Opacity class Null safety
A widget that makes its child partially transparent.
This class paints its child into an intermediate buffer and then blends the child back into the scene partially transparent.
For values of opacity other than 0.0 and 1.0, this class is relatively expensive because it requires painting the child into an intermediate buffer. For the value 0.0, the child is simply not painted at all. For the value 1.0, the child is painted immediately without an intermediate buffer.
The presence of the intermediate buffer which has a transparent background by default may cause some child widgets to behave differently. For example a BackdropFilter child will only be able to apply its filter to the content between this widget and the backdrop child and may require adjusting the BackdropFilter.blendMode property to produce the desired results.
_visible
member field is true, and
hides it when it is false:
Opacity(
opacity: _visible ? 1.0 : 0.0,
child: const Text("Now you see me, now you don't!"),
)
This is more efficient than adding and removing the child widget from the tree on demand.
Performance considerations for opacity animation
Animating an Opacity widget directly causes the widget (and possibly its subtree) to rebuild each frame, which is not very efficient. Consider using an AnimatedOpacity or a FadeTransition instead.
Transparent image
If only a single Image or Color needs to be composited with an opacity between 0.0 and 1.0, it's much faster to directly use them without Opacity widgets.
For example, Container(color: Color.fromRGBO(255, 0, 0, 0.5))
is much
faster than Opacity(opacity: 0.5, child: Container(color: Colors.red))
.
Image.network(
'https://raw.githubusercontent.com/flutter/assets-for-api-docs/master/packages/diagrams/assets/blend_mode_destination.jpeg',
color: const Color.fromRGBO(255, 255, 255, 0.5),
colorBlendMode: BlendMode.modulate
)
Directly drawing an Image or Color with opacity is faster than using Opacity on top of them because Opacity could apply the opacity to a group of widgets and therefore a costly offscreen buffer will be used. Drawing content into the offscreen buffer may also trigger render target switches and such switching is particularly slow in older GPUs.
See also:
- Visibility, which can hide a child more efficiently (albeit less subtly, because it is either visible or hidden, rather than allowing fractional opacity values).
- ShaderMask, which can apply more elaborate effects to its child.
- Transform, which applies an arbitrary transform to its child widget at paint time.
- AnimatedOpacity, which uses an animation internally to efficiently animate opacity.
- FadeTransition, which uses a provided animation to efficiently animate opacity.
- Image, which can directly provide a partially transparent image with much less performance hit.
- Inheritance
Constructors
Properties
- alwaysIncludeSemantics → bool
-
Whether the semantic information of the children is always included.
final
- child → Widget?
-
The widget below this widget in the tree.
finalinherited
- hashCode → int
- The hash code for this object.
- key → Key?
-
Controls how one widget replaces another widget in the tree.
finalinherited
- opacity → double
-
The fraction to scale the child's alpha value.
final
- runtimeType → Type
-
A representation of the runtime type of the object.
read-onlyinherited
Methods
-
createElement(
) → SingleChildRenderObjectElement -
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
inherited
-
createRenderObject(
BuildContext context) → RenderOpacity -
Creates an instance of the RenderObject class that this
RenderObjectWidget represents, using the configuration described by this
RenderObjectWidget.
override
-
debugDescribeChildren(
) → List< DiagnosticsNode> -
Returns a list of
DiagnosticsNode
objects describing this node's children.protected">@protectedinherited -
debugFillProperties(
DiagnosticPropertiesBuilder properties) → void -
Add additional properties associated with the node.
override
-
didUnmountRenderObject(
covariant RenderObject renderObject) → void -
A render object previously associated with this widget has been removed
from the tree. The given RenderObject will be of the same type as
returned by this object's createRenderObject.
protected">@protectedinherited
-
noSuchMethod(
Invocation invocation) → dynamic -
Invoked when a non-existent method or property is accessed.
inherited
-
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 short, textual description of this widget.
inherited
-
updateRenderObject(
BuildContext context, covariant RenderOpacity renderObject) → void -
Copies the configuration described by this RenderObjectWidget to the
given RenderObject, which will be of the same type as returned by this
object's createRenderObject.
override
Operators
-
operator ==(
Object other) → bool -
The equality operator.
nonVirtual">@nonVirtualinherited