Capture control topic

The host captures what is under its glass by painting that part of the tree a second time, into a picture of its own. Most of the time that is exactly right. GlassProxy is for the subtrees where it is not: it tells the capture what a subtree is, and changes nothing in the frame the user sees.

Declaration The capture… For
GlassProxy.replace paints a GlassProxyPainter's stand-in instead of the subtree a video, a camera preview, a map or any platform view: they record nothing, and the glass would show a hole
GlassProxy.hidden leaves the subtree out, and does not run its paint a subtree the glass should not show, or one whose paint has side effects
GlassProxy.opaque takes the subtree as covering its own box a full-bleed image or panel: the capture stops looking under it
GlassProxy.verbatim exempts the subtree from the shadow filter a highlight blurred through a MaskFilter on purpose

A stand-in for a video

Stack(
  fit: StackFit.expand,
  children: <Widget>[
    GlassProxy.replace(
      painter: const GradientProxyPainter(
        LinearGradient(colors: <Color>[Color(0xFF1B1F2A), Color(0xFFFF9F0A)]),
      ),
      child: videoPlayer, // a platform view
    ),
    const Positioned(left: 16, right: 16, bottom: 16, child: GlassBar(child: Text('Live'))),
  ],
)

Two stand-ins come with the package, SolidProxyPainter and GradientProxyPainter; anything else is a GlassProxyPainter of your own:

class PosterProxyPainter extends GlassProxyPainter {
  const PosterProxyPainter(this.poster);

  final ui.Image poster;

  @override
  void paint(Canvas canvas, Size size) =>
      paintImage(canvas: canvas, rect: Offset.zero & size, image: poster, fit: BoxFit.cover);

  @override
  bool shouldRepaint(PosterProxyPainter oldPainter) => oldPainter.poster != poster;
}

A stand-in changes what the glass sees, not when the host captures: a video that composites a new frame is still a change. Live, with each declaration: Capture control.

Classes

GlassProxy Capture control
Declares what its subtree is in the glass proxy.
GlassProxyPainter Capture control
Draws a stand-in for a subtree in the proxy, for GlassProxy.replace.
GradientProxyPainter Capture control
A stub of one gradient. Keeps a local mean where a flat colour would not — which is the axis a blur cannot restore (D29): a low-pass removes what is above its cutoff and cannot bring back a mean that is no longer there.
RenderGlassProxy Capture control
The marker the proxy pass reads. In the real frame it is a plain proxy box.
SolidProxyPainter Capture control
A stub of one flat colour — the cheapest thing that stands for a subtree.

Enums

GlassProxyRole Capture control
The four declarations, in the order of how much they subtract.