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 SceneBuilder.build to obtain a Scene. A Scene can then be painted using dart:ui.FlutterView.render.

Memory

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>();

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

  @override
  void paint(PaintingContext context, Offset offset) {
    _clipRectLayer.layer = context.pushClipRect(
      needsCompositing,
      offset,
      Offset.zero & size,
      super.paint,
      oldLayer: _clipRectLayer.layer,
    );
  }

  @override
  void dispose() {
    _clipRectLayer.layer = null;
    super.dispose();
  }
}

Constructors

Layer()

Properties

alwaysNeedsAddToScene → bool: Subclasses may override this to true to disable retained rendering.
protected">@protectedread-only
attached → bool: Whether this node is in a tree whose root is attached to something.
read-onlyinherited
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.
read-only
debugHandleCount → int: Returns the number of objects holding a LayerHandle to this layer.
read-only
debugSubtreeNeedsAddToScene → bool?: Whether this or any descendant layer in the subtree needs addToScene.
visibleForTesting">@visibleForTestingread-only
depth → int: The depth of this node in the tree.
read-onlyinherited
engineLayer ↔ EngineLayer?: Stores the engine layer created for this layer in order to reuse engine resources across frames for better app performance.
protected">@protectedprotected">@protectedvisibleForTesting">@visibleForTestingvisibleForTesting">@visibleForTestingread / write
hashCode → int: The hash code for this object.
read-onlyinherited
nextSibling → Layer?: This layer's next sibling in the parent layer's child list.
read-only
owner → Object?: The owner for this node (null if unattached).
read-onlyinherited
parent → ContainerLayer?: This layer's parent in the layer tree.
read-onlyoverride
previousSibling → Layer?: This layer's previous sibling in the parent layer's child list.
read-only
runtimeType → Type: A representation of the runtime type of the object.
read-onlyinherited
subtreeHasCompositionCallbacks → bool: Whether the subtree rooted at this layer has any composition callback observers.
read-only

Methods

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.
protected">@protected
adoptChild(covariant Layer child) → void: Mark the given node as being a child of this node.
override
attach(covariant Object owner) → void: Mark this node as attached to the given owner.
mustCallSuper">@mustCallSuperinherited
debugDescribeChildren() → List<DiagnosticsNode>: Returns a list of DiagnosticsNode objects describing this node's children.
inherited
debugFillProperties(DiagnosticPropertiesBuilder properties) → void: Add additional properties associated with the node.
override
debugMarkClean() → void: Mark that this layer is in sync with engine.
visibleForTesting">@visibleForTesting
describeClipBounds() → Rect?: Describes the clip that would be applied to contents of this layer, if any.
detach() → void: Mark this node as detached.
mustCallSuper">@mustCallSuperinherited
dispose() → void: Clears any retained resources that this layer holds.
mustCallSuper">@mustCallSuperprotected">@protectedvisibleForTesting">@visibleForTesting
dropChild(covariant Layer child) → void: Disconnect the given node from this node.
override
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.
protected">@protected
markNeedsAddToScene() → void: Mark that this layer has changed and addToScene needs to be called.
protected">@protectedvisibleForTesting">@visibleForTesting
noSuchMethod(Invocation invocation) → dynamic: Invoked when a non-existent method or property is accessed.
inherited
redepthChild(AbstractNode child) → void: Adjust the depth of the given child to be greater than this node's own depth.
protected">@protectedinherited
redepthChildren() → void: Adjust the depth of this node's children, if any.
inherited
remove() → void: Removes this layer from its parent layer's child list.
mustCallSuper">@mustCallSuper
supportsRasterization() → bool: Whether or not this layer, or any child layers, can be rasterized with Scene.toImage or Scene.toImageSync.
toDiagnosticsNode({String? name, DiagnosticsTreeStyle? style}) → DiagnosticsNode: Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep.
inherited
toString({DiagnosticLevel minLevel = DiagnosticLevel.info}) → String: A string representation of this object.
inherited
toStringDeep({String prefixLineOne = '', String? prefixOtherLines, DiagnosticLevel minLevel = DiagnosticLevel.debug}) → String: Returns a string representation of this node and its descendants.
inherited
toStringShallow({String joiner = ', ', DiagnosticLevel minLevel = DiagnosticLevel.debug}) → String: Returns a one-line detailed description of the object.
inherited
toStringShort() → String: A brief description of this object, usually just the runtimeType and the hashCode.
override
updateSubtreeNeedsAddToScene() → void: Traverses the layer subtree starting from this layer and determines whether it needs addToScene.
protected">@protectedvisibleForTesting">@visibleForTesting

Operators

operator ==(Object other) → bool: The equality operator.
inherited