packages/flutter/lib/src/widgets/snapshot_widget.dart - mirrors/flutter - Git at Google

 // Copyright 2014 The Flutter Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 import 'dart:ui' as ui;

 import 'package:flutter/rendering.dart';

 import 'basic.dart';
 import 'debug.dart';
 import 'framework.dart';
 import 'media_query.dart';

 /// Controls how the [SnapshotWidget] paints its child.
 enum SnapshotMode {
   /// The child is snapshotted, but only if all descendants can be snapshotted.
   ///
   /// If there is a platform view in the children of a snapshot widget, the
   /// snapshot will not be used and the child will be rendered using
   /// [SnapshotPainter.paint]. This uses an un-snapshotted child and by default
   /// paints it with no additional modification.
   permissive,

   /// An error is thrown if the child cannot be snapshotted.
   ///
   /// This setting is the default state of the [SnapshotWidget].
   normal,

   /// The child is snapshotted and any child platform views are ignored.
   ///
   /// This mode can be useful if there is a platform view descendant that does
   /// not need to be included in the snapshot.
   forced,
 }

 /// A controller for the [SnapshotWidget] that controls when the child image is displayed
 /// and when to regenerated the child image.
 ///
 /// When the value of [allowSnapshotting] is true, the [SnapshotWidget] will paint the child
 /// widgets based on the [SnapshotMode] of the snapshot widget.
 ///
 /// The controller notifies its listeners when the value of [allowSnapshotting] changes
 /// or when [clear] is called.
 ///
 /// To force [SnapshotWidget] to recreate the child image, call [clear].
 class SnapshotController extends ChangeNotifier {
   /// Create a new [SnapshotController].
   ///
   /// By default, [allowSnapshotting] is `false` and cannot be `null`.
   SnapshotController({
     bool allowSnapshotting = false,
   }) : _allowSnapshotting = allowSnapshotting;

   /// Reset the snapshot held by any listening [SnapshotWidget].
   ///
   /// This has no effect if [allowSnapshotting] is `false`.
   void clear() {
     notifyListeners();
   }

   /// Whether a snapshot of this child widget is painted in its place.
   bool get allowSnapshotting => _allowSnapshotting;
   bool _allowSnapshotting;
   set allowSnapshotting(bool value) {
     if (value == allowSnapshotting) {
       return;
     }
     _allowSnapshotting = value;
     notifyListeners();
   }
 }

 /// A widget that can replace its child with a snapshotted version of the child.
 ///
 /// A snapshot is a frozen texture-backed representation of all child pictures
 /// and layers stored as a [ui.Image].
 ///
 /// This widget is useful for performing short animations that would otherwise
 /// be expensive or that cannot rely on raster caching. For example, scale and
 /// skew animations are often expensive to perform on complex children, as are
 /// blurs. For a short animation, a widget that contains these expensive effects
 /// can be replaced with a snapshot of itself and manipulated instead.
 ///
 /// For example, the Android Q [ZoomPageTransitionsBuilder] uses a snapshot widget
 /// for the forward and entering route to avoid the expensive scale animation.
 /// This also has the effect of briefly pausing any animations on the page.
 ///
 /// Generally, this widget should not be used in places where users expect the
 /// child widget to continue animating or to be responsive, such as an unbounded
 /// animation.
 ///
 /// Caveats:
 ///
 /// * The contents of platform views cannot be captured by a snapshot
 ///   widget. If a platform view is encountered, then the snapshot widget will
 ///   determine how to render its children based on the [SnapshotMode]. This
 ///   defaults to [SnapshotMode.normal] which will throw an exception if a
 ///   platform view is encountered.
 ///
 /// * The snapshotting functionality of this widget is not supported on the HTML
 ///   backend of Flutter for the Web. Setting [SnapshotController.allowSnapshotting] to true
 ///   may cause an error to be thrown. On the CanvasKit backend of Flutter, the
 ///   performance of using this widget may regress performance due to the fact
 ///   that both the UI and engine share a single thread.
 class SnapshotWidget extends SingleChildRenderObjectWidget {
   /// Create a new [SnapshotWidget].
   ///
   /// The [controller] and [child] arguments are required.
   const SnapshotWidget({
     super.key,
     this.mode = SnapshotMode.normal,
     this.painter = const _DefaultSnapshotPainter(),
     required this.controller,
     required super.child
   });

