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

 // Copyright 2015 The Chromium 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:async';
 import 'dart:io' show File;
 import 'dart:typed_data';

 import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
 import 'package:flutter/services.dart';

 import 'basic.dart';
 import 'framework.dart';
 import 'localizations.dart';
 import 'media_query.dart';
 import 'ticker_provider.dart';

 export 'package:flutter/painting.dart' show
   AssetImage,
   ExactAssetImage,
   FileImage,
   ImageConfiguration,
   ImageInfo,
   ImageStream,
   ImageProvider,
   MemoryImage,
   NetworkImage;

 /// Creates an [ImageConfiguration] based on the given [BuildContext] (and
 /// optionally size).
 ///
 /// This is the object that must be passed to [BoxPainter.paint] and to
 /// [ImageProvider.resolve].
 ///
 /// If this is not called from a build method, then it should be reinvoked
 /// whenever the dependencies change, e.g. by calling it from
 /// [State.didChangeDependencies], so that any changes in the environment are
 /// picked up (e.g. if the device pixel ratio changes).
 ///
 /// See also:
 ///
 ///  * [ImageProvider], which has an example showing how this might be used.
 ImageConfiguration createLocalImageConfiguration(BuildContext context, { Size size }) {
   return new ImageConfiguration(
     bundle: DefaultAssetBundle.of(context),
     devicePixelRatio: MediaQuery.of(context, nullOk: true)?.devicePixelRatio ?? 1.0,
     locale: Localizations.localeOf(context, nullOk: true),
     textDirection: Directionality.of(context),
     size: size,
     platform: defaultTargetPlatform,
   );
 }

 /// Prefetches an image into the image cache.
 ///
 /// Returns a [Future] that will complete when the first image yielded by the
 /// [ImageProvider] is available.
 ///
 /// If the image is later used by an [Image] or [BoxDecoration] or [FadeInImage],
 /// it will probably be loaded faster. The consumer of the image does not need
 /// to use the same [ImageProvider] instance. The [ImageCache] will find the image
 /// as long as both images share the same key.
 ///
 /// The [BuildContext] and [Size] are used to select an image configuration
 /// (see [createLocalImageConfiguration]).
 ///
 /// See also:
 ///
 ///   * [ImageCache], which holds images that may be reused.
 Future<Null> precacheImage(ImageProvider provider, BuildContext context, { Size size }) {
   final ImageConfiguration config = createLocalImageConfiguration(context, size: size);
   final Completer<Null> completer = new Completer<Null>();
   final ImageStream stream = provider.resolve(config);
   void listener(ImageInfo image, bool sync) {
     completer.complete();
   }
   stream.addListener(listener);
   completer.future.then((Null _) { stream.removeListener(listener); });
   return completer.future;
 }

 /// A widget that displays an image.
 ///
 /// Several constructors are provided for the various ways that an image can be
 /// specified:
 ///
 ///  * [new Image], for obtaining an image from an [ImageProvider].
 ///  * [new Image.asset], for obtaining an image from an [AssetBundle]
 ///    using a key.
 ///  * [new Image.network], for obtaining an image from a URL.
 ///  * [new Image.file], for obtaining an image from a [File].
 ///  * [new Image.memory], for obtaining an image from a [Uint8List].
 ///
 /// The following image formats are supported: {@macro flutter.dart:ui.imageFormats}
 ///
 /// To automatically perform pixel-density-aware asset resolution, specify the
 /// image using an [AssetImage] and make sure that a [MaterialApp], [WidgetsApp],
 /// or [MediaQuery] widget exists above the [Image] widget in the widget tree.
 ///
 /// The image is painted using [paintImage], which describes the meanings of the
 /// various fields on this class in more detail.
 ///
 /// See also:
 ///
 ///  * [Icon], which shows an image from a font.
 ///  * [new Ink.image], which is the preferred way to show an image in a
 ///    material application (especially if the image is in a [Material] and will
 ///    have an [InkWell] on top of it).
 class Image extends StatefulWidget {
   /// Creates a widget that displays an image.
   ///
   /// To show an image from the network or from an asset bundle, consider using
   /// [new Image.network] and [new Image.asset] respectively.
   ///
   /// The [image], [alignment], [repeat], and [matchTextDirection] arguments
   /// must not be null.
   ///
   /// Either the [width] and [height] arguments should be specified, or the
   /// widget should be placed in a context that sets tight layout constraints.
   /// Otherwise, the image dimensions will change as the image is loaded, which
   /// will result in ugly layout changes.
   const Image({
     Key key,
     @required this.image,
     this.width,
     this.height,
     this.color,
     this.colorBlendMode,
     this.fit,
     this.alignment: Alignment.center,
     this.repeat: ImageRepeat.noRepeat,
     this.centerSlice,
     this.matchTextDirection: false,
     this.gaplessPlayback: false,
   }) : assert(image != null),
        assert(alignment != null),
        assert(repeat != null),
        assert(matchTextDirection != null),
        super(key: key);

