Refactored dynamic elements

pkg/stdlib/html/driver/common/lazy.go

@@ -0,0 +1,56 @@
+package common
+
+import (
+	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+	"sync"
+)
+
+type (
+	LazyFactory func() (core.Value, error)
+
+	LazyValue struct {
+		sync.Mutex
+		factory LazyFactory
+		ready   bool
+		value   core.Value
+		err     error
+	}
+)
+
+func NewLazyValue(factory LazyFactory) *LazyValue {
+	lz := new(LazyValue)
+	lz.ready = false
+	lz.factory = factory
+	lz.value = values.None
+
+	return lz
+}
+
+func (lv *LazyValue) Value() (core.Value, error) {
+	lv.Lock()
+	defer lv.Unlock()
+
+	if !lv.ready {
+		val, err := lv.factory()
+
+		if err == nil {
+			lv.value = val
+			lv.err = nil
+		} else {
+			lv.value = values.None
+			lv.err = err
+		}
+	}
+
+	return lv.value, lv.err
+}
+
+func (lv *LazyValue) Reset() {
+	lv.Lock()
+	defer lv.Unlock()
+
+	lv.ready = false
+	lv.value = values.None
+	lv.err = nil
+}