use filetree reader instead of filetree

imgbom/scope/scope.go

@@ -9,12 +9,12 @@ import (
 
 type Scope struct {
 	Option Option
-	Trees  []*tree.FileTree
+	Trees  []tree.FileTreeReader
 	Image  *image.Image
 }
 
 func NewScope(img *image.Image, option Option) (Scope, error) {
-	var trees = make([]*tree.FileTree, 0)
+	var trees = make([]tree.FileTreeReader, 0)
 
 	if img == nil {
 		return Scope{}, fmt.Errorf("no image given")

imgbom/scope/scope_test.go

@@ -81,7 +81,11 @@ func TestScope(t *testing.T) {
 			t.Fatalf("mismatched tree lengths: %d!=%d", len(actual.Trees), len(c.expectedTrees))
 		}
 
-		for idx, at := range actual.Trees {
+		for idx, atr := range actual.Trees {
+			at, ok := atr.(*tree.FileTree)
+			if !ok {
+				t.Fatalf("could not extract tree from reader")
+			}
 			if !at.Equal(c.expectedTrees[idx]) {
 				t.Error("mismatched tree @ idx", idx)
 			}