   /// Creates a widget that displays an [ImageStream] obtained from the network.
   ///
   /// The [src], [scale], and [repeat] arguments must not be null.
   ///
   /// Either the [width] and [height] arguments should be specified, or the
   /// widget should be placed in a context that sets tight layout constraints.
   /// Otherwise, the image dimensions will change as the image is loaded, which
   /// will result in ugly layout changes.
   ///
   /// All network images are cached regardless of HTTP headers.
   ///
   /// An optional [headers] argument can be used to send custom HTTP headers
   /// with the image request.
   Image.network(String src, {
     Key key,
     double scale: 1.0,
     this.width,
     this.height,
     this.color,
     this.colorBlendMode,
     this.fit,
     this.alignment: Alignment.center,
     this.repeat: ImageRepeat.noRepeat,
     this.centerSlice,
     this.matchTextDirection: false,
     this.gaplessPlayback: false,
     Map<String, String> headers,
   }) : image = new NetworkImage(src, scale: scale, headers: headers),
        assert(alignment != null),
        assert(repeat != null),
        assert(matchTextDirection != null),
        super(key: key);

   /// Creates a widget that displays an [ImageStream] obtained from a [File].
   ///
   /// The [file], [scale], and [repeat] arguments must not be null.
   ///
   /// Either the [width] and [height] arguments should be specified, or the
   /// widget should be placed in a context that sets tight layout constraints.
   /// Otherwise, the image dimensions will change as the image is loaded, which
   /// will result in ugly layout changes.
   ///
   /// On Android, this may require the
   /// `android.permission.READ_EXTERNAL_STORAGE` permission.
   Image.file(File file, {
     Key key,
     double scale: 1.0,
     this.width,
     this.height,
     this.color,
     this.colorBlendMode,
     this.fit,
     this.alignment: Alignment.center,
     this.repeat: ImageRepeat.noRepeat,
     this.centerSlice,
     this.matchTextDirection: false,
     this.gaplessPlayback: false,
   }) : image = new FileImage(file, scale: scale),
        assert(alignment != null),
        assert(repeat != null),
        assert(matchTextDirection != null),
        super(key: key);

