Memory leak in offscreen layout pass

[wanggaowan]

Steps to reproduce

The following is the off-screen rendering logic. As long as the Code sample code is executed, the Android Native memory will grow and will not be released afterwards.

import 'dart:async';
import 'dart:ui' as ui;

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

class OffScreenRender {
  static OffScreenRender? _singleton;

  final PipelineOwner pipelineOwner = PipelineOwner();
  final FocusManager focusManager = FocusManager();
  late final BuildOwner buildOwner;
  bool isDirty = false;

  OffScreenRender._inner() {
    buildOwner = BuildOwner(
      focusManager: focusManager,
      onBuildScheduled: () {
        isDirty = true;
      },
    );
  }

  static OffScreenRender get _instance {
    return _singleton ??= OffScreenRender._inner();
  }

  factory OffScreenRender() => _instance;

  /// Render the given Widget to [ui.Image]
  /// 
  /// [width] and [height] indicate the width and height of the imaged image. 
  /// If the child does not specify a size, this width and height is also 
  /// the maximum width and height of the child.
  Future<ui.Image> render(
    Widget child,
    double width,
    double height, {
    int maxTryCount = 10,
    double? outputPixelRatio,
    int renderDelay = 100,
  }) async {
    var view = PlatformDispatcher.instance.views.first;
    var devicePixelRatio = view.devicePixelRatio;

    final RenderRepaintBoundary repaintBoundary = RenderRepaintBoundary();
    final RenderView renderView = RenderView(
      view: view,
      configuration: ViewConfiguration(
        physicalConstraints:
            BoxConstraints.expand(width: width, height: height),
        logicalConstraints: BoxConstraints.expand(width: width, height: height),
        devicePixelRatio: devicePixelRatio,
      ),
      child: repaintBoundary,
    );

    pipelineOwner.rootNode = renderView;
    renderView.prepareInitialFrame();

    final RenderObjectToWidgetElement<RenderBox> rootElement =
        RenderObjectToWidgetAdapter<RenderBox>(
      container: repaintBoundary,
      child: Directionality(
        textDirection: TextDirection.ltr,
        child: MediaQuery.fromView(
          view: view,
          child: child,
        ),
      ),
    ).attachToRenderTree(buildOwner);

    ui.Image? image;
    int tryCount = 0;
    try {
      do {
        isDirty = false;
        if (image != null) {
          image.dispose();
        }
        image = await _rebuildChild(pipelineOwner, rootElement, buildOwner,
            repaintBoundary, outputPixelRatio ?? devicePixelRatio);
        await Future.delayed(Duration(milliseconds: renderDelay));
        tryCount++;
      } while (tryCount < maxTryCount && isDirty);
      return image;
    } finally {
      pipelineOwner.rootNode = null;
      focusManager.dispose();
    }
  }

  Future<ui.Image> _rebuildChild(
      PipelineOwner pipelineOwner,
      Element child,
      BuildOwner buildOwner,
      RenderRepaintBoundary repaintBoundary,
      double outputPixelRatio) async {
    buildOwner.buildScope(child);
    buildOwner.finalizeTree();
    pipelineOwner.flushLayout();
    pipelineOwner.flushCompositingBits();
    pipelineOwner.flushPaint();
    return await repaintBoundary.toImage(pixelRatio: outputPixelRatio);
  }
}

Expected results

I want the memory to be released correctly after rendering is finished and image.dispose is called

Actual results

Memory continues to grow and will not be released, eventually causing App to flash back.

Code sample

Code sample

// After calling this method, android Native memory will gradually grow and will not be released, 
// even if image.dispose has been called to release memory
void test() async {
    for (var i = 0; i < 100; ++i) {
      var image = await OffScreenRender().render(Container(
        width: 100,
        height: 100,
        color: Colors.white,
        alignment: Alignment.center,
        child: const Text('test offscreen render'),
      ),100,100);
      // Process image, then release
      image.dispose();
    }
  }

Screenshots or Video

Screenshots / Video demonstration

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Logs

Logs

[Paste your logs here]

Flutter Doctor output

Doctor output

[✓]  Flutter (Channel [user-branch], 3.29.3, on macOS 15.7.5 24G624 darwin-arm64, locale zh-Hans-CN)
[✓]  Android toolchain - develop for Android devices (Android SDK version 36.0.0)
[✓]  Xcode - develop for iOS and macOS (Xcode 26.0.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2025.2)
[✓] Android Studio (version 2024.3)
[✓] IntelliJ IDEA Ultimate Edition (version 2025.2.4)
[✓] IntelliJ IDEA Ultimate Edition (version 2026.1.2)
[✓] VS Code (version 1.118.1)
[✓] Network resources

[mbcorona]

Before we route this, one thing worth checking: OffScreenRender is a singleton, so pipelineOwner, buildOwner, and focusManager persist across all 100 renders, but each iteration creates a fresh RenderView, RenderRepaintBoundary, and attached element tree. The finally block sets pipelineOwner.rootNode = null but doesn't dispose the RenderView / RenderRepaintBoundary or unmount the element tree from attachToRenderTree.
Could you confirm whether explicitly tearing those down between iterations changes the memory behavior? If the leak persists with proper teardown, that points at something worth investigating in the framework. If it goes away, the leak is in the harness.
Separately: focusManager.dispose() is called in finally, but focusManager is reused across calls — so iterations 2+ are using a disposed FocusManager.

