Fix #63: add experimental support for setting custom app font

ui/theme/theme.go

@@ -1,7 +1,11 @@
 package theme
 
 import (
+	"errors"
 	"image/color"
+	"io/ioutil"
+	"log"
+	"strings"
 
 	"fyne.io/fyne/v2"
 	"fyne.io/fyne/v2/theme"
@@ -9,11 +13,19 @@ import (
 
 const ColorNamePageBackground fyne.ThemeColorName = "PageBackground"
 
-type MyTheme struct{}
+var (
+	normalFont fyne.Resource
+	boldFont   fyne.Resource
+)
+
+type MyTheme struct {
+	NormalFont string
+	BoldFont   string
+}
 
 var _ fyne.Theme = (*MyTheme)(nil)
 
-func (m MyTheme) Color(name fyne.ThemeColorName, variant fyne.ThemeVariant) color.Color {
+func (m *MyTheme) Color(name fyne.ThemeColorName, variant fyne.ThemeVariant) color.Color {
 	switch name {
 	case ColorNamePageBackground:
 		return color.RGBA{R: 15, G: 15, B: 15, A: 255}
@@ -29,14 +41,56 @@ func (m MyTheme) Color(name fyne.ThemeColorName, variant fyne.ThemeVariant) colo
 	return theme.DarkTheme().Color(name, variant)
 }
 
-func (m MyTheme) Icon(name fyne.ThemeIconName) fyne.Resource {
+func (m *MyTheme) Icon(name fyne.ThemeIconName) fyne.Resource {
 	return theme.DefaultTheme().Icon(name)
 }
 
-func (m MyTheme) Font(style fyne.TextStyle) fyne.Resource {
+func (m *MyTheme) Font(style fyne.TextStyle) fyne.Resource {
+	switch style {
+	case fyne.TextStyle{}:
+		if m.NormalFont != "" && normalFont == nil {
+			if content, err := readTTFFile(m.NormalFont); err != nil {
+				m.NormalFont = ""
+				m.BoldFont = ""
+			} else {
+				normalFont = fyne.NewStaticResource("normalFont", content)
+			}
+		}
+		if normalFont != nil {
+			return normalFont
+		}
+	case fyne.TextStyle{Bold: true}:
+		if m.BoldFont != "" && boldFont == nil {
+			if content, err := ioutil.ReadFile(m.BoldFont); err != nil {
+				m.BoldFont = ""
+			} else {
+				normalFont = fyne.NewStaticResource("boldFont", content)
+			}
+		}
+		if boldFont != nil {
+			return boldFont
+		}
+		if normalFont != nil {
+			return normalFont
+		}
+	}
+
 	return theme.DefaultTheme().Font(style)
 }
 
-func (m MyTheme) Size(name fyne.ThemeSizeName) float32 {
+func (m *MyTheme) Size(name fyne.ThemeSizeName) float32 {
 	return theme.DefaultTheme().Size(name)
 }
+
+func readTTFFile(filepath string) ([]byte, error) {
+	if !strings.HasSuffix(filepath, ".ttf") {
+		err := errors.New("only .ttf fonts are supported")
+		log.Printf("error loading custom font %q: %s", filepath, err.Error())
+		return nil, err
+	}
+	content, err := ioutil.ReadFile(filepath)
+	if err != nil {
+		log.Printf("error loading custom font %q: %s", filepath, err.Error())
+	}
+	return content, err
+}