   /// The controller that determines when to display the children as a snapshot.
   final SnapshotController controller;

   /// Configuration that controls how the snapshot widget decides to paint its children.
   ///
   /// Defaults to [SnapshotMode.normal], which throws an error when a platform view
   /// or texture view is encountered.
   ///
   /// See [SnapshotMode] for more information.
   final SnapshotMode mode;

   /// The painter used to paint the child snapshot or child widgets.
   final SnapshotPainter painter;

   @override
   RenderObject createRenderObject(BuildContext context) {
     debugCheckHasMediaQuery(context);
     return _RenderSnapshotWidget(
       controller: controller,
       mode: mode,
       devicePixelRatio: MediaQuery.of(context).devicePixelRatio,
       painter: painter,
     );
   }

   @override
   void updateRenderObject(BuildContext context, covariant RenderObject renderObject) {
     debugCheckHasMediaQuery(context);
     (renderObject as _RenderSnapshotWidget)
       ..controller = controller
       ..mode = mode
       ..devicePixelRatio = MediaQuery.of(context).devicePixelRatio
       ..painter = painter;
   }
 }

 // A render object that conditionally converts its child into a [ui.Image]
 // and then paints it in place of the child.
 class _RenderSnapshotWidget extends RenderProxyBox {
   // Create a new [_RenderSnapshotWidget].
   _RenderSnapshotWidget({
     required double devicePixelRatio,
     required SnapshotController controller,
     required SnapshotMode mode,
     required SnapshotPainter painter,
   }) : _devicePixelRatio = devicePixelRatio,
        _controller = controller,
        _mode = mode,
        _painter = painter;

   /// The device pixel ratio used to create the child image.
   double get devicePixelRatio => _devicePixelRatio;
   double _devicePixelRatio;
   set devicePixelRatio(double value) {
     if (value == devicePixelRatio) {
       return;
     }
     _devicePixelRatio = value;
     if (_childRaster == null) {
       return;
     } else {
       _childRaster?.dispose();
       _childRaster = null;
       markNeedsPaint();
     }
   }

   /// The painter used to paint the child snapshot or child widgets.
   SnapshotPainter get painter => _painter;
   SnapshotPainter _painter;
   set painter(SnapshotPainter value) {
     if (value == painter) {
       return;
     }
     final SnapshotPainter oldPainter = painter;
     oldPainter.removeListener(markNeedsPaint);
     _painter = value;
     if (oldPainter.runtimeType != painter.runtimeType ||
         painter.shouldRepaint(oldPainter)) {
       markNeedsPaint();
     }
     if (attached) {
       painter.addListener(markNeedsPaint);
     }
   }

   /// A controller that determines whether to paint the child normally or to
   /// paint a snapshotted version of that child.
   SnapshotController get controller => _controller;
   SnapshotController _controller;
   set controller(SnapshotController value) {
     if (value == controller) {
       return;
     }
     controller.removeListener(_onRasterValueChanged);
     final bool oldValue = controller.allowSnapshotting;
     _controller = value;
     if (attached) {
       controller.addListener(_onRasterValueChanged);
       if (oldValue != controller.allowSnapshotting) {
         _onRasterValueChanged();
       }
     }
   }

   /// How the snapshot widget will handle platform views in child layers.
   SnapshotMode get mode => _mode;
   SnapshotMode _mode;
   set mode(SnapshotMode value) {
     if (value == _mode) {
       return;
     }
     _mode = value;
     markNeedsPaint();
   }

   ui.Image? _childRaster;
   // Set to true if the snapshot mode was not forced and a platform view
   // was encountered while attempting to snapshot the child.
   bool _disableSnapshotAttempt = false;

   @override
   void attach(covariant PipelineOwner owner) {
     controller.addListener(_onRasterValueChanged);
     painter.addListener(markNeedsPaint);
     super.attach(owner);
   }

   @override
   void detach() {
     _disableSnapshotAttempt = false;
     controller.removeListener(_onRasterValueChanged);
     painter.removeListener(markNeedsPaint);
     _childRaster?.dispose();
     _childRaster = null;
     super.detach();
   }

   @override
   void dispose() {
     controller.removeListener(_onRasterValueChanged);
     painter.removeListener(markNeedsPaint);
     _childRaster?.dispose();
     _childRaster = null;
     super.dispose();
   }

   void _onRasterValueChanged() {
     _disableSnapshotAttempt = false;
     _childRaster?.dispose();
     _childRaster = null;
     markNeedsPaint();
   }