[wanggaowan]

I adjusted the component unloading according to what you said, but the memory will still leak after trying. If there is still something wrong with my writing, please point it out for me.

import 'dart:async';
import 'dart:ui' as ui;

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

class OffScreenRender {
  static OffScreenRender? _singleton;

  OffScreenRender._inner();

  static OffScreenRender get _instance {
    return _singleton ??= OffScreenRender._inner();
  }

  factory OffScreenRender() => _instance;

  /// Render the given Widget to [ui.Image]
  /// 
  /// [width] and [height] indicate the width and height of the imaged image. 
  /// If the child does not specify a size, this width and height is also 
  /// the maximum width and height of the child.
  Future<ui.Image> render(
    Widget child,
    double width,
    double height, {
    int maxTryCount = 10,
    double? outputPixelRatio,
    int renderDelay = 100,
  }) async {
    var view = PlatformDispatcher.instance.views.first;
    var devicePixelRatio = view.devicePixelRatio;

    final RenderRepaintBoundary repaintBoundary = RenderRepaintBoundary();
    final RenderView renderView = RenderView(
      view: view,
      configuration: ViewConfiguration(
        physicalConstraints:
            BoxConstraints.expand(width: width, height: height),
        logicalConstraints: BoxConstraints.expand(width: width, height: height),
        devicePixelRatio: devicePixelRatio,
      ),
      child: repaintBoundary,
    );

    bool isDirty = false;
    final PipelineOwner pipelineOwner = PipelineOwner();
    final FocusManager focusManager = FocusManager();
    final BuildOwner buildOwner = BuildOwner(
      focusManager: focusManager,
      onBuildScheduled: () {
        isDirty = true;
      },
    );

    pipelineOwner.rootNode = renderView;
    renderView.prepareInitialFrame();

    final RenderObjectToWidgetElement<RenderBox> rootElement =
        RenderObjectToWidgetAdapter<RenderBox>(
      container: repaintBoundary,
      child: Directionality(
        textDirection: TextDirection.ltr,
        child: MediaQuery.fromView(
          view: view,
          child: child,
        ),
      ),
    ).attachToRenderTree(buildOwner);

    ui.Image? image;
    int tryCount = 0;
    try {
      do {
        isDirty = false;
        if (image != null) {
          image.dispose();
        }
        image = await _rebuildChild(pipelineOwner, rootElement, buildOwner,
            repaintBoundary, outputPixelRatio ?? devicePixelRatio);
        await Future.delayed(Duration(milliseconds: renderDelay));
        tryCount++;
      } while (tryCount < maxTryCount && isDirty);
      return image;
    } finally {
      pipelineOwner.rootNode = null;
      pipelineOwner.dispose();

      buildOwner.lockState(() {
        rootElement.deactivate();
        rootElement.unmount();
      });
      buildOwner.finalizeTree();

      renderView.child = null;
      focusManager.dispose();
    }
  }

  Future<ui.Image> _rebuildChild(
      PipelineOwner pipelineOwner,
      Element child,
      BuildOwner buildOwner,
      RenderRepaintBoundary repaintBoundary,
      double outputPixelRatio) async {
    buildOwner.buildScope(child);
    buildOwner.finalizeTree();
    pipelineOwner.flushLayout();
    pipelineOwner.flushCompositingBits();
    pipelineOwner.flushPaint();
    return await repaintBoundary.toImage(pixelRatio: outputPixelRatio);
  }
}

// This is the logic executed by calling 'pipelineOwner.rootNode = null'
set rootNode(RenderObject? value) {
    if (_rootNode == value) {
      return;
    }
    _rootNode?.detach();
    _rootNode = value;
    _rootNode?.attach(this);
  }

//  This is the logic executed by calling 'renderView.child = null'
set child(ChildType? value) {
    if (_child != null) {
      dropChild(_child!);
    }
    _child = value;
    if (_child != null) {
      adoptChild(_child!);
    }
  }

[mbcorona]

HI @wanggaowan
Thanks for trying that. The element tree and PipelineOwner are torn down now, but the RenderView and RenderRepaintBoundary themselves are never disposed — renderView.child = null just drops the boundary, it doesn't release its layer. RenderObject.dispose() is what clears the LayerHandle, and the RepaintBoundary's OffsetLayer (plus the picture from the last toImage()) is the likeliest holder of the native memory you're seeing.
After detaching, could you add repaintBoundary.dispose() and renderView.dispose() and re-measure? If native memory still grows once the render objects are disposed, that points at the framework and we'll route it for investigation.