Layer class Null safety

A composited layer.

During painting, the render tree generates a tree of composited layers that are uploaded into the engine and displayed by the compositor. This class is the base class for all composited layers.

Most layers can have their properties mutated, and layers can be moved to different parents. The scene must be explicitly recomposited after such changes are made; the layer tree does not maintain its own dirty state.

To composite the tree, create a SceneBuilder object, pass it to the root Layer object's addToScene method, and then call to obtain a Scene. A Scene can then be painted using dart:ui.FlutterView.render.


Layers retain resources between frames to speed up rendering. A layer will retain these resources until all LayerHandles referring to the layer have nulled out their references.

Layers must not be used after disposal. If a RenderObject needs to maintain a layer for later usage, it must create a handle to that layer. This is handled automatically for the RenderObject.layer property, but additional layers must use their own LayerHandle.

This RenderObject is a repaint boundary that pushes an additional ClipRectLayer.
class ClippingRenderObject extends RenderBox {
  final LayerHandle<ClipRectLayer> _clipRectLayer = LayerHandle<ClipRectLayer>();

  bool get isRepaintBoundary => true; // The [layer] property will be used.

  void paint(PaintingContext context, Offset offset) {
    _clipRectLayer.layer = context.pushClipRect(
      offset, & size,
      oldLayer: _clipRectLayer.layer,

  void dispose() {
    _clipRectLayer.layer = null;

See also:

Mixed in types




alwaysNeedsAddToScene bool
Subclasses may override this to true to disable retained rendering.
@protected, read-only
attached bool
Whether this node is in a tree whose root is attached to something.
read-only, inherited
debugCreator Object?
The object responsible for creating this layer.
read / write
debugDisposed bool
If asserts are enabled, returns whether dispose has been called since the last time any retained resources were created.
debugHandleCount int
Returns the number of objects holding a LayerHandle to this layer.
debugSubtreeNeedsAddToScene bool?
Whether this or any descendant layer in the subtree needs addToScene.
@visibleForTesting, read-only
depth int
The depth of this node in the tree.
read-only, inherited
engineLayer EngineLayer?
Stores the engine layer created for this layer in order to reuse engine resources across frames for better app performance.
hashCode int
The hash code for this object.
read-only, inherited
nextSibling Layer?
This layer's next sibling in the parent layer's child list.
owner Object?
The owner for this node (null if unattached).
read-only, inherited
parent ContainerLayer?
This layer's parent in the layer tree.
read-only, override
previousSibling Layer?
This layer's previous sibling in the parent layer's child list.
runtimeType Type
A representation of the runtime type of the object.
read-only, inherited
subtreeHasCompositionCallbacks bool
Whether the subtree rooted at this layer has any composition callback observers.


addCompositionCallback(CompositionCallback callback) VoidCallback
Adds a callback for when the layer tree that this layer is part of gets composited, or when it is detached and will not be rendered again.
addToScene(SceneBuilder builder) → void
Override this method to upload this layer to the engine.
adoptChild(covariant Layer child) → void
Mark the given node as being a child of this node.
attach(covariant Object owner) → void
Mark this node as attached to the given owner.
@mustCallSuper, inherited
debugDescribeChildren() List<DiagnosticsNode>
Returns a list of DiagnosticsNode objects describing this node's children.
debugFillProperties(DiagnosticPropertiesBuilder properties) → void
Add additional properties associated with the node.
debugMarkClean() → void
Mark that this layer is in sync with engine.
describeClipBounds() Rect?
Describes the clip that would be applied to contents of this layer, if any.
detach() → void
Mark this node as detached.
@mustCallSuper, inherited
dispose() → void
Clears any retained resources that this layer holds.
dropChild(covariant Layer child) → void
Disconnect the given node from this node.
find<S extends Object>(Offset localPosition) → S?
Search this layer and its subtree for the first annotation of type S under the point described by localPosition.
findAllAnnotations<S extends Object>(Offset localPosition) AnnotationResult<S>
Search this layer and its subtree for all annotations of type S under the point described by localPosition.
findAnnotations<S extends Object>(AnnotationResult<S> result, Offset localPosition, {required bool onlyFirst}) bool
Search this layer and its subtree for annotations of type S at the location described by localPosition.
markNeedsAddToScene() → void
Mark that this layer has changed and addToScene needs to be called.
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent method or property is accessed.
redepthChild(AbstractNode child) → void
Adjust the depth of the given child to be greater than this node's own depth.
@protected, inherited
redepthChildren() → void
Adjust the depth of this node's children, if any.
remove() → void
Removes this layer from its parent layer's child list.
toDiagnosticsNode({String? name, DiagnosticsTreeStyle? style}) DiagnosticsNode
Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep.
toString({DiagnosticLevel minLevel =}) String
A string representation of this object.
toStringDeep({String prefixLineOne = '', String? prefixOtherLines, DiagnosticLevel minLevel = DiagnosticLevel.debug}) String
Returns a string representation of this node and its descendants.
toStringShallow({String joiner = ', ', DiagnosticLevel minLevel = DiagnosticLevel.debug}) String
Returns a one-line detailed description of the object.
toStringShort() String
A brief description of this object, usually just the runtimeType and the hashCode.
updateSubtreeNeedsAddToScene() → void
Traverses the layer subtree starting from this layer and determines whether it needs addToScene.


operator ==(Object other) bool
The equality operator.