flutter_shimmer2.dart

import 'package:flutter/rendering.dart';

class _Shimmer extends SingleChildRenderObjectWidget {
  final Gradient gradient;

  _Shimmer({Widget child, this.gradient})
      : super(child: child);

  @override
  _ShimmerFilter createRenderObject(BuildContext context) {
    return _ShimmerFilter(gradient);
  }
}

class _ShimmerFilter extends RenderProxyBox {
  final _clearPaint = Paint();
  final Paint _gradientPaint;
  final Gradient _gradient;

  _ShimmerFilter(this._gradient)
      : _gradientPaint = Paint()..blendMode = BlendMode.srcIn;

  @override
  bool get alwaysNeedsCompositing => child != null;
  
  @override
  void paint(PaintingContext context, Offset offset) {
    if (child != null) {
      assert(needsCompositing);

      final rect = offset & child.size;
      _gradientPaint.shader = _gradient.createShader(rect);

      context.canvas.saveLayer(rect, _clearPaint);
      context.paintChild(child, offset);
      context.canvas.drawRect(rect, _gradientPaint);
      context.canvas.restore();
    }
  }
}