Support centering the `*container.Scroll` content on a position (when content can scroll)

[Nazgand]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

I have working code to do so here, but the feature should be built into Fyne.

func PositionRelativeTo(descendant, ancestor fyne.CanvasObject) fyne.Position {
	if descendant == nil || ancestor == nil || descendant == ancestor {
		return fyne.NewPos(0, 0)
	}

	path := findPathToDescendant(ancestor, descendant)
	if len(path) == 0 {
		return fyne.NewPos(0, 0)
	}

	relPos := fyne.NewPos(0, 0)
	for _, node := range path[1:] { // Skip ancestor.
		relPos = relPos.Add(node.RelPos)
	}

	return relPos
}

func findPathToDescendant(root, target fyne.CanvasObject) []PathNode {
	if root == nil || target == nil {
		return nil
	}

	path := []PathNode{{Obj: root, RelPos: fyne.NewPos(0, 0)}}

	if root == target {
		return path
	}

	if cont, ok := root.(*fyne.Container); ok {
		for _, child := range cont.Objects {
			if childPath := findPathToDescendant(child, target); childPath != nil {
				childRelPos := child.Position()
				childPath[0].RelPos = childRelPos
				return append(path, childPath...)
			}
		}
	}

	return nil
}

type PathNode struct {
	Obj    fyne.CanvasObject
	RelPos fyne.Position
}

func CenterDescendantOfScroll(scroll *container.Scroll, descendant fyne.CanvasObject) {
	TargetOffset := PositionRelativeTo(descendant, scroll.Content)
	TargetOffset = TargetOffset.AddXY(descendant.Size().Width/2,
		descendant.Size().Height/2).SubtractXY(scroll.Size().Width/2,
		scroll.Size().Height/2)
	TargetOffset.X = max(TargetOffset.X, 0)
	TargetOffset.X = min(TargetOffset.X,
		scroll.Content.Size().Width-scroll.Size().Width)
	TargetOffset.Y = max(TargetOffset.Y, 0)
	TargetOffset.Y = min(TargetOffset.Y,
		scroll.Content.Size().Height-scroll.Size().Height)
	scroll.Offset = TargetOffset
}

Is it possible to construct a solution with the existing API?

Possible, but it should be 1 line of code.

Describe the solution you'd like to see.

With

var scroll *container.Scroll
var descendant fyne.CanvasObject

I should be able to simply call

scroll.CenterDescendant(descendant)

[andydotxyz]

I don't understand what this is for.

Centering content of a scroll does not seem like something scroll was built to do. If you want an item of content centered would you not just put that inside a Center container? (inside the Scroll)

[Nazgand]

I have a game-tree built with many nested container.NewHBox(), container.NewVBox(), widget.NewButton(). I have the arrow keys navigate the game-tree, and when the current node changes, I center the button that corresponds to the current node/move.
It is very useful. I do not see how a Center container could replicate what I need.
Also, the code I provided does not exactly center the descendant if the descendant is too close to the edge, but it does bring the descendant into view, which is the primary purpose.

[andydotxyz]

Thanks, i now understand what you mean. Perhaps this is a restatement of the request to "ensure object visible" but with an addition that you'd like it to appear as close to the center as possible?

I.e. if no scrolling do nothing but id scroll is possible then try to center on a specific point?

[Nazgand]

Indeed.

[andydotxyz]

Marking it as related to #3029