opacity property

@override double get opacity

The fraction to scale the child's alpha value.

An opacity of 1.0 is fully opaque. An opacity of 0.0 is fully transparent (i.e., invisible).

The opacity must not be null.

Values 1.0 and 0.0 are painted with a fast path. Other values require painting the child into an intermediate buffer, which is expensive.

Implementation

@override
double get opacity => _opacity ?? 1.0;

set opacity (double? value)

Implementation

set opacity(double? value) {
  if (_opacity == value) return;

  final double previousOpacity = opacity;
  final int previousAlpha = ui.Color.getAlphaFromOpacity(previousOpacity);
  final bool previousCreatesStackingContext = previousOpacity < 1.0;
  final bool previousNeedsCompositing = previousAlpha != 0 && previousAlpha != 255;

  _opacity = value;
  final double nextOpacity = opacity;
  final int alpha = ui.Color.getAlphaFromOpacity(nextOpacity);
  final bool nextCreatesStackingContext = nextOpacity < 1.0;
  final bool nextNeedsCompositing = alpha != 0 && alpha != 255;
  getSelfRenderBoxValue((renderBoxModel, _) {
    renderBoxModel.alpha = alpha;
  });

  // Opacity only changes compositing requirements when it crosses the
  // intermediate alpha range that uses an opacity layer.
  if (previousNeedsCompositing != nextNeedsCompositing) {
    markNeedsCompositingBitsUpdate();
  }

  // Opacity affects stacking order only when it starts or stops creating a
  // stacking context. Fractional opacity changes within the same range do not
  // require resorting siblings every frame.
  if (previousCreatesStackingContext != nextCreatesStackingContext) {
    getAttachedRenderParentRenderStyle()?.markChildrenNeedsSort();
  }

  markNeedsPaint();
}