add support for adding box by var or const (#60)

builder/visitor.go

@@ -192,6 +192,16 @@ func (v *visitor) evalSelector(expr *ast.CallExpr, sel *ast.SelectorExpr) error
 		for _, e := range expr.Args {
 			switch at := e.(type) {
 			case *ast.Ident:
+				switch at.Obj.Kind {
+				case ast.Var:
+					if as, ok := at.Obj.Decl.(*ast.AssignStmt); ok {
+						v.addVariable(as)
+					}
+				case ast.Con:
+					if vs, ok := at.Obj.Decl.(*ast.ValueSpec); ok {
+						v.addConstant(vs)
+					}
+				}
 				return v.evalIdent(at)
 			case *ast.BasicLit:
 				v.addBox(at.Value)
@@ -218,3 +228,21 @@ func (v *visitor) addBox(b string) {
 	b = strings.Replace(b, "\"", "", -1)
 	v.Boxes = append(v.Boxes, b)
 }
+
+func (v *visitor) addVariable(as *ast.AssignStmt) error {
+	if len(as.Rhs) == 1 {
+		if bs, ok := as.Rhs[0].(*ast.BasicLit); ok {
+			v.addBox(bs.Value)
+		}
+	}
+	return nil
+}
+
+func (v *visitor) addConstant(vs *ast.ValueSpec) error {
+	if len(vs.Values) == 1 {
+		if bs, ok := vs.Values[0].(*ast.BasicLit); ok {
+			v.addBox(bs.Value)
+		}
+	}
+	return nil
+}

builder/visitor_test.go

@@ -13,6 +13,6 @@ func Test_Visitor(t *testing.T) {
 
 	r.Equal("example", v.Package)
 	r.Len(v.Errors, 0)
-	r.Len(v.Boxes, 5)
-	r.Equal([]string{"./assets", "./bar", "./foo", "./sf", "./templates"}, v.Boxes)
+	r.Len(v.Boxes, 7)
+	r.Equal([]string{"./assets", "./bar", "./constant", "./foo", "./sf", "./templates", "./variable"}, v.Boxes)
 }

example/example.go

@@ -6,15 +6,26 @@ import (
 
 var a = packr.NewBox("./foo")
 
+const constString = "./constant"
+
 type S struct{}
 
 func (S) f(packr.Box) {}
 
 func init() {
 	// packr.NewBox("../idontexists")
 
-	b := "./baz"
-	packr.NewBox(b) // won't work, no variables allowed, only strings
+	b := "./variable"
+	packr.NewBox(b)
+
+	packr.NewBox(constString)
+
+	// Cannot work from a function
+	packr.NewBox(strFromFunc())
+
+	// This variable should not be added
+	fromFunc := strFromFunc()
+	packr.NewBox(fromFunc)
 
 	foo("/templates", packr.NewBox("./templates"))
 	packr.NewBox("./assets")
@@ -25,4 +36,8 @@ func init() {
 	s.f(packr.NewBox("./sf"))
 }
 
+func strFromFunc() string {
+	return "./fromFunc"
+}
+
 func foo(s string, box packr.Box) {}