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// 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:developer' as developer;
import 'dart:ui' as ui show Image;
import 'package:flutter/foundation.dart';
import 'package:flutter/scheduler.dart';
import 'alignment.dart';
import 'basic_types.dart';
import 'borders.dart';
import 'box_fit.dart';
import 'debug.dart';
import 'image_provider.dart';
import 'image_stream.dart';
/// How to paint any portions of a box not covered by an image.
enum ImageRepeat {
/// Repeat the image in both the x and y directions until the box is filled.
/// Repeat the image in the x direction until the box is filled horizontally.
/// Repeat the image in the y direction until the box is filled vertically.
/// Leave uncovered portions of the box transparent.
/// An image for a box decoration.
/// The image is painted using [paintImage], which describes the meanings of the
/// various fields on this class in more detail.
class DecorationImage {
/// Creates an image to show in a [BoxDecoration].
/// The [image], [alignment], [repeat], and [matchTextDirection] arguments
/// must not be null.
const DecorationImage({
required this.image,
this.alignment =,
this.repeat = ImageRepeat.noRepeat,
this.matchTextDirection = false,
this.scale = 1.0
}) : assert(image != null),
assert(alignment != null),
assert(repeat != null),
assert(matchTextDirection != null),
assert(scale != null);
/// The image to be painted into the decoration.
/// Typically this will be an [AssetImage] (for an image shipped with the
/// application) or a [NetworkImage] (for an image obtained from the network).
final ImageProvider image;
/// An optional error callback for errors emitted when loading [image].
final ImageErrorListener? onError;
/// A color filter to apply to the image before painting it.
final ColorFilter? colorFilter;
/// How the image should be inscribed into the box.
/// The default is [BoxFit.scaleDown] if [centerSlice] is null, and
/// [BoxFit.fill] if [centerSlice] is not null.
/// See the discussion at [paintImage] for more details.
final BoxFit? fit;
/// How to align the image within its bounds.
/// The alignment aligns the given position in the image to the given position
/// in the layout bounds. For example, an [Alignment] alignment of (-1.0,
/// -1.0) aligns the image to the top-left corner of its layout bounds, while a
/// [Alignment] alignment of (1.0, 1.0) aligns the bottom right of the
/// image with the bottom right corner of its layout bounds. Similarly, an
/// alignment of (0.0, 1.0) aligns the bottom middle of the image with the
/// middle of the bottom edge of its layout bounds.
/// To display a subpart of an image, consider using a [CustomPainter] and
/// [Canvas.drawImageRect].
/// If the [alignment] is [TextDirection]-dependent (i.e. if it is a
/// [AlignmentDirectional]), then a [TextDirection] must be available
/// when the image is painted.
/// Defaults to [].
/// See also:
/// * [Alignment], a class with convenient constants typically used to
/// specify an [AlignmentGeometry].
/// * [AlignmentDirectional], like [Alignment] for specifying alignments
/// relative to text direction.
final AlignmentGeometry alignment;
/// The center slice for a nine-patch image.
/// The region of the image inside the center slice will be stretched both
/// horizontally and vertically to fit the image into its destination. The
/// region of the image above and below the center slice will be stretched
/// only horizontally and the region of the image to the left and right of
/// the center slice will be stretched only vertically.
/// The stretching will be applied in order to make the image fit into the box
/// specified by [fit]. When [centerSlice] is not null, [fit] defaults to
/// [BoxFit.fill], which distorts the destination image size relative to the
/// image's original aspect ratio. Values of [BoxFit] which do not distort the
/// destination image size will result in [centerSlice] having no effect
/// (since the nine regions of the image will be rendered with the same
/// scaling, as if it wasn't specified).
final Rect? centerSlice;
/// How to paint any portions of the box that would not otherwise be covered
/// by the image.
final ImageRepeat repeat;
/// Whether to paint the image in the direction of the [TextDirection].
/// If this is true, then in [TextDirection.ltr] contexts, the image will be
/// drawn with its origin in the top left (the "normal" painting direction for
/// images); and in [TextDirection.rtl] contexts, the image will be drawn with
/// a scaling factor of -1 in the horizontal direction so that the origin is
/// in the top right.
final bool matchTextDirection;
/// Defines image pixels to be shown per logical pixels.