   // Paint [child] with this painting context, then convert to a raster and detach all
   // children from this layer.
   ui.Image? _paintAndDetachToImage() {
     final OffsetLayer offsetLayer = OffsetLayer();
     final PaintingContext context = PaintingContext(offsetLayer, Offset.zero & size);
     super.paint(context, Offset.zero);
     // This ignore is here because this method is protected by the `PaintingContext`. Adding a new
     // method that performs the work of `_paintAndDetachToImage` would avoid the need for this, but
     // that would conflict with our goals of minimizing painting context.
     // ignore: invalid_use_of_protected_member
     context.stopRecordingIfNeeded();
     if (mode != SnapshotMode.forced && !offsetLayer.supportsRasterization()) {
       if (mode == SnapshotMode.normal) {
         throw FlutterError('SnapshotWidget used with a child that contains a PlatformView.');
       }
       _disableSnapshotAttempt = true;
       return null;
     }
     final ui.Image image = offsetLayer.toImageSync(Offset.zero & size, pixelRatio: devicePixelRatio);
     offsetLayer.dispose();
     return image;
   }

   @override
   void paint(PaintingContext context, Offset offset) {
     if (size.isEmpty) {
       _childRaster?.dispose();
       _childRaster = null;
       return;
     }
     if (!controller.allowSnapshotting || _disableSnapshotAttempt) {
       _childRaster?.dispose();
       _childRaster = null;
       painter.paint(context, offset, size, super.paint);
       return;
     }
     _childRaster ??= _paintAndDetachToImage();
     if (_childRaster == null) {
       painter.paint(context, offset, size, super.paint);
     } else {
       painter.paintSnapshot(context, offset, size, _childRaster!, devicePixelRatio);
     }
     return;
   }
 }

 /// A painter used to paint either a snapshot or the child widgets that
 /// would be a snapshot.
 ///
 /// The painter can call [notifyListeners] to have the [SnapshotWidget]
 /// re-paint (re-using the same raster). This allows animations to be performed
 /// without re-snapshotting of children. For certain scale or perspective changing
 /// transforms, such as a rotation, this can be significantly faster than performing
 /// the same animation at the widget level.
 ///
 /// By default, the [SnapshotWidget] includes a delegate that draws the child raster
 /// exactly as the child widgets would have been drawn. Nevertheless, this can
 /// also be used to efficiently transform the child raster and apply complex paint
 /// effects.
 ///
 /// {@tool snippet}
 ///
 /// The following method shows how to efficiently rotate the child raster.
 ///
 /// ```dart
 /// void paint(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
 ///   const double radians = 0.5; // Could be driven by an animation.
 ///   final Matrix4 transform = Matrix4.rotationZ(radians);
 ///   context.canvas.transform(transform.storage);
 ///   final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
 ///   final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
 ///   final Paint paint = Paint()
 ///     ..filterQuality = FilterQuality.low;
 ///   context.canvas.drawImageRect(image, src, dst, paint);
 /// }
 /// ```
 /// {@end-tool}
 abstract class SnapshotPainter extends ChangeNotifier  {
   /// Called whenever the [image] that represents a [SnapshotWidget]s child should be painted.
   ///
   /// The image is rasterized at the physical pixel resolution and should be scaled down by
   /// [pixelRatio] to account for device independent pixels.
   ///
   /// {@tool snippet}
   ///
   /// The following method shows how the default implementation of the delegate used by the
   /// [SnapshotPainter] paints the snapshot. This must account for the fact that the image
   /// width and height will be given in physical pixels, while the image must be painted with
   /// device independent pixels. That is, the width and height of the image is the widget and
   /// height of the provided `size`, multiplied by the `pixelRatio`:
   ///
   /// ```dart
   /// void paint(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
   ///   final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
   ///   final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
   ///   final Paint paint = Paint()
   ///     ..filterQuality = FilterQuality.low;
   ///   context.canvas.drawImageRect(image, src, dst, paint);
   /// }
   /// ```
   /// {@end-tool}
   void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio);