   /// Creates a widget that displays an [ImageStream] obtained from an asset
   /// bundle. The key for the image is given by the `name` argument.
   ///
   /// The `package` argument must be non-null when displaying an image from a
   /// package and null otherwise. See the `Assets in packages` section for
   /// details.
   ///
   /// If the `bundle` argument is omitted or null, then the
   /// [DefaultAssetBundle] will be used.
   ///
   /// By default, the pixel-density-aware asset resolution will be attempted. In
   /// addition:
   ///
   /// * If the `scale` argument is provided and is not null, then the exact
   /// asset specified will be used.
   //
   // TODO(ianh): Implement the following (see ../services/image_resolution.dart):
   // ///
   // /// * If [width] and [height] are both specified, and [scale] is not, then
   // ///   size-aware asset resolution will be attempted also, with the given
   // ///   dimensions interpreted as logical pixels.
   // ///
   // /// * If the images have platform, locale, or directionality variants, the
   // ///   current platform, locale, and directionality are taken into account
   // ///   during asset resolution as well.
   ///
   /// The [name] and [repeat] arguments must not be null.
   ///
   /// Either the [width] and [height] arguments should be specified, or the
   /// widget should be placed in a context that sets tight layout constraints.
   /// Otherwise, the image dimensions will change as the image is loaded, which
   /// will result in ugly layout changes.
   ///
   /// ## Sample code
   ///
   /// Suppose that the project's `pubspec.yaml` file contains the following:
   ///
   /// ```yaml
   /// flutter:
   ///   assets:
   ///     - images/cat.png
   ///     - images/2x/cat.png
   ///     - images/3.5x/cat.png
   /// ```
   ///
   /// On a screen with a device pixel ratio of 2.0, the following widget would
   /// render the `images/2x/cat.png` file:
   ///
   /// ```dart
   /// new Image.asset('images/cat.png')
   /// ```
   ///
   /// This corresponds to the file that is in the project's `images/2x/`
   /// directory with the name `cat.png` (the paths are relative to the
   /// `pubspec.yaml` file).
   ///
   /// On a device with a 4.0 device pixel ratio, the `images/3.5x/cat.png` asset
   /// would be used. On a device with a 1.0 device pixel ratio, the
   /// `images/cat.png` resource would be used.
   ///
   /// The `images/cat.png` image can be omitted from disk (though it must still
   /// be present in the manifest). If it is omitted, then on a device with a 1.0
   /// device pixel ratio, the `images/2x/cat.png` image would be used instead.
   ///
   ///
   /// ## Assets in packages
   ///
   /// To create the widget with an asset from a package, the [package] argument
   /// must be provided. For instance, suppose a package called `my_icons` has
   /// `icons/heart.png` .
   ///
   /// Then to display the image, use:
   ///
   /// ```dart
   /// new Image.asset('icons/heart.png', package: 'my_icons')
   /// ```
   ///
   /// Assets used by the package itself should also be displayed using the
   /// [package] argument as above.
   ///
   /// If the desired asset is specified in the `pubspec.yaml` of the package, it
   /// is bundled automatically with the app. In particular, assets used by the
   /// package itself must be specified in its `pubspec.yaml`.
   ///
   /// A package can also choose to have assets in its 'lib/' folder that are not
   /// specified in its `pubspec.yaml`. In this case for those images to be
   /// bundled, the app has to specify which ones to include. For instance a
   /// package named `fancy_backgrounds` could have:
   ///
   /// ```
   /// lib/backgrounds/background1.png
   /// lib/backgrounds/background2.png
   /// lib/backgrounds/background3.png
   ///```
   ///
   /// To include, say the first image, the `pubspec.yaml` of the app should
   /// specify it in the assets section:
   ///
   /// ```yaml
   ///  assets:
   ///    - packages/fancy_backgrounds/backgrounds/background1.png
   /// ```
   ///
   /// The `lib/` is implied, so it should not be included in the asset path.
   ///
   ///
   /// See also:
   ///
   ///  * [AssetImage], which is used to implement the behavior when the scale is
   ///    omitted.
   ///  * [ExactAssetImage], which is used to implement the behavior when the
   ///    scale is present.
   ///  * <https://flutter.io/assets-and-images/>, an introduction to assets in
   ///    Flutter.
   Image.asset(String name, {
     Key key,
     AssetBundle bundle,
     double scale,
     this.width,
     this.height,
     this.color,
     this.colorBlendMode,
     this.fit,
     this.alignment: Alignment.center,
     this.repeat: ImageRepeat.noRepeat,
     this.centerSlice,
     this.matchTextDirection: false,
     this.gaplessPlayback: false,
     String package,
   }) : image = scale != null
          ? new ExactAssetImage(name, bundle: bundle, scale: scale, package: package)
          : new AssetImage(name, bundle: bundle, package: package),
        assert(alignment != null),
        assert(repeat != null),
        assert(matchTextDirection != null),
        super(key: key);

   /// Creates a widget that displays an [ImageStream] obtained from a [Uint8List].
   ///
   /// The [bytes], [scale], and [repeat] arguments must not be null.
   ///
   /// Either the [width] and [height] arguments should be specified, or the
   /// widget should be placed in a context that sets tight layout constraints.
   /// Otherwise, the image dimensions will change as the image is loaded, which
   /// will result in ugly layout changes.
   Image.memory(Uint8List bytes, {
     Key key,
     double scale: 1.0,
     this.width,
     this.height,
     this.color,
     this.colorBlendMode,
     this.fit,
     this.alignment: Alignment.center,
     this.repeat: ImageRepeat.noRepeat,
     this.centerSlice,
     this.matchTextDirection: false,
     this.gaplessPlayback: false,
   }) : image = new MemoryImage(bytes, scale: scale),
        assert(alignment != null),
        assert(repeat != null),
        assert(matchTextDirection != null),
        super(key: key);

   /// The image to display.
   final ImageProvider image;

   /// If non-null, require the image to have this width.
   ///
   /// If null, the image will pick a size that best preserves its intrinsic
   /// aspect ratio.
   ///
   /// It is strongly recommended that either both the [width] and the [height]
   /// be specified, or that the widget be placed in a context that sets tight
   /// layout constraints, so that the image does not change size as it loads.
   /// Consider using [fit] to adapt the image's rendering to fit the given width
   /// and height if the exact image dimensions are not known in advance.
   final double width;

   /// If non-null, require the image to have this height.
   ///
   /// If null, the image will pick a size that best preserves its intrinsic
   /// aspect ratio.
   ///
   /// It is strongly recommended that either both the [width] and the [height]
   /// be specified, or that the widget be placed in a context that sets tight
   /// layout constraints, so that the image does not change size as it loads.
   /// Consider using [fit] to adapt the image's rendering to fit the given width
   /// and height if the exact image dimensions are not known in advance.
   final double height;

