Building a Custom Layout
In a Fyne application each Container arranges its child elements using a simple layout algorithm.
Fyne defines many layouts available in the fyne.io/fyne/v2/layout package.
If you look at the code you will see that they all implement the Layout interface.
type Layout interface {
Layout([]CanvasObject, Size)
MinSize(objects []CanvasObject) Size
}
Any application can provide a custom layout to arrange widgets in a non-standard manner. To do this you need to implement the interface above in your own code. To illustrate this we will create a new layout that arranges elements in a diagonal line and is arranged to the bottom left of its container
First we will define a new type, diagonal, and define what its minimum size will be. To calculate this we just add the width and height of all child elements (specified as the []fyne.CanvasObject parameter to MinSize.
import "fyne.io/fyne/v2"
type diagonal struct {
}
func (d *diagonal) MinSize(objects []fyne.CanvasObject) fyne.Size {
w, h := float32(0), float32(0)
for _, o := range objects {
childSize := o.MinSize()
w += childSize.Width
h += childSize.Height
}
return fyne.NewSize(w, h)
}
To this type we add a Layout() function that should move all of the specified objects into the fyne.Size specified in the second parameter.
In our implementation we calculate the top-left of the widgets (this is 0 x parameter and has a y position that is the height of the container less the total of all child item heights).
From the top position we simply advance each item position by the size of the previous child.
func (d *diagonal) Layout(objects []fyne.CanvasObject, containerSize fyne.Size) {
pos := fyne.NewPos(0, containerSize.Height - d.MinSize(objects).Height)
for _, o := range objects {
size := o.MinSize()
o.Resize(size)
o.Move(pos)
pos = pos.Add(fyne.NewPos(size.Width, size.Height))
}
}
That’s all there is to creating a custom layout.
Now that the code is all written we can use it as the layout parameter to container.New.
The code below sets up 3 Label widgets and place them in a container with our new layout.
If you run this application you will see the diagonal widget arrangement and, upon resizing the window, they will align to the bottom left of the available space.
package main
import (
"fyne.io/fyne/v2"
"fyne.io/fyne/v2/app"
"fyne.io/fyne/v2/container"
"fyne.io/fyne/v2/widget"
)
func main() {
a := app.New()
w := a.NewWindow("Diagonal")
text1 := widget.NewLabel("topleft")
text2 := widget.NewLabel("Middle Label")
text3 := widget.NewLabel("bottomright")
w.SetContent(container.New(&diagonal{}, text1, text2, text3))
w.ShowAndRun()
}