   /// Paint the child via [painter], applying any effects that would have been painted
   /// in [SnapshotPainter.paintSnapshot].
   ///
   /// This method is called when snapshotting is disabled, or when [SnapshotMode.permissive]
   /// is used and a child platform view prevents snapshotting.
   ///
   /// The [offset] and [size] are the location and dimensions of the render object.
   void paint(PaintingContext context, Offset offset, Size size, PaintingContextCallback painter);

   /// Called whenever a new instance of the snapshot widget delegate class is
   /// provided to the [SnapshotWidget] object, or any time that a new
   /// [SnapshotPainter] object is created with a new instance of the
   /// delegate class (which amounts to the same thing, because the latter is
   /// implemented in terms of the former).
   ///
   /// If the new instance represents different information than the old
   /// instance, then the method should return true, otherwise it should return
   /// false.
   ///
   /// If the method returns false, then the [paint] call might be optimized
   /// away.
   ///
   /// It's possible that the [paint] method will get called even if
   /// [shouldRepaint] returns false (e.g. if an ancestor or descendant needed to
   /// be repainted). It's also possible that the [paint] method will get called
   /// without [shouldRepaint] being called at all (e.g. if the box changes
   /// size).
   ///
   /// Changing the delegate will not cause the child image retained by the
   /// [SnapshotWidget] to be updated. Instead, [SnapshotController.clear] can
   /// be used to force the generation of a new image.
   ///
   /// The `oldPainter` argument will never be null.
   bool shouldRepaint(covariant SnapshotPainter oldPainter);
 }

 class _DefaultSnapshotPainter implements SnapshotPainter {
   const _DefaultSnapshotPainter();

   @override
   void addListener(ui.VoidCallback listener) { }

   @override
   void dispose() { }

   @override
   bool get hasListeners => false;

   @override
   void notifyListeners() { }

   @override
   void paint(PaintingContext context, ui.Offset offset, ui.Size size, PaintingContextCallback painter) {
     painter(context, offset);
   }

   @override
   void paintSnapshot(PaintingContext context, ui.Offset offset, ui.Size size, ui.Image image, double pixelRatio) {
     final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
     final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
     final Paint paint = Paint()
       ..filterQuality = FilterQuality.low;
     context.canvas.drawImageRect(image, src, dst, paint);
   }

   @override
   void removeListener(ui.VoidCallback listener) { }

   @override
   bool shouldRepaint(covariant _DefaultSnapshotPainter oldPainter) => false;
 }
	// Copyright 2014 The Flutter Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	import 'dart:ui' as ui;

	import 'package:flutter/rendering.dart';

	import 'basic.dart';
	import 'debug.dart';
	import 'framework.dart';
	import 'media_query.dart';

	/// Controls how the [SnapshotWidget] paints its child.
	enum SnapshotMode {
	/// The child is snapshotted, but only if all descendants can be snapshotted.
	///
	/// If there is a platform view in the children of a snapshot widget, the
	/// snapshot will not be used and the child will be rendered using
	/// [SnapshotPainter.paint]. This uses an un-snapshotted child and by default
	/// paints it with no additional modification.
	permissive,

	/// An error is thrown if the child cannot be snapshotted.
	///
	/// This setting is the default state of the [SnapshotWidget].
	normal,

	/// The child is snapshotted and any child platform views are ignored.
	///
	/// This mode can be useful if there is a platform view descendant that does
	/// not need to be included in the snapshot.
	forced,
	}

	/// A controller for the [SnapshotWidget] that controls when the child image is displayed
	/// and when to regenerated the child image.
	///
	/// When the value of [allowSnapshotting] is true, the [SnapshotWidget] will paint the child
	/// widgets based on the [SnapshotMode] of the snapshot widget.
	///
	/// The controller notifies its listeners when the value of [allowSnapshotting] changes
	/// or when [clear] is called.
	///
	/// To force [SnapshotWidget] to recreate the child image, call [clear].
	class SnapshotController extends ChangeNotifier {
	/// Create a new [SnapshotController].
	///
	/// By default, [allowSnapshotting] is `false` and cannot be `null`.
	SnapshotController({
	bool allowSnapshotting = false,
	}) : _allowSnapshotting = allowSnapshotting;

	/// Reset the snapshot held by any listening [SnapshotWidget].
	///
	/// This has no effect if [allowSnapshotting] is `false`.
	void clear() {
	notifyListeners();
	}