/// By default the value of scale is 1.0. The scale for the image is
/// calculated by multiplying [scale] with `scale` of the given [ImageProvider].
final double scale;
/// Creates a [DecorationImagePainter] for this [DecorationImage].
/// The `onChanged` argument must not be null. It will be called whenever the
/// image needs to be repainted, e.g. because it is loading incrementally or
/// because it is animated.
DecorationImagePainter createPainter(VoidCallback onChanged) {
assert(onChanged != null);
return DecorationImagePainter._(this, onChanged);
bool operator ==(Object other) {
if (identical(this, other))
return true;
if (other.runtimeType != runtimeType)
return false;
return other is DecorationImage
&& other.image == image
&& other.colorFilter == colorFilter
&& == fit
&& other.alignment == alignment
&& other.centerSlice == centerSlice
&& other.repeat == repeat
&& other.matchTextDirection == matchTextDirection
&& other.scale == scale;
int get hashCode => hashValues(image, colorFilter, fit, alignment, centerSlice, repeat, matchTextDirection, scale);
String toString() {
final List<String> properties = <String>[
if (colorFilter != null)
if (fit != null &&
!(fit == BoxFit.fill && centerSlice != null) &&
!(fit == BoxFit.scaleDown && centerSlice == null))
if (centerSlice != null)
'centerSlice: $centerSlice',
if (repeat != ImageRepeat.noRepeat)
if (matchTextDirection)
'match text direction',
'scale: $scale'
return '${objectRuntimeType(this, 'DecorationImage')}(${properties.join(", ")})';
/// The painter for a [DecorationImage].
/// To obtain a painter, call [DecorationImage.createPainter].
/// To paint, call [paint]. The `onChanged` callback passed to
/// [DecorationImage.createPainter] will be called if the image needs to paint
/// again (e.g. because it is animated or because it had not yet loaded the
/// first time the [paint] method was called).
/// This object should be disposed using the [dispose] method when it is no
/// longer needed.
class DecorationImagePainter {
DecorationImagePainter._(this._details, this._onChanged) : assert(_details != null);
final DecorationImage _details;
final VoidCallback _onChanged;
ImageStream? _imageStream;
ImageInfo? _image;
/// Draw the image onto the given canvas.
/// The image is drawn at the position and size given by the `rect` argument.
/// The image is clipped to the given `clipPath`, if any.
/// The `configuration` object is used to resolve the image (e.g. to pick
/// resolution-specific assets), and to implement the
/// [DecorationImage.matchTextDirection] feature.
/// If the image needs to be painted again, e.g. because it is animated or
/// because it had not yet been loaded the first time this method was called,
/// then the `onChanged` callback passed to [DecorationImage.createPainter]
/// will be called.
void paint(Canvas canvas, Rect rect, Path? clipPath, ImageConfiguration configuration) {
assert(canvas != null);
assert(rect != null);
assert(configuration != null);
bool flipHorizontally = false;
if (_details.matchTextDirection) {
assert(() {
// We check this first so that the assert will fire immediately, not just
// when the image is ready.
if (configuration.textDirection == null) {
throw FlutterError.fromParts(<DiagnosticsNode>[
ErrorSummary('DecorationImage.matchTextDirection can only be used when a TextDirection is available.'),
'When DecorationImagePainter.paint() was called, there was no text direction provided '
'in the ImageConfiguration object to match.'
