ShaderMask needs to play nicely with compositing

Something like the following code used to work, and now no longer works.

This code should put a fading edge at the top of the block so the children fade away to the background color as they go under the mask. Now the entire block goes away. In my use case I see a chunk of area that is fully faded away that isn't faded away when just the shader mask is removed.

Future main() async =>
  runApp(new Container(
      decoration: new BoxDecoration(backgroundColor: new Color(0xFF808080)),
      child: new ShaderMask(
          shaderCallback: _createShader,
          child: new Block([
            _createColoredBox(0xFFFF0000),
            _createColoredBox(0xFF0FF000),
            _createColoredBox(0xFF0000FF),
            _createColoredBox(0xFFFFFF00)
          ]))));

Widget _createColoredBox(int hexColor) => new Container(
    height: 300.0,
    decoration: new BoxDecoration(backgroundColor: new Color(hexColor)));

// Fading edge colors need to be pure white with some value of alpha to properly fade the children.
ui.Shader _createShader(Rect bounds) => new LinearGradient(
    begin: Point.origin,
    end: new Point(0.0, 100.0),
    colors: [const Color(0x00FFFFFF), const Color(0xFFFFFFFF)],
    stops: [0.0, 1.0]).createShader();

This problem can also be demonstrated with the "Swipe Away" (card_collection.dart) widget example. Select "Let the sun shine" from the drawer:

android: I/flutter : -- EXCEPTION CAUGHT BY RENDERING LIBRARY -------------------------------
android: I/flutter : The following exception was raised during paint():
android: I/flutter : 'packages/flutter/src/rendering/object.dart': Failed assertion: line 330: '!needsCompositing' is not true.
android: I/flutter : The following RenderObject was being processed when the exception was fired:
android: I/flutter : RenderShaderMask
android: I/flutter : That RenderObject had the following owner:
android: I/flutter : ShaderMask ← Stack ← Padding ← DecoratedBox ← ⋯
android: I/flutter : Stack trace:
android: I/flutter : #0      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:27)
android: I/flutter : #1      PaintingContext.pushShaderMask (packages/flutter/src/rendering/object.dart:330)
android: I/flutter : #2      RenderShaderMask.paint (packages/flutter/src/rendering/proxy_box.dart:591)
android: I/flutter : #3      RenderObject._paintWithContext (packages/flutter/src/rendering/object.dart:1042)
android: I/flutter : #4      PaintingContext.paintChild (packages/flutter/src/rendering/object.dart:93)
android: I/flutter : #5      RenderBox&ContainerRenderObjectMixin&RenderBoxContainerDefaultsMixin.defaultPaint (packages/flutter/src/rendering/box.dart:819)
android: I/flutter : #6      RenderStack.paintStack (packages/flutter/src/rendering/stack.dart:428)
android: I/flutter : #7      RenderStackBase.paint (packages/flutter/src/rendering/stack.dart:383)

We've started using compositing more. I don't think we can afford to have paint operations that the compositor doesn't understand anymore.

Step 1: flutter-team-archive/engine#2248

Updating the example to match the new API (flutter#1155)

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.

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ShaderMask needs to play nicely with compositing #1155

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ShaderMask needs to play nicely with compositing #1155

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HansMuller commented on Jan 9, 2016

abarth commented on Jan 9, 2016

abarth commented on Jan 11, 2016

github-actions commented on Sep 6, 2021

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