DOM: Expose copy of layers from overlay document

There are use-cases to access them individually

Signed-off-by: Richard Kosegi <richard.kosegi@gmail.com>

dom/overlay.go

@@ -69,9 +69,11 @@ func (m *overlayDocument) Search(fn SearchValueFunc) Coordinates {
 	return r
 }
 
-func (m *overlayDocument) Layers() []string {
-	c := make([]string, len(m.names))
-	copy(c, m.names)
+func (m *overlayDocument) Layers() map[string]Container {
+	c := make(map[string]Container, len(m.names))
+	for n, v := range m.overlays {
+		c[n] = v.Clone().(Container)
+	}
 	return c
 }
 

dom/overlay_test.go

@@ -136,3 +136,12 @@ func TestOverlaySearch(t *testing.T) {
 	res = d.Search(SearchEqual(2))
 	assert.Equal(t, 0, len(res))
 }
+
+func TestOverlayLayers(t *testing.T) {
+	d := NewOverlayDocument()
+	d.Put("layer1", "root.next.next2", LeafNode(1))
+	d.Put("layer2", "root.other", LeafNode(5))
+	m := d.Layers()
+	assert.Equal(t, 2, len(m))
+	assert.Equal(t, 5, m["layer2"].Children()["root"].(Container).Children()["other"].(Leaf).Value())
+}

dom/types.go

@@ -213,6 +213,7 @@ type OverlayDocument interface {
 	Put(overlay, path string, value Node)
 	// Merged returns read-only, merged view of all overlays
 	Merged(option ...MergeOption) Container
-	// Layers return copy of list with all layer names
-	Layers() []string
+	// Layers returns a copy of mapping between layer name and its associated Container.
+	// Containers are cloned using Node.Clone()
+	Layers() map[string]Container
 }