UIGraphicsGetCurrentContext()
UIGraphicsPushContext(context!)

// Setup for the path appearance
UIColor.greenColor().setStroke()
UIColor.whiteColor().setFill()

// Draw an oval
let rect = CGRectMake(2, 2, 96, 96)
let path = UIBezierPath(ovalInRect: rect)
path.lineWidth = 4.0
path.fill()
path.stroke()

// Convert to UIImage
let cgimage = CGBitmapContextCreateImage(context);
let uiimage = UIImage(CGImage: cgimage!)

// End the graphics context
UIGraphicsPopContext()
UIGraphicsEndImageContext()

image.setImage(uiimage)

You can check the sample codes here.