DiagnosticsProperty<DecorationImage>('The DecorationImage was', _details, style: DiagnosticsTreeStyle.errorProperty),
DiagnosticsProperty<ImageConfiguration>('The ImageConfiguration was', configuration, style: DiagnosticsTreeStyle.errorProperty),
return true;
if (configuration.textDirection == TextDirection.rtl)
flipHorizontally = true;
final ImageStream newImageStream = _details.image.resolve(configuration);
if (newImageStream.key != _imageStream?.key) {
final ImageStreamListener listener = ImageStreamListener(
onError: _details.onError,
_imageStream = newImageStream;
if (_image == null)
if (clipPath != null) {;
canvas: canvas,
rect: rect,
image: _image!.image,
debugImageLabel: _image!.debugLabel,
scale: _details.scale * _image!.scale,
colorFilter: _details.colorFilter,
alignment: _details.alignment.resolve(configuration.textDirection),
centerSlice: _details.centerSlice,
repeat: _details.repeat,
flipHorizontally: flipHorizontally,
filterQuality: FilterQuality.low,
if (clipPath != null)
void _handleImage(ImageInfo value, bool synchronousCall) {
if (_image == value)
_image = value;
assert(_onChanged != null);
if (!synchronousCall)
/// Releases the resources used by this painter.
/// This should be called whenever the painter is no longer needed.
/// After this method has been called, the object is no longer usable.
void dispose() {
onError: _details.onError,
String toString() {
return '${objectRuntimeType(this, 'DecorationImagePainter')}(stream: $_imageStream, image: $_image) for $_details';
/// Used by [paintImage] to report image sizes drawn at the end of the frame.
Map<String, ImageSizeInfo> _pendingImageSizeInfo = <String, ImageSizeInfo>{};
/// [ImageSizeInfo]s that were reported on the last frame.
/// Used to prevent duplicative reports from frame to frame.
Set<ImageSizeInfo> _lastFrameImageSizeInfo = <ImageSizeInfo>{};
/// Flushes inter-frame tracking of image size information from [paintImage].
/// Has no effect if asserts are disabled.
void debugFlushLastFrameImageSizeInfo() {
assert(() {
_lastFrameImageSizeInfo = <ImageSizeInfo>{};
return true;
/// Paints an image into the given rectangle on the canvas.
/// The arguments have the following meanings:
/// * `canvas`: The canvas onto which the image will be painted.
/// * `rect`: The region of the canvas into which the image will be painted.
/// The image might not fill the entire rectangle (e.g., depending on the
/// `fit`). If `rect` is empty, nothing is painted.
/// * `image`: The image to paint onto the canvas.
/// * `scale`: The number of image pixels for each logical pixel.
/// * `colorFilter`: If non-null, the color filter to apply when painting the
/// image.
/// * `fit`: How the image should be inscribed into `rect`. If null, the
/// default behavior depends on `centerSlice`. If `centerSlice` is also null,
/// the default behavior is [BoxFit.scaleDown]. If `centerSlice` is
/// non-null, the default behavior is [BoxFit.fill]. See [BoxFit] for
/// details.
/// * `alignment`: How the destination rectangle defined by applying `fit` is
/// aligned within `rect`. For example, if `fit` is [BoxFit.contain] and
/// `alignment` is [Alignment.bottomRight], the image will be as large
/// as possible within `rect` and placed with its bottom right corner at the
/// bottom right corner of `rect`. Defaults to [].
/// * `centerSlice`: The image is drawn in nine portions described by splitting
/// the image by drawing two horizontal lines and two vertical lines, where
/// `centerSlice` describes the rectangle formed by the four points where
/// these four lines intersect each other. (This forms a 3-by-3 grid
/// of regions, the center region being described by `centerSlice`.)
/// The four regions in the corners are drawn, without scaling, in the four
/// corners of the destination rectangle defined by applying `fit`. The
/// remaining five regions are drawn by stretching them to fit such that they
/// exactly cover the destination rectangle while maintaining their relative
/// positions.
/// * `repeat`: If the image does not fill `rect`, whether and how the image
/// should be repeated to fill `rect`. By default, the image is not repeated.
/// See [ImageRepeat] for details.
/// * `flipHorizontally`: Whether to flip the image horizontally. This is
/// occasionally used with images in right-to-left environments, for images
/// that were designed for left-to-right locales (or vice versa). Be careful,
/// when using this, to not flip images with integral shadows, text, or other
/// effects that will look incorrect when flipped.
/// * `invertColors`: Inverting the colors of an image applies a new color
/// filter to the paint. If there is another specified color filter, the
/// invert will be applied after it. This is primarily used for implementing
/// smart invert on iOS.
/// * `filterQuality`: Use this to change the quality when scaling an image.
