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 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.

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 one of these alternative widgets instead:

• AnimatedOpacity, which uses an animation internally to efficiently animate opacity.
• FadeTransition, which uses a provided animation to efficiently animate opacity.

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/main/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.

Hit testing

Setting the opacity to zero does not prevent hit testing from being applied to the descendants of the Opacity widget. This can be confusing for the user, who may not see anything, and may believe the area of the interface where the Opacity is hiding a widget to be non-interactive.

With certain widgets, such as Flow, that compute their positions only when they are painted, this can actually lead to bugs (from unexpected geometry to exceptions), because those widgets are not painted by the Opacity widget at all when the opacity is zero.

To avoid such problems, it is generally a good idea to use an IgnorePointer widget when setting the opacity to zero. This prevents interactions with any children in the subtree.

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). Specifically, the Visibility.maintain constructor is equivalent to using an opacity widget with values of 0.0 or 1.0.
• ShaderMask, which can apply more elaborate effects to its child.
• Transform, which applies an arbitrary transform to its child widget at paint time.
• SliverOpacity, the sliver version of this widget.