   /// If non-null, this color is blended with each image pixel using [colorBlendMode].
   final Color color;

   /// Used to combine [color] with this image.
   ///
   /// The default is [BlendMode.srcIn]. In terms of the blend mode, [color] is
   /// the source and this image is the destination.
   ///
   /// See also:
   ///
   ///  * [BlendMode], which includes an illustration of the effect of each blend mode.
   final BlendMode colorBlendMode;

   /// How to inscribe the image into the space allocated during layout.
   ///
   /// The default varies based on the other fields. See the discussion at
   /// [paintImage].
   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, a [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 an ambient [Directionality] widget
   /// must be in scope.
   ///
   /// Defaults to [Alignment.center].
   ///
   /// 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;

   /// How to paint any portions of the layout bounds not covered by the image.
   final ImageRepeat repeat;

   /// 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.
   final Rect centerSlice;

   /// 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.
   ///
   /// This is occasionally used with images in right-to-left environments, for
   /// images that were designed for left-to-right locales. Be careful, when
   /// using this, to not flip images with integral shadows, text, or other
   /// effects that will look incorrect when flipped.
   ///
   /// If this is true, there must be an ambient [Directionality] widget in
   /// scope.
   final bool matchTextDirection;

   /// Whether to continue showing the old image (true), or briefly show nothing
   /// (false), when the image provider changes.
   final bool gaplessPlayback;

   @override
   _ImageState createState() => new _ImageState();

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(new DiagnosticsProperty<ImageProvider>('image', image));
     properties.add(new DoubleProperty('width', width, defaultValue: null));
     properties.add(new DoubleProperty('height', height, defaultValue: null));
     properties.add(new DiagnosticsProperty<Color>('color', color, defaultValue: null));
     properties.add(new EnumProperty<BlendMode>('colorBlendMode', colorBlendMode, defaultValue: null));
     properties.add(new EnumProperty<BoxFit>('fit', fit, defaultValue: null));
     properties.add(new DiagnosticsProperty<AlignmentGeometry>('alignment', alignment, defaultValue: null));
     properties.add(new EnumProperty<ImageRepeat>('repeat', repeat, defaultValue: ImageRepeat.noRepeat));
     properties.add(new DiagnosticsProperty<Rect>('centerSlice', centerSlice, defaultValue: null));
     properties.add(new FlagProperty('matchTextDirection', value: matchTextDirection, ifTrue: 'match text direction'));
   }
 }

 class _ImageState extends State<Image> {
   ImageStream _imageStream;
   ImageInfo _imageInfo;
   bool _isListeningToStream = false;

   @override
   void didChangeDependencies() {
     _resolveImage();

     if (TickerMode.of(context))
       _listenToStream();
     else
       _stopListeningToStream();

     super.didChangeDependencies();
   }

   @override
   void didUpdateWidget(Image oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (widget.image != oldWidget.image)
       _resolveImage();
   }

   @override
   void reassemble() {
     _resolveImage(); // in case the image cache was flushed
     super.reassemble();
   }

   void _resolveImage() {
     final ImageStream newStream =
       widget.image.resolve(createLocalImageConfiguration(
           context,
           size: widget.width != null && widget.height != null ? new Size(widget.width, widget.height) : null
       ));
     assert(newStream != null);
     _updateSourceStream(newStream);
   }

   void _handleImageChanged(ImageInfo imageInfo, bool synchronousCall) {
     setState(() {
       _imageInfo = imageInfo;
     });
   }

   // Update _imageStream to newStream, and moves the stream listener
   // registration from the old stream to the new stream (if a listener was
   // registered).
   void _updateSourceStream(ImageStream newStream) {
     if (_imageStream?.key == newStream?.key)
       return;

     if (_isListeningToStream)
       _imageStream.removeListener(_handleImageChanged);

     if (!widget.gaplessPlayback)
       setState(() { _imageInfo = null; });

     _imageStream = newStream;
     if (_isListeningToStream)
       _imageStream.addListener(_handleImageChanged);
   }

   void _listenToStream() {
     if (_isListeningToStream)
       return;
     _imageStream.addListener(_handleImageChanged);
     _isListeningToStream = true;
   }

   void _stopListeningToStream() {
     if (!_isListeningToStream)
       return;
     _imageStream.removeListener(_handleImageChanged);
     _isListeningToStream = false;
   }

   @override
   void dispose() {
     assert(_imageStream != null);
     _stopListeningToStream();
     super.dispose();
   }