/// Use the [FilterQuality.low] quality setting to scale the image, which corresponds to
/// bilinear interpolation, rather than the default [FilterQuality.none] which corresponds
/// to nearest-neighbor.
/// The `canvas`, `rect`, `image`, `scale`, `alignment`, `repeat`, `flipHorizontally` and `filterQuality`
/// arguments must not be null.
/// See also:
/// * [paintBorder], which paints a border around a rectangle on a canvas.
/// * [DecorationImage], which holds a configuration for calling this function.
/// * [BoxDecoration], which uses this function to paint a [DecorationImage].
void paintImage({
required Canvas canvas,
required Rect rect,
required ui.Image image,
String? debugImageLabel,
double scale = 1.0,
ColorFilter? colorFilter,
BoxFit? fit,
Alignment alignment =,
Rect? centerSlice,
ImageRepeat repeat = ImageRepeat.noRepeat,
bool flipHorizontally = false,
bool invertColors = false,
FilterQuality filterQuality = FilterQuality.low,
bool isAntiAlias = false,
}) {
assert(canvas != null);
assert(image != null);
assert(alignment != null);
assert(repeat != null);
assert(flipHorizontally != null);
assert(isAntiAlias != null);
if (rect.isEmpty)
Size outputSize = rect.size;
Size inputSize = Size(image.width.toDouble(), image.height.toDouble());
Offset? sliceBorder;
if (centerSlice != null) {
sliceBorder = Offset(
centerSlice.left + inputSize.width - centerSlice.right, + inputSize.height - centerSlice.bottom,
outputSize = outputSize - sliceBorder as Size;
inputSize = inputSize - sliceBorder as Size;
fit ??= centerSlice == null ? BoxFit.scaleDown : BoxFit.fill;
assert(centerSlice == null || (fit != BoxFit.none && fit != BoxFit.cover));
final FittedSizes fittedSizes = applyBoxFit(fit, inputSize / scale, outputSize);
final Size sourceSize = fittedSizes.source * scale;
Size destinationSize = fittedSizes.destination;
if (centerSlice != null) {
outputSize += sliceBorder!;
destinationSize += sliceBorder;
// We don't have the ability to draw a subset of the image at the same time
// as we apply a nine-patch stretch.
assert(sourceSize == inputSize, 'centerSlice was used with a BoxFit that does not guarantee that the image is fully visible.');
if (repeat != ImageRepeat.noRepeat && destinationSize == outputSize) {
// There's no need to repeat the image because we're exactly filling the
// output rect with the image.
repeat = ImageRepeat.noRepeat;
final Paint paint = Paint()..isAntiAlias = isAntiAlias;
if (colorFilter != null)
paint.colorFilter = colorFilter;
if (sourceSize != destinationSize) {
paint.filterQuality = filterQuality;
paint.invertColors = invertColors;
final double halfWidthDelta = (outputSize.width - destinationSize.width) / 2.0;
final double halfHeightDelta = (outputSize.height - destinationSize.height) / 2.0;
final double dx = halfWidthDelta + (flipHorizontally ? -alignment.x : alignment.x) * halfWidthDelta;
final double dy = halfHeightDelta + alignment.y * halfHeightDelta;
final Offset destinationPosition = rect.topLeft.translate(dx, dy);
final Rect destinationRect = destinationPosition & destinationSize;
// Set to true if we added a saveLayer to the canvas to invert/flip the image.
bool invertedCanvas = false;
// Output size and destination rect are fully calculated.
if (!kReleaseMode) {
final ImageSizeInfo sizeInfo = ImageSizeInfo(
// Some ImageProvider implementations may not have given this.
source: debugImageLabel ?? '<Unknown Image(${image.width}×${image.height})>',
imageSize: Size(image.width.toDouble(), image.height.toDouble()),
displaySize: outputSize,
assert(() {
if (debugInvertOversizedImages &&
sizeInfo.decodedSizeInBytes > sizeInfo.displaySizeInBytes + debugImageOverheadAllowance) {
final int overheadInKilobytes = (sizeInfo.decodedSizeInBytes - sizeInfo.displaySizeInBytes) ~/ 1024;
final int outputWidth = outputSize.width.toInt();
final int outputHeight = outputSize.height.toInt();
exception: 'Image $debugImageLabel has a display size of '
'$outputWidth×$outputHeight but a decode size of '
'${image.width}×${image.height}, which uses an additional '
'Consider resizing the asset ahead of time, supplying a cacheWidth '
'parameter of $outputWidth, a cacheHeight parameter of '
'$outputHeight, or using a ResizeImage.',
library: 'painting library',
context: ErrorDescription('while painting an image'),
// Invert the colors of the canvas.
