Drawing Text on NSImage is Slow - How to improve performance?

You should update in the main thread ; here you create a new thread each time. And UI must occur in main thread.

You could also simplify code:

override func controlTextDidChange(_ obj: Notification) {
    if let textdata = obj.object as! NSTextField {
        if txtfield.identifier?.rawValue == "txt_field"
        {
            DispatchQueue.main.async {
                self.textdata = self.txtbox.stringValue // To be done on main thread as well
                self.img_view.image = self.drawText(image: NSImage(byReferencingFile: self.selectedfilename)!);
            }
            
        }
    }
}

But a question: why do you call textDidChange ? Can user modify ?

Why not call at the end, once editing is finished ?

In drawText:

why do you need the conversion, which may be time consuming:

let rep : NSBitmapImageRep = NSBitmapImageRep(bitmapDataPlanes: nil, pixelsWide: Int(image.size.width), pixelsHigh: Int(image.size.height), bitsPerSample: 8, samplesPerPixel: 4, hasAlpha: true, isPlanar: false, colorSpaceName: NSColorSpaceName.calibratedRGB, bytesPerRow: 0, bitsPerPixel: 0)!

Note: you could make the signature of drawText more explicit, such as:

func drawText(over image :NSImage) ->NSImage

in order to call:

self.img_view.image = self.drawText(over: NSImage(byReferencingFile: self.selectedfilename)!);

Thanks for your response.

The user can modify the text and based on that the drawn string should be modified.

Can this be called when the editing of NSTextBox if finished? rather than for every character change?

I will modify the drawtext based on your comments and get back.

I am not sure what explains the slowliness.

You could use profiler, but I would suggest a simple start:

Test with different code to see when it slows down

I have a question: what is the type of text ? NSAttributedString ?

1. just simpler text

func drawText(image :NSImage) ->NSImage
    { 
        let text = text          // useless ?
        let textRect = CGRect(x: (image.size.width/2-fontsize.width/2), y: image.size.height/2, width: fontsize.width, height: fontsize.height)
        text.draw(in: textRect)
        return im
    }

2. Attributed text and basic image

func drawText(image :NSImage) ->NSImage
    {
        let text = text          // useless ?
        let font = NSFont(name:String(combo_font.stringValue), size: 50)
        let imageRect = CGRect(x: 0, y: 0, width: image.size.width, height: image.size.height)
  
        let fontAttributes = [NSAttributedStringKey.font: font]
        let fontsize = (text as NSString).size(withAttributes: fontAttributes)

        let textRect = CGRect(x: (image.size.width/2-fontsize.width/2), y: image.size.height/2, width: fontsize.width, height: fontsize.height)
     
        let textStyle = NSMutableParagraphStyle.default.mutableCopy() as! NSMutableParagraphStyle
        let textFontAttributes = [
            NSAttributedStringKey.font: font,
            NSAttributedStringKey.foregroundColor: NSColor.white,
            NSAttributedStringKey.paragraphStyle: textStyle
        ]
        image.draw(in: imageRect)
        text.draw(in: textRect, withAttributes: textFontAttributes)
        return im
    }

If this is not slow, the problem comes probably from the bitmap handling in NSBitmapImageRep (which would not be totally surprising for very large images)

3 If that's the case,

Why do you need this bitmap processing ?

you may do the drawing at the end of text change, not evrytime some text change

Removal of the conversion code has speed up things a little bit. The type of text is string,the assignation is useless.Will use the global variable.How can i hook to end of textchange?