   @override
   Widget build(BuildContext context) {
     return new RawImage(
       image: _imageInfo?.image,
       width: widget.width,
       height: widget.height,
       scale: _imageInfo?.scale ?? 1.0,
       color: widget.color,
       colorBlendMode: widget.colorBlendMode,
       fit: widget.fit,
       alignment: widget.alignment,
       repeat: widget.repeat,
       centerSlice: widget.centerSlice,
       matchTextDirection: widget.matchTextDirection,
     );
   }

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder description) {
     super.debugFillProperties(description);
     description.add(new DiagnosticsProperty<ImageStream>('stream', _imageStream));
     description.add(new DiagnosticsProperty<ImageInfo>('pixels', _imageInfo));
   }
 }
	// Copyright 2015 The Chromium 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:async';
	import 'dart:io' show File;
	import 'dart:typed_data';

	import 'package:flutter/foundation.dart';
	import 'package:flutter/painting.dart';
	import 'package:flutter/services.dart';

	import 'basic.dart';
	import 'framework.dart';
	import 'localizations.dart';
	import 'media_query.dart';
	import 'ticker_provider.dart';

	export 'package:flutter/painting.dart' show
	AssetImage,
	ExactAssetImage,
	FileImage,
	ImageConfiguration,
	ImageInfo,
	ImageStream,
	ImageProvider,
	MemoryImage,
	NetworkImage;

	/// Creates an [ImageConfiguration] based on the given [BuildContext] (and
	/// optionally size).
	///
	/// This is the object that must be passed to [BoxPainter.paint] and to
	/// [ImageProvider.resolve].
	///
	/// If this is not called from a build method, then it should be reinvoked
	/// whenever the dependencies change, e.g. by calling it from
	/// [State.didChangeDependencies], so that any changes in the environment are
	/// picked up (e.g. if the device pixel ratio changes).
	///
	/// See also:
	///
	/// * [ImageProvider], which has an example showing how this might be used.
	ImageConfiguration createLocalImageConfiguration(BuildContext context, { Size size }) {
	return new ImageConfiguration(
	bundle: DefaultAssetBundle.of(context),
	devicePixelRatio: MediaQuery.of(context, nullOk: true)?.devicePixelRatio ?? 1.0,
	locale: Localizations.localeOf(context, nullOk: true),
	textDirection: Directionality.of(context),
	size: size,
	platform: defaultTargetPlatform,
	);
	}

	/// Prefetches an image into the image cache.
	///
	/// Returns a [Future] that will complete when the first image yielded by the
	/// [ImageProvider] is available.
	///
	/// If the image is later used by an [Image] or [BoxDecoration] or [FadeInImage],
	/// it will probably be loaded faster. The consumer of the image does not need
	/// to use the same [ImageProvider] instance. The [ImageCache] will find the image
	/// as long as both images share the same key.
	///
	/// The [BuildContext] and [Size] are used to select an image configuration
	/// (see [createLocalImageConfiguration]).
	///
	/// See also:
	///
	/// * [ImageCache], which holds images that may be reused.
	Future<Null> precacheImage(ImageProvider provider, BuildContext context, { Size size }) {
	final ImageConfiguration config = createLocalImageConfiguration(context, size: size);
	final Completer<Null> completer = new Completer<Null>();
	final ImageStream stream = provider.resolve(config);
	void listener(ImageInfo image, bool sync) {
	completer.complete();
	}
	stream.addListener(listener);
	completer.future.then((Null _) { stream.removeListener(listener); });
	return completer.future;
	}

	/// A widget that displays an image.
	///
	/// Several constructors are provided for the various ways that an image can be
	/// specified:
	///
	/// * [new Image], for obtaining an image from an [ImageProvider].
	/// * [new Image.asset], for obtaining an image from an [AssetBundle]
	/// using a key.
	/// * [new Image.network], for obtaining an image from a URL.
	/// * [new Image.file], for obtaining an image from a [File].
	/// * [new Image.memory], for obtaining an image from a [Uint8List].
	///
	/// The following image formats are supported: {@macro flutter.dart:ui.imageFormats}
	///
	/// To automatically perform pixel-density-aware asset resolution, specify the
	/// image using an [AssetImage] and make sure that a [MaterialApp], [WidgetsApp],
	/// or [MediaQuery] widget exists above the [Image] widget in the widget tree.
	///
	/// The image is painted using [paintImage], which describes the meanings of the
	/// various fields on this class in more detail.
	///
	/// See also:
	///
	/// * [Icon], which shows an image from a font.
	/// * [new Ink.image], which is the preferred way to show an image in a
	/// material application (especially if the image is in a [Material] and will
	/// have an [InkWell] on top of it).
	class Image extends StatefulWidget {
	/// Creates a widget that displays an image.
	///
	/// To show an image from the network or from an asset bundle, consider using
	/// [new Image.network] and [new Image.asset] respectively.
	///
	/// The [image], [alignment], [repeat], and [matchTextDirection] arguments
	/// must not be null.
	///
	/// Either the [width] and [height] arguments should be specified, or the
	/// widget should be placed in a context that sets tight layout constraints.
	/// Otherwise, the image dimensions will change as the image is loaded, which
	/// will result in ugly layout changes.
	const Image({
	Key key,
	@required this.image,
	this.width,
	this.height,
	this.color,
	this.colorBlendMode,
	this.fit,
	this.alignment: Alignment.center,
	this.repeat: ImageRepeat.noRepeat,
	this.centerSlice,
	this.matchTextDirection: false,
	this.gaplessPlayback: false,
	}) : assert(image != null),
	assert(alignment != null),
	assert(repeat != null),
	assert(matchTextDirection != null),
	super(key: key);