Paint()..colorFilter = const ColorFilter.matrix(<double>[
-1, 0, 0, 0, 255,
0, -1, 0, 0, 255,
0, 0, -1, 0, 255,
0, 0, 0, 1, 0,
// Flip the canvas vertically.
final double dy = -( + rect.height / 2.0);
canvas.translate(0.0, -dy);
canvas.scale(1.0, -1.0);
canvas.translate(0.0, dy);
invertedCanvas = true;
return true;
// Avoid emitting events that are the same as those emitted in the last frame.
if (!_lastFrameImageSizeInfo.contains(sizeInfo)) {
final ImageSizeInfo? existingSizeInfo = _pendingImageSizeInfo[sizeInfo.source];
if (existingSizeInfo == null || existingSizeInfo.displaySizeInBytes < sizeInfo.displaySizeInBytes) {
_pendingImageSizeInfo[sizeInfo.source!] = sizeInfo;
if (debugOnPaintImage != null) {
SchedulerBinding.instance!.addPostFrameCallback((Duration timeStamp) {
_lastFrameImageSizeInfo = _pendingImageSizeInfo.values.toSet();
if (_pendingImageSizeInfo.isEmpty) {
<String, Object>{
for (ImageSizeInfo imageSizeInfo in _pendingImageSizeInfo.values)
imageSizeInfo.source!: imageSizeInfo.toJson()
_pendingImageSizeInfo = <String, ImageSizeInfo>{};
final bool needSave = repeat != ImageRepeat.noRepeat || flipHorizontally;
if (needSave);
if (repeat != ImageRepeat.noRepeat)
if (flipHorizontally) {
final double dx = -(rect.left + rect.width / 2.0);
canvas.translate(-dx, 0.0);
canvas.scale(-1.0, 1.0);
canvas.translate(dx, 0.0);
if (centerSlice == null) {
final Rect sourceRect = alignment.inscribe(
sourceSize, & inputSize,
if (repeat == ImageRepeat.noRepeat) {
canvas.drawImageRect(image, sourceRect, destinationRect, paint);
} else {
for (final Rect tileRect in _generateImageTileRects(rect, destinationRect, repeat))
canvas.drawImageRect(image, sourceRect, tileRect, paint);
} else {
if (repeat == ImageRepeat.noRepeat) {
canvas.drawImageNine(image, centerSlice, destinationRect, paint);
} else {
for (final Rect tileRect in _generateImageTileRects(rect, destinationRect, repeat))
canvas.drawImageNine(image, centerSlice, tileRect, paint);
if (needSave)
if (invertedCanvas) {
Iterable<Rect> _generateImageTileRects(Rect outputRect, Rect fundamentalRect, ImageRepeat repeat) sync* {
int startX = 0;
int startY = 0;
int stopX = 0;
int stopY = 0;
final double strideX = fundamentalRect.width;
final double strideY = fundamentalRect.height;
if (repeat == ImageRepeat.repeat || repeat == ImageRepeat.repeatX) {
startX = ((outputRect.left - fundamentalRect.left) / strideX).floor();
stopX = ((outputRect.right - fundamentalRect.right) / strideX).ceil();
if (repeat == ImageRepeat.repeat || repeat == ImageRepeat.repeatY) {
startY = (( - / strideY).floor();
stopY = ((outputRect.bottom - fundamentalRect.bottom) / strideY).ceil();
for (int i = startX; i <= stopX; ++i) {
for (int j = startY; j <= stopY; ++j)
yield fundamentalRect.shift(Offset(i * strideX, j * strideY));