	/// Whether a snapshot of this child widget is painted in its place.
	bool get allowSnapshotting => _allowSnapshotting;
	bool _allowSnapshotting;
	set allowSnapshotting(bool value) {
	if (value == allowSnapshotting) {
	return;
	}
	_allowSnapshotting = value;
	notifyListeners();
	}
	}

	/// A widget that can replace its child with a snapshotted version of the child.
	///
	/// A snapshot is a frozen texture-backed representation of all child pictures
	/// and layers stored as a [ui.Image].
	///
	/// This widget is useful for performing short animations that would otherwise
	/// be expensive or that cannot rely on raster caching. For example, scale and
	/// skew animations are often expensive to perform on complex children, as are
	/// blurs. For a short animation, a widget that contains these expensive effects
	/// can be replaced with a snapshot of itself and manipulated instead.
	///
	/// For example, the Android Q [ZoomPageTransitionsBuilder] uses a snapshot widget
	/// for the forward and entering route to avoid the expensive scale animation.
	/// This also has the effect of briefly pausing any animations on the page.
	///
	/// Generally, this widget should not be used in places where users expect the
	/// child widget to continue animating or to be responsive, such as an unbounded
	/// animation.
	///
	/// Caveats:
	///
	/// * The contents of platform views cannot be captured by a snapshot
	/// widget. If a platform view is encountered, then the snapshot widget will
	/// determine how to render its children based on the [SnapshotMode]. This
	/// defaults to [SnapshotMode.normal] which will throw an exception if a
	/// platform view is encountered.
	///
	/// * The snapshotting functionality of this widget is not supported on the HTML
	/// backend of Flutter for the Web. Setting [SnapshotController.allowSnapshotting] to true
	/// may cause an error to be thrown. On the CanvasKit backend of Flutter, the
	/// performance of using this widget may regress performance due to the fact
	/// that both the UI and engine share a single thread.
	class SnapshotWidget extends SingleChildRenderObjectWidget {
	/// Create a new [SnapshotWidget].
	///
	/// The [controller] and [child] arguments are required.
	const SnapshotWidget({
	super.key,
	this.mode = SnapshotMode.normal,
	this.painter = const _DefaultSnapshotPainter(),
	required this.controller,
	required super.child
	});

	/// The controller that determines when to display the children as a snapshot.
	final SnapshotController controller;

	/// Configuration that controls how the snapshot widget decides to paint its children.
	///
	/// Defaults to [SnapshotMode.normal], which throws an error when a platform view
	/// or texture view is encountered.
	///
	/// See [SnapshotMode] for more information.
	final SnapshotMode mode;

	/// The painter used to paint the child snapshot or child widgets.
	final SnapshotPainter painter;

	@override
	RenderObject createRenderObject(BuildContext context) {
	debugCheckHasMediaQuery(context);
	return _RenderSnapshotWidget(
	controller: controller,
	mode: mode,
	devicePixelRatio: MediaQuery.of(context).devicePixelRatio,
	painter: painter,
	);
	}

	@override
	void updateRenderObject(BuildContext context, covariant RenderObject renderObject) {
	debugCheckHasMediaQuery(context);
	(renderObject as _RenderSnapshotWidget)
	..controller = controller
	..mode = mode
	..devicePixelRatio = MediaQuery.of(context).devicePixelRatio
	..painter = painter;
	}
	}

	// A render object that conditionally converts its child into a [ui.Image]
	// and then paints it in place of the child.
	class _RenderSnapshotWidget extends RenderProxyBox {
	// Create a new [_RenderSnapshotWidget].
	_RenderSnapshotWidget({
	required double devicePixelRatio,
	required SnapshotController controller,
	required SnapshotMode mode,
	required SnapshotPainter painter,
	}) : _devicePixelRatio = devicePixelRatio,
	_controller = controller,
	_mode = mode,
	_painter = painter;

	/// The device pixel ratio used to create the child image.
	double get devicePixelRatio => _devicePixelRatio;
	double _devicePixelRatio;
	set devicePixelRatio(double value) {
	if (value == devicePixelRatio) {
	return;
	}
	_devicePixelRatio = value;
	if (_childRaster == null) {
	return;
	} else {
	_childRaster?.dispose();
	_childRaster = null;
	markNeedsPaint();
	}
	}