	/// Creates a widget that displays an [ImageStream] obtained from the network.
	///
	/// The [src], [scale], and [repeat] arguments must not be null.
	///
	/// Either the [width] and [height] arguments should be specified, or the
	/// widget should be placed in a context that sets tight layout constraints.
	/// Otherwise, the image dimensions will change as the image is loaded, which
	/// will result in ugly layout changes.
	///
	/// All network images are cached regardless of HTTP headers.
	///
	/// An optional [headers] argument can be used to send custom HTTP headers
	/// with the image request.
	Image.network(String src, {
	Key key,
	double scale: 1.0,
	this.width,
	this.height,
	this.color,
	this.colorBlendMode,
	this.fit,
	this.alignment: Alignment.center,
	this.repeat: ImageRepeat.noRepeat,
	this.centerSlice,
	this.matchTextDirection: false,
	this.gaplessPlayback: false,
	Map<String, String> headers,
	}) : image = new NetworkImage(src, scale: scale, headers: headers),
	assert(alignment != null),
	assert(repeat != null),
	assert(matchTextDirection != null),
	super(key: key);

	/// Creates a widget that displays an [ImageStream] obtained from a [File].
	///
	/// The [file], [scale], and [repeat] arguments must not be null.
	///
	/// Either the [width] and [height] arguments should be specified, or the
	/// widget should be placed in a context that sets tight layout constraints.
	/// Otherwise, the image dimensions will change as the image is loaded, which
	/// will result in ugly layout changes.
	///
	/// On Android, this may require the
	/// `android.permission.READ_EXTERNAL_STORAGE` permission.
	Image.file(File file, {
	Key key,
	double scale: 1.0,
	this.width,
	this.height,
	this.color,
	this.colorBlendMode,
	this.fit,
	this.alignment: Alignment.center,
	this.repeat: ImageRepeat.noRepeat,
	this.centerSlice,
	this.matchTextDirection: false,
	this.gaplessPlayback: false,
	}) : image = new FileImage(file, scale: scale),
	assert(alignment != null),
	assert(repeat != null),
	assert(matchTextDirection != null),
	super(key: key);

