Garage-SDKs/garage_ui/lib/app_frame_capture.dart

import "dart:ui" as ui;

import "package:flutter/foundation.dart";
import "package:flutter/rendering.dart";
import "package:flutter/widgets.dart";

// Wraps the whole app in a RepaintBoundary we can snapshot on demand, so an
// eyedropper has something to read pixels out of on platforms with no system
// colour sampler (see platform/eyedropper.dart for the ones that do).
//
// This obviously only covers whats inside the flutter window - thats the
// tradeoff for it working everywhere. Mounted once, in main.dart, just under
// CustomCursorLayer so the fake cursor doesnt end up baked into the snapshot
// and get sampled by the very thing thats drawing it.
class AppFrameCapture extends StatelessWidget {
  const AppFrameCapture({super.key, required this.child});

  static final GlobalKey _boundaryKey = GlobalKey(
    debugLabel: "app frame capture",
  );

  final Widget child;

  @override
  Widget build(BuildContext context) {
    return RepaintBoundary(key: _boundaryKey, child: child);
  }

  /// Grabs the current frame. Returns null (and logs why) if theres nothing
  /// to grab - no boundary mounted yet, or the raster came back empty.
  static Future<AppFrameSnapshot?> capture() async {
    final object = _boundaryKey.currentContext?.findRenderObject();
    if (object is! RenderRepaintBoundary) {
      debugPrint(
        "AppFrameCapture.capture: no boundary mounted, is AppFrameCapture in "
        "the tree?",
      );
      return null;
    }

    ui.Image? image;
    try {
      // pixelRatio 1 on purpose - it keeps image pixels and logical pixels the
      // same thing, so colorAt can index straight off a pointer position with
      // no dpr maths, and it keeps the byte buffer a quarter of the size on a
      // retina display.
      image = await object.toImage();
      final width = image.width;
      final height = image.height;
      final bytes = await image.toByteData(format: ui.ImageByteFormat.rawRgba);
      if (bytes == null) {
        debugPrint("AppFrameCapture.capture: toByteData returned null");
        return null;
      }

      return AppFrameSnapshot._(
        bytes.buffer.asUint8List(),
        width,
        height,
        object.localToGlobal(Offset.zero),
      );
    } catch (error, stack) {
      debugPrint("AppFrameCapture.capture failed: $error\n$stack");
      return null;
    } finally {
      image?.dispose();
    }
  }
}

/// A frozen copy of the app window you can read single pixels out of.
class AppFrameSnapshot {
  AppFrameSnapshot._(this._pixels, this.width, this.height, this.origin);

  /// straight-from-bytes ctor, only so the sampling maths can be probed
  /// without standing up a whole render tree.
  @visibleForTesting
  AppFrameSnapshot.fromRawRgba({
    required Uint8List pixels,
    required this.width,
    required this.height,
    this.origin = Offset.zero,
  }) : _pixels = pixels;

  final Uint8List _pixels;
  final int width;
  final int height;

  /// where the captured boundary sits in global coordinates. normally zero,
  /// but not worth assuming.
  final Offset origin;

  /// The colour at [globalPosition], or null if thats outside the captured
  /// area or lands on a fully transparent pixel.
  Color? colorAt(Offset globalPosition) {
    final local = globalPosition - origin;
    final x = local.dx.floor();
    final y = local.dy.floor();
    if (x < 0 || y < 0 || x >= width || y >= height) return null;

    final i = (y * width + x) * 4;
    final a = _pixels[i + 3];
    if (a == 0) return null;

    // rawRgba is premultiplied, so anything drawn over a translucent layer
    // reads darker than it looks unless we undo that first.
    int channel(int offset) {
      final value = _pixels[i + offset];
      if (a == 255) return value;
      return (value * 255 / a).round().clamp(0, 255);
    }

    return Color.fromARGB(255, channel(0), channel(1), channel(2));
  }
}