	/// The painter used to paint the child snapshot or child widgets.
	SnapshotPainter get painter => _painter;
	SnapshotPainter _painter;
	set painter(SnapshotPainter value) {
	if (value == painter) {
	return;
	}
	final SnapshotPainter oldPainter = painter;
	oldPainter.removeListener(markNeedsPaint);
	_painter = value;
	if (oldPainter.runtimeType != painter.runtimeType \|\|
	painter.shouldRepaint(oldPainter)) {
	markNeedsPaint();
	}
	if (attached) {
	painter.addListener(markNeedsPaint);
	}
	}

	/// A controller that determines whether to paint the child normally or to
	/// paint a snapshotted version of that child.
	SnapshotController get controller => _controller;
	SnapshotController _controller;
	set controller(SnapshotController value) {
	if (value == controller) {
	return;
	}
	controller.removeListener(_onRasterValueChanged);
	final bool oldValue = controller.allowSnapshotting;
	_controller = value;
	if (attached) {
	controller.addListener(_onRasterValueChanged);
	if (oldValue != controller.allowSnapshotting) {
	_onRasterValueChanged();
	}
	}
	}

	/// How the snapshot widget will handle platform views in child layers.
	SnapshotMode get mode => _mode;
	SnapshotMode _mode;
	set mode(SnapshotMode value) {
	if (value == _mode) {
	return;
	}
	_mode = value;
	markNeedsPaint();
	}

	ui.Image? _childRaster;
	// Set to true if the snapshot mode was not forced and a platform view
	// was encountered while attempting to snapshot the child.
	bool _disableSnapshotAttempt = false;

	@override
	void attach(covariant PipelineOwner owner) {
	controller.addListener(_onRasterValueChanged);
	painter.addListener(markNeedsPaint);
	super.attach(owner);
	}

	@override
	void detach() {
	_disableSnapshotAttempt = false;
	controller.removeListener(_onRasterValueChanged);
	painter.removeListener(markNeedsPaint);
	_childRaster?.dispose();
	_childRaster = null;
	super.detach();
	}

	@override
	void dispose() {
	controller.removeListener(_onRasterValueChanged);
	painter.removeListener(markNeedsPaint);
	_childRaster?.dispose();
	_childRaster = null;
	super.dispose();
	}

	void _onRasterValueChanged() {
	_disableSnapshotAttempt = false;
	_childRaster?.dispose();
	_childRaster = null;
	markNeedsPaint();
	}

	// Paint [child] with this painting context, then convert to a raster and detach all
	// children from this layer.
	ui.Image? _paintAndDetachToImage() {
	final OffsetLayer offsetLayer = OffsetLayer();
	final PaintingContext context = PaintingContext(offsetLayer, Offset.zero & size);
	super.paint(context, Offset.zero);
	// This ignore is here because this method is protected by the `PaintingContext`. Adding a new
	// method that performs the work of `_paintAndDetachToImage` would avoid the need for this, but
	// that would conflict with our goals of minimizing painting context.
	// ignore: invalid_use_of_protected_member
	context.stopRecordingIfNeeded();
	if (mode != SnapshotMode.forced && !offsetLayer.supportsRasterization()) {
	if (mode == SnapshotMode.normal) {
	throw FlutterError('SnapshotWidget used with a child that contains a PlatformView.');
	}
	_disableSnapshotAttempt = true;
	return null;
	}
	final ui.Image image = offsetLayer.toImageSync(Offset.zero & size, pixelRatio: devicePixelRatio);
	offsetLayer.dispose();
	return image;
	}

	@override
	void paint(PaintingContext context, Offset offset) {
	if (size.isEmpty) {
	_childRaster?.dispose();
	_childRaster = null;
	return;
	}
	if (!controller.allowSnapshotting \|\| _disableSnapshotAttempt) {
	_childRaster?.dispose();
	_childRaster = null;
	painter.paint(context, offset, size, super.paint);
	return;
	}
	_childRaster ??= _paintAndDetachToImage();
	if (_childRaster == null) {
	painter.paint(context, offset, size, super.paint);
	} else {
	painter.paintSnapshot(context, offset, size, _childRaster!, devicePixelRatio);
	}
	return;
	}
	}