	/// Creates a widget that displays an [ImageStream] obtained from an asset
	/// bundle. The key for the image is given by the `name` argument.
	///
	/// The `package` argument must be non-null when displaying an image from a
	/// package and null otherwise. See the `Assets in packages` section for
	/// details.
	///
	/// If the `bundle` argument is omitted or null, then the
	/// [DefaultAssetBundle] will be used.
	///
	/// By default, the pixel-density-aware asset resolution will be attempted. In
	/// addition:
	///
	/// * If the `scale` argument is provided and is not null, then the exact
	/// asset specified will be used.
	//
	// TODO(ianh): Implement the following (see ../services/image_resolution.dart):
	// ///
	// /// * If [width] and [height] are both specified, and [scale] is not, then
	// /// size-aware asset resolution will be attempted also, with the given
	// /// dimensions interpreted as logical pixels.
	// ///
	// /// * If the images have platform, locale, or directionality variants, the
	// /// current platform, locale, and directionality are taken into account
	// /// during asset resolution as well.
	///
	/// The [name] and [repeat] arguments must not be null.
	///
	/// Either the [width] and [height] arguments should be specified, or the
	/// widget should be placed in a context that sets tight layout constraints.
	/// Otherwise, the image dimensions will change as the image is loaded, which
	/// will result in ugly layout changes.
	///
	/// ## Sample code
	///
	/// Suppose that the project's `pubspec.yaml` file contains the following:
	///
	/// ```yaml
	/// flutter:
	/// assets:
	/// - images/cat.png
	/// - images/2x/cat.png
	/// - images/3.5x/cat.png
	/// ```
	///
	/// On a screen with a device pixel ratio of 2.0, the following widget would
	/// render the `images/2x/cat.png` file:
	///
	/// ```dart
	/// new Image.asset('images/cat.png')
	/// ```
	///
	/// This corresponds to the file that is in the project's `images/2x/`
	/// directory with the name `cat.png` (the paths are relative to the
	/// `pubspec.yaml` file).
	///
	/// On a device with a 4.0 device pixel ratio, the `images/3.5x/cat.png` asset
	/// would be used. On a device with a 1.0 device pixel ratio, the
	/// `images/cat.png` resource would be used.
	///
	/// The `images/cat.png` image can be omitted from disk (though it must still
	/// be present in the manifest). If it is omitted, then on a device with a 1.0
	/// device pixel ratio, the `images/2x/cat.png` image would be used instead.
	///
	///
	/// ## Assets in packages
	///
	/// To create the widget with an asset from a package, the [package] argument
	/// must be provided. For instance, suppose a package called `my_icons` has
	/// `icons/heart.png` .
	///
	/// Then to display the image, use:
	///
	/// ```dart
	/// new Image.asset('icons/heart.png', package: 'my_icons')
	/// ```
	///
	/// Assets used by the package itself should also be displayed using the
	/// [package] argument as above.
	///
	/// If the desired asset is specified in the `pubspec.yaml` of the package, it
	/// is bundled automatically with the app. In particular, assets used by the
	/// package itself must be specified in its `pubspec.yaml`.
	///
	/// A package can also choose to have assets in its 'lib/' folder that are not
	/// specified in its `pubspec.yaml`. In this case for those images to be
	/// bundled, the app has to specify which ones to include. For instance a
	/// package named `fancy_backgrounds` could have:
	///
	/// ```
	/// lib/backgrounds/background1.png
	/// lib/backgrounds/background2.png
	/// lib/backgrounds/background3.png
	///```
	///
	/// To include, say the first image, the `pubspec.yaml` of the app should
	/// specify it in the assets section:
	///
	/// ```yaml
	/// assets:
	/// - packages/fancy_backgrounds/backgrounds/background1.png
	/// ```
	///
	/// The `lib/` is implied, so it should not be included in the asset path.
	///
	///
	/// See also:
	///
	/// * [AssetImage], which is used to implement the behavior when the scale is
	/// omitted.
	/// * [ExactAssetImage], which is used to implement the behavior when the
	/// scale is present.
	/// * <https://flutter.io/assets-and-images/>, an introduction to assets in
	/// Flutter.
	Image.asset(String name, {
	Key key,
	AssetBundle bundle,
	double scale,
	this.width,
	this.height,
	this.color,
	this.colorBlendMode,
	this.fit,
	this.alignment: Alignment.center,
	this.repeat: ImageRepeat.noRepeat,
	this.centerSlice,
	this.matchTextDirection: false,
	this.gaplessPlayback: false,
	String package,
	}) : image = scale != null
	? new ExactAssetImage(name, bundle: bundle, scale: scale, package: package)
	: new AssetImage(name, bundle: bundle, package: package),
	assert(alignment != null),
	assert(repeat != null),
	assert(matchTextDirection != null),
	super(key: key);

	/// Creates a widget that displays an [ImageStream] obtained from a [Uint8List].
	///
	/// The [bytes], [scale], and [repeat] arguments must not be null.
	///
	/// Either the [width] and [height] arguments should be specified, or the
	/// widget should be placed in a context that sets tight layout constraints.
	/// Otherwise, the image dimensions will change as the image is loaded, which
	/// will result in ugly layout changes.
	Image.memory(Uint8List bytes, {
	Key key,
	double scale: 1.0,
	this.width,
	this.height,
	this.color,
	this.colorBlendMode,
	this.fit,
	this.alignment: Alignment.center,
	this.repeat: ImageRepeat.noRepeat,
	this.centerSlice,
	this.matchTextDirection: false,
	this.gaplessPlayback: false,
	}) : image = new MemoryImage(bytes, scale: scale),
	assert(alignment != null),
	assert(repeat != null),
	assert(matchTextDirection != null),
	super(key: key);

	/// The image to display.
	final ImageProvider image;

	/// If non-null, require the image to have this width.
	///
	/// If null, the image will pick a size that best preserves its intrinsic
	/// aspect ratio.
	///
	/// It is strongly recommended that either both the [width] and the [height]
	/// be specified, or that the widget be placed in a context that sets tight
	/// layout constraints, so that the image does not change size as it loads.
	/// Consider using [fit] to adapt the image's rendering to fit the given width
	/// and height if the exact image dimensions are not known in advance.
	final double width;

