Fyne: dynamically added widgets disappear after hiding/unhiding a custom widget

The issue happens because Fyne discards the renderer of hidden widgets after a cache timeout, and then recreates it using your CreateRenderer() method.

Any objects added after CreateRenderer() are lost unless you store them persistently.

You can fix this by:

• Keeping a persistent slice of your child objects (items []fyne.CanvasObject)

• Rebuilding your container from that slice in CreateRenderer()

• Adding new widgets to both the container and the slice when you create them dynamically

Here’s the full example that works:

package main

import (
    "fyne.io/fyne/v2"
    "fyne.io/fyne/v2/app"
    "fyne.io/fyne/v2/container"
    "fyne.io/fyne/v2/widget"
)

// Base widget A
type A struct {
    widget.BaseWidget
    body  *fyne.Container
    items []fyne.CanvasObject // persist dynamically added items
}

func NewA() *A {
    a := &A{}
    a.ExtendBaseWidget(a)
    return a
}

func (a *A) CreateRenderer() fyne.WidgetRenderer {
    a.body = container.NewVBox()
    for _, item := range a.items {
        a.body.Add(item)
    }
    return widget.NewSimpleRenderer(a.body)
}

// Derived widget B that adds dynamic items
type B struct {
    A
}

func NewB() *B {
    b := &B{}
    b.ExtendBaseWidget(b)
    return b
}

func (b *B) AddItem(item fyne.CanvasObject) {
    b.items = append(b.items, item) // persist
    b.body.Add(item)
    b.body.Refresh()
}

func main() {
    myApp := app.New()
    w := myApp.NewWindow("Dynamic Fyne Widget")

    b := NewB()
    addBtn := widget.NewButton("Add Label", func() {
        b.AddItem(widget.NewLabel("Dynamic Item"))
    })

    w.SetContent(container.NewVBox(addBtn, b))
    w.ShowAndRun()
}

1. Fyne may destroy and recreate renderers for hidden widgets after a cache timeout (to save memory).

2. If you added new child widgets after CreateRenderer() was first called, they are not known to the new renderer.

3. By keeping your items slice persistent in the widget itself, you can rebuild your UI inside CreateRenderer() when Fyne recreates it.

4. Using widget.NewSimpleRenderer(a.body) is fine here, but for more advanced cases, you can implement a custom renderer struct.