Opacity class

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.

This example shows some Text when the _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 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)).

The following example draws an Image with 0.5 opacity without using Opacity: 
Image.network(
  'https://github.com/flutter/flutter_gallery_assets/raw/master/lib/products/backpack.png',
  color: 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

Opacity({Key key, @required double opacity, bool alwaysIncludeSemantics: false, Widget child })
Creates a widget that makes its child partially transparent. [...]
const

Properties

alwaysIncludeSemantics → bool
Whether the semantic information of the children is always included. [...]
final
opacity → double
The fraction to scale the child's alpha value. [...]
final
child → Widget
The widget below this widget in the tree. [...]
final, inherited
hashCode → int
The hash code for this object. [...]
read-only, inherited
key → Key
Controls how one widget replaces another widget in the tree. [...]
final, inherited
runtimeType → Type
A representation of the runtime type of the object.
read-only, inherited

Methods

createRenderObject(BuildContext context) → RenderOpacity
Creates an instance of the RenderObject class that this RenderObjectWidget represents, using the configuration described by this RenderObjectWidget. [...]
override
debugFillProperties(DiagnosticPropertiesBuilder properties) → void
Add additional properties associated with the node. [...]
override
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
createElement() → SingleChildRenderObjectElement
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
inherited
debugDescribeChildren() → List<DiagnosticsNode>
Returns a list of DiagnosticsNode objects describing this node's children. [...]
@protected, inherited
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, inherited
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 toStringDeep. [...]
inherited
toString({DiagnosticLevel minLevel: DiagnosticLevel.debug }) → String
Returns 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

Operators

operator ==(dynamic other) → bool
The equality operator. [...]
inherited