Developing apps with Fyne

Using the Preferences API


Storing user configurations and values is a common task for application developers, but implementing it across multiple platforms can be tedious and time-consuming. To make it easier, Fyne has an API for storing values on the filesystem in a clean and understandable way while the complex parts are handled for you.

Lets start with the setup of the API. It is part of the Preferences interface where storage and loading functions exist for values of Bool, Float, Int and String. They each consist of three different functions, one for loading, one loading with a fallback value and lastly, one for storing values. An example of the three functions and their behaviour can be seen below for the String type:

// String looks up a string value for the key
String(key string) string
// StringWithFallback looks up a string value and returns the given fallback if not found
StringWithFallback(key, fallback string) string
// SetString saves a string value for the given key
SetString(key string, value string)

These functions can be accessed through the created application variable and calling the Preferences() method on. Please note that it is necessary to create the apps with a unique ID (usually like a reversed url). This means that the application will need to be created using app.NewWithID() to have its own place to store values. It can roughly be used like the example below:

a := app.NewWithID("com.example.tutorial.preferences")
[...]
a.Preferences().SetBool("Boolean", true)
number := a.Preferences().IntWithFallback("ApplicationLuckyNumber", 21)
expression := a.Preferences().String("RegularExpression")
[...]

To show this, we are going to build a simple little app that always closes after a set amount of time. This timeout should be user changeable and applied on the next start of the application.

Let us start by creating a variable called timeout that will be used to store time in the form of time.Duration.

var timeout time.Duration

Then we could create a select widget to let the user select the timeout from a couple pre-defined strings and then multiplying the timeout by the number of seconds that the string relates to. Lastly, the "AppTimeout" key is used to set the string value to the selected one.

timeoutSelector := widget.NewSelect([]string{"10 seconds", "30 seconds", "1 minute"}, func(selected string) {
    switch selected {
    case "10 seconds":
        timeout = 10 * time.Second
    case "30 seconds":
        timeout = 30 * time.Second
    case "1 minute":
        timeout = time.Minute
    }

    a.Preferences().SetString("AppTimeout", selected)
})

Now we want to grab the set value and if none exists, we want to have a fallback that sets the timeout to the shortest one possible to save the user time when waiting. This can be done by setting the selected value of timeoutSelector to the loaded value or the fallback if that happens to be the case. By doing it this way, the code inside the select widget will run for that specific value.

timeoutSelector.SetSelected(a.Preferences().StringWithFallback("AppTimeout", "10 seconds"))

The last part will just be to have a function that starts in a separate goroutine and tells the application to quit after the selected timeout.

go func() {
    time.Sleep(timeout)
    a.Quit()
}()

In the end, the resulting code should look something like this:

package main

import (
    "time"

    "fyne.io/fyne/app"
    "fyne.io/fyne/widget"
)

func main() {
    a := app.NewWithID("com.example.tutorial.preferences")
    w := a.NewWindow("Timeout")

    var timeout time.Duration

    timeoutSelector := widget.NewSelect([]string{"10 seconds", "30 seconds", "1 minute"}, func(selected string) {
        switch selected {
        case "10 seconds":
            timeout = 10 * time.Second
        case "30 seconds":
            timeout = 30 * time.Second
        case "1 minute":
            timeout = time.Minute
        }

        a.Preferences().SetString("AppTimeout", selected)
    })

    timeoutSelector.SetSelected(a.Preferences().StringWithFallback("AppTimeout", "10 seconds"))

    go func() {
        time.Sleep(timeout)
        a.Quit()
    }()

    w.SetContent(timeoutSelector)
    w.ShowAndRun()
}

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