text/v2: let NewMultiFace return an error

Updates #2845

examples/mixedfont/main.go

@@ -74,7 +74,11 @@ func (g *Game) Update() error {
 			Size:   32,
 		}
 
-		g.face = text.NewMultiFace(en, ja)
+		f, err := text.NewMultiFace(en, ja)
+		if err != nil {
+			return err
+		}
+		g.face = f
 	}
 	return nil
 }

text/v2/multi.go

@@ -15,6 +15,7 @@
 package text
 
 import (
+	"errors"
 	"unicode/utf8"
 
 	"github.com/hajimehoshi/ebiten/v2/vector"
@@ -31,11 +32,24 @@ type MultiFace struct {
 }
 
 // NewMultiFace creates a new MultiFace from the given faces.
-func NewMultiFace(faces ...Face) *MultiFace {
+//
+// NewMultiFace returns an error when no faces are given, or the faces' directions don't agree.
+func NewMultiFace(faces ...Face) (*MultiFace, error) {
+	if len(faces) == 0 {
+		return nil, errors.New("text: no faces are given at NewMultiFace")
+	}
+
+	d := faces[0].direction()
+	for _, f := range faces[1:] {
+		if f.direction() != d {
+			return nil, errors.New("text: all the faces' directions must agree")
+		}
+	}
+
 	m := &MultiFace{}
 	m.faces = make([]Face, len(faces))
 	copy(m.faces, faces)
-	return m
+	return m, nil
 }
 
 // Metrics implements Face.

text/v2/text_test.go

@@ -243,7 +243,10 @@ func TestUnhashableFace(t *testing.T) {
 
 func TestMultiFace(t *testing.T) {
 	faces := []text.Face{text.NewStdFace(bitmapfont.Face)}
-	f := text.NewMultiFace(faces...)
+	f, err := text.NewMultiFace(faces...)
+	if err != nil {
+		t.Fatal(err)
+	}
 	img := ebiten.NewImage(30, 30)
 	text.Draw(img, "Hello", f, nil)