swift - draw() iOS chart with gradient fill to PDFContext

Question

I am using the iOS Charts library to generate some line charts with a gradient fill (below the curve) and everything is working as expected on screenenter code here. Saving these charts by calling the charts library builtin save method works as expected for PNG and JPEG.

graphController.chartView.save(to: "chart7.png", format: .png)

but now I am looking into writing them to a PDF file and was inspired by this SO post. unfortunately the gradient isn't showing up in the PDF file, I get a solid fill instead. here is my code

func generatePdfWithFilePath(thefilePath: String, nDays: Int, renderingRect: CGRect) {
        UIGraphicsBeginPDFContextToFile(thefilePath, CGRect.zero, nil)
        //Start a new page.
        let usLetter = CGRect(x: 0.0, y: 0.0, width: 612.0, height: 792.0)
        UIGraphicsBeginPDFPageWithInfo(usLetter, nil)

        // Create a view for the Graph
        let graphController = LineChartController(rect: renderingRect, nDays: nDays)

        if let currentContext = UIGraphicsGetCurrentContext() {
            let frame = graphController.chartView.frame
            graphController.chartView.draw(frame)
            //graphController.chartView.layer.render(in: currentContext)
        }
        // Close the PDF context and write the contents out.
        UIGraphicsEndPDFContext()
    }

if i uncomment the line with layer.render instead, the chart is saved properly with gradient fill.

i was hoping the draw() method will save my chart in a way that would make it scalable (vs a bitmap) since i am looking to integrate it later in another document and want to play around with the chart size . the gradient is the result of this

//set a gradient fill under the line
let gradientColors = [ChartColorTemplates.colorFromString("#00ff0000").cgColor,
                      ChartColorTemplates.colorFromString("#ffff0000").cgColor]
let gradient = CGGradient(colorsSpace: nil, colors: gradientColors as CFArray, locations: nil)!
set.fillAlpha = 0.75
set.fill = Fill(linearGradient: gradient, angle: 90) //.linearGradient(gradient, angle: 90)
set.drawFilledEnabled = true // set to false to disable gradient fill

maybe my understanding of how draw() works is wrong... any help will be appreciated.

Answer 1

等待大神答复