Make sure that bundled icons are no larger than they need to be

Helps to keep app size down too.
Fixes fyne-io#5272

app/meta_development.go

@@ -37,7 +37,7 @@ func checkLocalMetadata() {
 	if data.Details.Icon != "" {
 		res, err := fyne.LoadResourceFromPath(data.Details.Icon)
 		if err == nil {
-			meta.Icon = res
+			meta.Icon = metadata.ScaleIcon(res, 512)
 		}
 	}
 

cmd/fyne/internal/commands/build.go

@@ -344,6 +344,8 @@ func createMetadataInitFile(srcdir string, app *appData) (func(), error) {
 			return func() { os.Remove(metadataInitFilePath) }, err
 		}
 
+		res = metadata.ScaleIcon(res, 512)
+
 		// The return type of fyne.LoadResourceFromPath is always a *fyne.StaticResource.
 		app.ResGoString = res.(*fyne.StaticResource).GoString()
 	}

internal/metadata/icon.go

@@ -0,0 +1,41 @@
+package metadata
+
+import (
+	"bytes"
+	"image/png"
+	"strconv"
+
+	"github.com/nfnt/resize"
+
+	"fyne.io/fyne/v2"
+)
+
+func ScaleIcon(data fyne.Resource, size int) fyne.Resource {
+	img, err := png.Decode(bytes.NewReader(data.Content()))
+	if err != nil {
+		fyne.LogError("Failed to decode app icon", err)
+		return data
+	}
+
+	if img.Bounds().Dx() <= size {
+		return data
+	}
+
+	sized := resize.Resize(uint(size), uint(size), img, resize.Lanczos3)
+	smallData := &bytes.Buffer{}
+	err = png.Encode(smallData, sized)
+	if err != nil {
+		fyne.LogError("Failed to encode smaller app icon", err)
+		return data
+	}
+
+	name := data.Name()
+	nameLen := len(name)
+	suffix := "-" + strconv.Itoa(size) + ".png"
+	if nameLen <= 4 || name[nameLen-4] != '.' {
+		name = "appicon" + suffix
+	} else {
+		name = name[:nameLen-4] + suffix
+	}
+	return fyne.NewStaticResource(name, smallData.Bytes())
+}

internal/metadata/icon_test.go

@@ -0,0 +1,28 @@
+package metadata
+
+import (
+	"bytes"
+	"image/png"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"fyne.io/fyne/v2"
+)
+
+func TestScaleIcon(t *testing.T) {
+	data, err := fyne.LoadResourceFromPath("./testdata/fyne.png")
+	assert.Nil(t, err)
+
+	assert.Equal(t, "fyne.png", data.Name())
+	img, err := png.Decode(bytes.NewReader(data.Content()))
+	assert.Nil(t, err)
+	assert.Equal(t, 512, img.Bounds().Dx())
+
+	smallData := ScaleIcon(data, 256)
+
+	assert.Equal(t, "fyne-256.png", smallData.Name())
+	img, err = png.Decode(bytes.NewReader(smallData.Content()))
+	assert.Nil(t, err)
+	assert.Equal(t, 256, img.Bounds().Dx())
+}