	/// A painter used to paint either a snapshot or the child widgets that
	/// would be a snapshot.
	///
	/// The painter can call [notifyListeners] to have the [SnapshotWidget]
	/// re-paint (re-using the same raster). This allows animations to be performed
	/// without re-snapshotting of children. For certain scale or perspective changing
	/// transforms, such as a rotation, this can be significantly faster than performing
	/// the same animation at the widget level.
	///
	/// By default, the [SnapshotWidget] includes a delegate that draws the child raster
	/// exactly as the child widgets would have been drawn. Nevertheless, this can
	/// also be used to efficiently transform the child raster and apply complex paint
	/// effects.
	///
	/// {@tool snippet}
	///
	/// The following method shows how to efficiently rotate the child raster.
	///
	/// ```dart
	/// void paint(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
	/// const double radians = 0.5; // Could be driven by an animation.
	/// final Matrix4 transform = Matrix4.rotationZ(radians);
	/// context.canvas.transform(transform.storage);
	/// final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
	/// final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
	/// final Paint paint = Paint()
	/// ..filterQuality = FilterQuality.low;
	/// context.canvas.drawImageRect(image, src, dst, paint);
	/// }
	/// ```
	/// {@end-tool}
	abstract class SnapshotPainter extends ChangeNotifier {
	/// Called whenever the [image] that represents a [SnapshotWidget]s child should be painted.
	///
	/// The image is rasterized at the physical pixel resolution and should be scaled down by
	/// [pixelRatio] to account for device independent pixels.
	///
	/// {@tool snippet}
	///
	/// The following method shows how the default implementation of the delegate used by the
	/// [SnapshotPainter] paints the snapshot. This must account for the fact that the image
	/// width and height will be given in physical pixels, while the image must be painted with
	/// device independent pixels. That is, the width and height of the image is the widget and
	/// height of the provided `size`, multiplied by the `pixelRatio`:
	///
	/// ```dart
	/// void paint(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
	/// final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
	/// final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
	/// final Paint paint = Paint()
	/// ..filterQuality = FilterQuality.low;
	/// context.canvas.drawImageRect(image, src, dst, paint);
	/// }
	/// ```
	/// {@end-tool}
	void paintSnapshot(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio);

	/// Paint the child via [painter], applying any effects that would have been painted
	/// in [SnapshotPainter.paintSnapshot].
	///
	/// This method is called when snapshotting is disabled, or when [SnapshotMode.permissive]
	/// is used and a child platform view prevents snapshotting.
	///
	/// The [offset] and [size] are the location and dimensions of the render object.
	void paint(PaintingContext context, Offset offset, Size size, PaintingContextCallback painter);

	/// Called whenever a new instance of the snapshot widget delegate class is
	/// provided to the [SnapshotWidget] object, or any time that a new
	/// [SnapshotPainter] object is created with a new instance of the
	/// delegate class (which amounts to the same thing, because the latter is
	/// implemented in terms of the former).
	///
	/// If the new instance represents different information than the old
	/// instance, then the method should return true, otherwise it should return
	/// false.
	///
	/// If the method returns false, then the [paint] call might be optimized
	/// away.
	///
	/// It's possible that the [paint] method will get called even if
	/// [shouldRepaint] returns false (e.g. if an ancestor or descendant needed to
	/// be repainted). It's also possible that the [paint] method will get called
	/// without [shouldRepaint] being called at all (e.g. if the box changes
	/// size).
	///
	/// Changing the delegate will not cause the child image retained by the
	/// [SnapshotWidget] to be updated. Instead, [SnapshotController.clear] can
	/// be used to force the generation of a new image.
	///
	/// The `oldPainter` argument will never be null.
	bool shouldRepaint(covariant SnapshotPainter oldPainter);
	}

	class _DefaultSnapshotPainter implements SnapshotPainter {
	const _DefaultSnapshotPainter();

	@override
	void addListener(ui.VoidCallback listener) { }

	@override
	void dispose() { }

	@override
	bool get hasListeners => false;

	@override
	void notifyListeners() { }

	@override
	void paint(PaintingContext context, ui.Offset offset, ui.Size size, PaintingContextCallback painter) {
	painter(context, offset);
	}

	@override
	void paintSnapshot(PaintingContext context, ui.Offset offset, ui.Size size, ui.Image image, double pixelRatio) {
	final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
	final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
	final Paint paint = Paint()
	..filterQuality = FilterQuality.low;
	context.canvas.drawImageRect(image, src, dst, paint);
	}

	@override
	void removeListener(ui.VoidCallback listener) { }

	@override
	bool shouldRepaint(covariant _DefaultSnapshotPainter oldPainter) => false;
	}