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Fun with Fractals: Multithreading for Free!

One of the challenges of using (or writing) a GUI toolkit is concurrency and thread safety. It may not be immediately obvious but threads and concurrent execution are everywhere: you need a renderer to keep the interface looking correct, some event handling to manage user input, a main thread to control the lifecycle of the application and probably more… Even the simplest of apps - a hello world window with a quit button - would potentially spawn a thread to handle the click event.

Toolkits also have various restrictions that ensure things work correctly. For Example, iOS requires that code which updates the user interface is run on the application main thread. Similarly EFL asserts that graphical objects are owned by the main thread which can limit portability. In Fyne we aim to remove these restrictions for developers - it should be possible to influence the GUI from anywhere within an application and have the toolkit work out the rest!

Concurrency

To show off how easy threading is we’ve made a couple of examples - and the most interesting is a fractal renderer. As each pixel of a fractal image can be calculated independently it’s a perfect candidate for multi-threading. Thankfully Fyne has a helpful image type to set up a dynamic rendering - see the NewRaster documentation for the method signature. The renderer for this automatically splits the image into multiple regions that are handled by different threads.

    window.SetContent(canvas.NewRaster(mandelbrot))

If you want to see the content of the mandelbrot method then check the fractal source code. Running it locally should look something like this:

Fractal window

Async Interface Updates

To demonstrate how easy working with multiple application threads can be there is another little demo of a simple RSS reader. If you’re unfamiliar with why updating the user interface can be problematic take a look at this stack overflow thread or many others on the topic - we don’t want developers to worry about which thread their code is running on or how to communicate with the “UI thread”.

Now take a look at the blog example code which spawns a new go routine (this may run on a different thread, depending on many factors) that then updates the user interface. The relevant code is:

    func parse(url string, titleList *widget.List) {
    	feed := gofeed.NewParser().parseURL(url)

    	for i := range feed.Items {
    		buttonList.Append(widget.NewLabel(item.Title))
    	}
    }

    func main() {
    	url := "http://fyne.io/feed.xml"

    	// ...	

    	list := widget.NewList()
    	window.SetContent(list)

    	go parse(url, list)

    	window.Show()
    }

If you run the (full) example you will notice that the list of article titles gets populated after the user interface shows. The downloading and parsing of the feed takes a little time and that should not slow down the user interface. The bonus here is that they async code had no additional complications!

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