List does not render contents when forwarding to a container

[virtuald]

Describe the bug:

I'd like to include more complex widgets in a list. I tried using interface composition to make my type pretend to be a container, but that didn't work. Then I implemented the CanvasObject interface, forwarding everything to an internal container, and that also didn't work.

If I return just the container from the create callback for the list, it works fine. However, then I need to type assert for each of the objects in the container in the update function. It would be much more convenient if I only needed to do a single type assert that gave me my custom type.

I'm new to fyne so it's possible that this isn't the right way to create a custom widget.

To Reproduce:

Steps to reproduce the behaviour:

1. Run example code
2. Notice that the rendered list is definitely there (mouseover highlights each element), but each list item is not visible

Screenshots:

Example code:

Interface composition version

package main

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

type myWidget struct {
	*fyne.Container

	// real usage obviously has multiple widgets here
	containedLabel *widget.Label
}

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

	l := []string{"1", "2", "3"}

	// does not work
	list := widget.NewList(
		func() int {
			return len(l)
		},
		func() fyne.CanvasObject {
			label := widget.NewLabel("")
			return &myWidget{
				containedLabel: label,
				Container:      container.NewVBox(label),
			}
		},
		func(id widget.ListItemID, obj fyne.CanvasObject) {
			w := obj.(*myWidget)
			w.containedLabel.Text = l[id]
		},
	)

	w.SetContent(list)

	w.ShowAndRun()
}

Forwarding version

package main

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

type myWidget struct {
	container *fyne.Container

	// real usage obviously has multiple widgets here
	containedLabel *widget.Label
}

func (w *myWidget) MinSize() fyne.Size      { return w.container.MinSize() }
func (w *myWidget) Move(p fyne.Position)    { w.container.Move(p) }
func (w *myWidget) Position() fyne.Position { return w.container.Position() }
func (w *myWidget) Resize(s fyne.Size)      { w.container.Resize(s) }
func (w *myWidget) Size() fyne.Size         { return w.container.Size() }
func (w *myWidget) Hide()                   { w.container.Hide() }
func (w *myWidget) Visible() bool           { return w.container.Visible() }
func (w *myWidget) Show()                   { w.container.Show() }
func (w *myWidget) Refresh()                { w.container.Refresh() }

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

	l := []string{"1", "2", "3"}

	// does not work
	list := widget.NewList(
		func() int {
			return len(l)
		},
		func() fyne.CanvasObject {
			label := widget.NewLabel("")
			return &myWidget{
				containedLabel: label,
				container:      container.NewVBox(label),
			}
		},
		func(id widget.ListItemID, obj fyne.CanvasObject) {
			w := obj.(*myWidget)
			w.containedLabel.Text = l[id]
		},
	)

	w.SetContent(list)

	w.ShowAndRun()
}

Device (please complete the following information):

• OS: MacOS
• Version: 10.14
• Go version: 1.15.6
• Fyne version: fyne.io/fyne/v2 v2.0.0

[fpabl0]

Hello :), if you want to hold different widgets, you could return just container.NewVBox in CreateItem callback:

	list := widget.NewList(
		func() int {
			return len(l)
		},
		func() fyne.CanvasObject {
			return container.NewVBox(
				widget.NewLabel(""),
				widget.NewLabel("Other text"),
			)
		},
		func(id widget.ListItemID, obj fyne.CanvasObject) {
			c := obj.(*fyne.Container)
			c.Objects[0].(*widget.Label).SetText(l[id])
			c.Objects[1].(*widget.Label).SetText("Other text " + l[id])
		},
	)

If you want to create custom widgets, you need to implement fyne.Widget or extend widget.BaseWidget or extend an existing widget. You can find more information in the docs:
https://developer.fyne.io/tutorial/extending-widgets
https://developer.fyne.io/tutorial/write-custom-widget

[andydotxyz]

Hello :), if you want to hold different widgets, you could return just container.NewVBox in CreateItem callback:

@fpabl0 is correct here, containers do not support embedding in that way - only widgets can be extended.
Some element of typecasting will be needed unless you define a custom widget, so it's your choice which seems easier for your usage.

[virtuald]

Hello :), if you want to hold different widgets, you could return just container.NewVBox in CreateItem callback:

Yes, I alluded to that solution in my original question. That's terrible, especially as the number of widgets goes up.

@fpabl0 is correct here, containers do not support embedding in that way - only widgets can be extended.

I guess though why doesn't it work? The returned object is implementing the interface, seems like that should be good enough? Seems very unexpected.

[andydotxyz]

That's terrible, especially as the number of widgets goes up.

A container is merely for containing items, the widgets are the smart elements.
You could use code like you have if your "myWidget" was actually a widget.

I guess though why doesn't it work? The returned object is implementing the interface,

Container is not an interface, it is a concrete type.
Our design is that Widgets can be extended - but canvas primitives and containers cannot. So the above should not be unexpected if we have got the documentation correct.

[virtuald]

I don't understand why I can't extend a container (or easily embed it into a widget as far as I can tell), that seems like an oversight. Being able to build complex widgets that are groups of widgets seems like a pretty basic UI thing.

[andydotxyz]

Widgets are a super set of containers. They provide that functionality but also more. Maybe #709 is relevant?