Add a transform.ChangeParent(child, parent, keepWorldPosition) Functi…

…on (#111)

* add change parent

* fix tests

* fix children test

features/hierarchy/example_test.go

@@ -172,6 +172,49 @@ func TestFindChildren(t *testing.T) {
 	}
 }
 
+func TestChangeParent(t *testing.T) {
+	w := donburi.NewWorld()
+
+	parent1 := donburi.NewTag().SetName("parent1")
+	parent2 := donburi.NewTag().SetName("parent2")
+	child := donburi.NewTag().SetName("child")
+
+	p1e := w.Entry(w.Create(parent1))
+	p2e := w.Entry(w.Create(parent2))
+	ce := w.Entry(w.Create(child))
+
+	// no parent exists
+	ChangeParent(ce, p1e)
+	testChildren(t, []childrenTest{
+		{
+			Parent:   p1e,
+			Children: []*donburi.Entry{ce},
+		},
+	})
+
+	// change to same parent
+	ChangeParent(ce, p1e)
+	testChildren(t, []childrenTest{
+		{
+			Parent:   p1e,
+			Children: []*donburi.Entry{ce},
+		},
+	})
+
+	// change parent
+	ChangeParent(ce, p2e)
+	testChildren(t, []childrenTest{
+		{
+			Parent:   p1e,
+			Children: []*donburi.Entry{},
+		},
+		{
+			Parent:   p2e,
+			Children: []*donburi.Entry{ce},
+		},
+	})
+}
+
 type childrenTest struct {
 	Parent   *donburi.Entry
 	Children []*donburi.Entry

features/hierarchy/parent.go

@@ -89,6 +89,39 @@ func HasParent(entry *donburi.Entry) bool {
 	return entry.HasComponent(parentComponent)
 }
 
+// ChangeParent changes a parent of the entry.
+func ChangeParent(child *donburi.Entry, newParent *donburi.Entry) {
+	if !newParent.Valid() {
+		panic("newParent is not valid")
+	}
+	if !child.Valid() {
+		panic("child is not valid")
+	}
+	if !newParent.HasComponent(childrenComponent) {
+		newParent.AddComponent(childrenComponent)
+	}
+
+	if oldParent, ok := GetParent(child); ok {
+		if oldParent == newParent {
+			return
+		}
+
+		if oldParent.Valid() {
+			oldChildren := donburi.Get[childrenData](oldParent, childrenComponent)
+			for i, c := range oldChildren.Children {
+				if c == child {
+					oldChildren.Children = append(oldChildren.Children[:i], oldChildren.Children[i+1:]...)
+					break
+				}
+			}
+		}
+
+		child.RemoveComponent(parentComponent)
+	}
+
+	SetParent(child, newParent)
+}
+
 func getParentData(entry *donburi.Entry) (*parentData, bool) {
 	if HasParent(entry) {
 		p := donburi.Get[parentData](entry, parentComponent)

features/transform/transform.go

@@ -41,6 +41,16 @@ func AppendChild(parent, child *donburi.Entry, keepWorldPosition bool) {
 	SetParent(child, parent, keepWorldPosition)
 }
 
+// ChangeParent changes parent of the entry.
+func ChangeParent(child, parent *donburi.Entry, keepWorldPosition bool) {
+	if !child.HasComponent(Transform) {
+		panic("entry does not have transform component")
+	}
+	RemoveParent(child, keepWorldPosition)
+	hierarchy.ChangeParent(child, parent)
+	SetParent(child, parent, keepWorldPosition)
+}
+
 // SetParent sets parent to the entry.
 func SetParent(entry, parent *donburi.Entry, keepWorldPosition bool) {
 	if !entry.HasComponent(Transform) {
@@ -73,6 +83,30 @@ func GetParent(entry *donburi.Entry) (*donburi.Entry, bool) {
 	return hierarchy.GetParent(entry)
 }
 
+// RemoveParent removes parent of the entry.
+func RemoveParent(entry *donburi.Entry, keepWorldPosition bool) {
+	d := GetTransform(entry)
+	if !d.hasParent {
+		return
+	}
+	parent, _ := GetParent(entry)
+	d.hasParent = false
+	if keepWorldPosition {
+		parentPos := WorldPosition(parent)
+
+		d.LocalPosition = d.LocalPosition.Add(parentPos)
+		d.LocalRotation += WorldRotation(parent)
+
+		ws := WorldScale(parent)
+		if ws.X != 0 {
+			d.LocalScale.X = d.LocalScale.X * ws.X
+		}
+		if ws.Y != 0 {
+			d.LocalScale.Y = d.LocalScale.Y * ws.Y
+		}
+	}
+}
+
 // GetChildren returns children of the entry.
 func GetChildren(entry *donburi.Entry) ([]*donburi.Entry, bool) {
 	return hierarchy.GetChildren(entry)

features/transform/transform_test.go

@@ -39,6 +39,14 @@ func TestTransform(t *testing.T) {
 		Rotation: 180,
 		Scale:    dmath.Vec2{X: 2, Y: 3},
 	})
+
+	transform.RemoveParent(parent, false)
+
+	testWorldTransform(t, child, &testTransform{
+		Position: dmath.Vec2{X: 1, Y: 2},
+		Rotation: 180,
+		Scale:    dmath.Vec2{X: 2, Y: 3},
+	})
 }
 
 func TestTransformKeepWorldPosition(t *testing.T) {
@@ -64,6 +72,14 @@ func TestTransformKeepWorldPosition(t *testing.T) {
 		Rotation: 90,
 		Scale:    dmath.Vec2{X: 1.5, Y: 1.5},
 	})
+
+	transform.RemoveParent(parent, true)
+
+	testWorldTransform(t, child, &testTransform{
+		Position: dmath.Vec2{X: 1, Y: 2},
+		Rotation: 90,
+		Scale:    dmath.Vec2{X: 1.5, Y: 1.5},
+	})
 }
 
 func TestTransformDefaultValue(t *testing.T) {