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In the traditional sense, GUI programs have used callbacks to customize actions for widgets. Fyne does not expose inserting custom callbacks to capture events on widgets, but it does not need to. The Go language is plenty extensible to make this work.

Instead we can simply use Type Embedding and extend the widget to only make it possible to enter numerical values.

First create a new type struct, we will call it numericalEntry.

type numericalEntry struct {
    widget.Entry
}

As mentioned in Extending existing widgets, we follow good practice and create a constructor function that extends the BaseWidget.

func newNumericalEntry() *numericalEntry {
    entry := &numericalEntry{}
    entry.ExtendBaseWidget(entry)
    return entry
}

Now we need to make the entry accept only numbers. This can be done by overriding the TypedRune(rune) method that’s part of the fyne.Focusable interface. This will allow us to intercept the standard handling of runes received from key presses and only pass through those that we want. Inside this method, we will use a conditional to check if the rune matches any of the numbers between zero and nine. If they do, we delegate it to the standard TypedRune(rune) method of the embeded entry. If they do not, we just ignore the inputs. This implementation will only allow integers to be entered, but can easily be extended to check for other keys in the future if necessary.

func (e *numericalEntry) TypedRune(r rune) {
	switch r {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		e.Entry.TypedRune(r)
	}
}

If we want to update the implementation to allow for decimal numers as well, we can simply add . and , to the list of allowed runes (some languages use commas over dots for decimal notations).

func (e *numericalEntry) TypedRune(r rune) {
	switch r {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.', ',':
		e.Entry.TypedRune(r)
	}
}

With this, the entry now only allows the user to enter numerical values when keys are pressed. However, the paste shortcut will still allow text to be entered. To fix this, we can overwrite the TypedShortcut(fyne.Shortcut) method that is part of the fyne.Shortcutable interface. First we need to do a type assertion to check if the given shortcut is of the type *fyne.ShortcutPaste. If it is not, we can just delegate the shortcut back to the embeded entry. If it is, we check if the clipboard content is numerical, by using strconv.ParseFloat() (if you want to only allow integers, strconv.Atoi() will be just fine), and then delegating the shortcut back to the embeded entry if the clipboard content could be parsed without errors.

func (e *numericalEntry) TypedShortcut(shortcut fyne.Shortcut) {
	paste, ok := shortcut.(*fyne.ShortcutPaste)
	if !ok {
		e.Entry.TypedShortcut(shortcut)
		return
	}

	content := paste.Clipboard.Content()
	if _, err := strconv.ParseFloat(content, 64); err == nil {
		e.Entry.TypedShortcut(shortcut)
	}
}

As a bonus, we can also make sure that mobile operating systems open the numerical keyboard instead of the default keyboard. This can be done by by first importng the fyne.io/fyne/v2/driver/mobile package and overwriting the Keyboard() mobile.KeyboardType method that is part of the m̀obile.Keyboardable interface. Inside the function, we then simply return the mobile.NumberKeyboard type.

func (e *numericalEntry) Keyboard() mobile.KeyboardType {
	return mobile.NumberKeyboard
}

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

package main

import (
	"strconv"

	"fyne.io/fyne/v2"
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/driver/mobile"
	"fyne.io/fyne/v2/widget"
)

type numericalEntry struct {
	widget.Entry
}

func newNumericalEntry() *numericalEntry {
	entry := &numericalEntry{}
	entry.ExtendBaseWidget(entry)
	return entry
}

func (e *numericalEntry) TypedRune(r rune) {
	switch r {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '.', ',':
		e.Entry.TypedRune(r)
	}
}

func (e *numericalEntry) TypedShortcut(shortcut fyne.Shortcut) {
	paste, ok := shortcut.(*fyne.ShortcutPaste)
	if !ok {
		e.Entry.TypedShortcut(shortcut)
		return
	}

	content := paste.Clipboard.Content()
	if _, err := strconv.ParseFloat(content, 64); err == nil {
		e.Entry.TypedShortcut(shortcut)
	}
}

func (e *numericalEntry) Keyboard() mobile.KeyboardType {
	return mobile.NumberKeyboard
}

func main() {
	a := app.New()
	w := a.NewWindow("Numerical")

	entry := newNumericalEntry()

	w.SetContent(entry)
	w.ShowAndRun()
}