	/// If non-null, require the image to have this height.
	///
	/// If null, the image will pick a size that best preserves its intrinsic
	/// aspect ratio.
	///
	/// It is strongly recommended that either both the [width] and the [height]
	/// be specified, or that the widget be placed in a context that sets tight
	/// layout constraints, so that the image does not change size as it loads.
	/// Consider using [fit] to adapt the image's rendering to fit the given width
	/// and height if the exact image dimensions are not known in advance.
	final double height;

	/// If non-null, this color is blended with each image pixel using [colorBlendMode].
	final Color color;

	/// Used to combine [color] with this image.
	///
	/// The default is [BlendMode.srcIn]. In terms of the blend mode, [color] is
	/// the source and this image is the destination.
	///
	/// See also:
	///
	/// * [BlendMode], which includes an illustration of the effect of each blend mode.
	final BlendMode colorBlendMode;

	/// How to inscribe the image into the space allocated during layout.
	///
	/// The default varies based on the other fields. See the discussion at
	/// [paintImage].
	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, a [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 an ambient [Directionality] widget
	/// must be in scope.
	///
	/// Defaults to [Alignment.center].
	///
	/// 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;

	/// How to paint any portions of the layout bounds not covered by the image.
	final ImageRepeat repeat;

	/// 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.
	final Rect centerSlice;

	/// 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.
	///
	/// This is occasionally used with images in right-to-left environments, for
	/// images that were designed for left-to-right locales. Be careful, when
	/// using this, to not flip images with integral shadows, text, or other
	/// effects that will look incorrect when flipped.
	///
	/// If this is true, there must be an ambient [Directionality] widget in
	/// scope.
	final bool matchTextDirection;

	/// Whether to continue showing the old image (true), or briefly show nothing
	/// (false), when the image provider changes.
	final bool gaplessPlayback;

	@override
	_ImageState createState() => new _ImageState();

	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties) {
	super.debugFillProperties(properties);
	properties.add(new DiagnosticsProperty<ImageProvider>('image', image));
	properties.add(new DoubleProperty('width', width, defaultValue: null));
	properties.add(new DoubleProperty('height', height, defaultValue: null));
	properties.add(new DiagnosticsProperty<Color>('color', color, defaultValue: null));
	properties.add(new EnumProperty<BlendMode>('colorBlendMode', colorBlendMode, defaultValue: null));
	properties.add(new EnumProperty<BoxFit>('fit', fit, defaultValue: null));
	properties.add(new DiagnosticsProperty<AlignmentGeometry>('alignment', alignment, defaultValue: null));
	properties.add(new EnumProperty<ImageRepeat>('repeat', repeat, defaultValue: ImageRepeat.noRepeat));
	properties.add(new DiagnosticsProperty<Rect>('centerSlice', centerSlice, defaultValue: null));
	properties.add(new FlagProperty('matchTextDirection', value: matchTextDirection, ifTrue: 'match text direction'));
	}
	}

	class _ImageState extends State<Image> {
	ImageStream _imageStream;
	ImageInfo _imageInfo;
	bool _isListeningToStream = false;

	@override
	void didChangeDependencies() {
	_resolveImage();

	if (TickerMode.of(context))
	_listenToStream();
	else
	_stopListeningToStream();

	super.didChangeDependencies();
	}

	@override
	void didUpdateWidget(Image oldWidget) {
	super.didUpdateWidget(oldWidget);
	if (widget.image != oldWidget.image)
	_resolveImage();
	}

	@override
	void reassemble() {
	_resolveImage(); // in case the image cache was flushed
	super.reassemble();
	}

	void _resolveImage() {
	final ImageStream newStream =
	widget.image.resolve(createLocalImageConfiguration(
	context,
	size: widget.width != null && widget.height != null ? new Size(widget.width, widget.height) : null
	));
	assert(newStream != null);
	_updateSourceStream(newStream);
	}

	void _handleImageChanged(ImageInfo imageInfo, bool synchronousCall) {
	setState(() {
	_imageInfo = imageInfo;
	});
	}

	// Update _imageStream to newStream, and moves the stream listener
	// registration from the old stream to the new stream (if a listener was
	// registered).
	void _updateSourceStream(ImageStream newStream) {
	if (_imageStream?.key == newStream?.key)
	return;

	if (_isListeningToStream)
	_imageStream.removeListener(_handleImageChanged);

	if (!widget.gaplessPlayback)
	setState(() { _imageInfo = null; });

	_imageStream = newStream;
	if (_isListeningToStream)
	_imageStream.addListener(_handleImageChanged);
	}

	void _listenToStream() {
	if (_isListeningToStream)
	return;
	_imageStream.addListener(_handleImageChanged);
	_isListeningToStream = true;
	}

	void _stopListeningToStream() {
	if (!_isListeningToStream)
	return;
	_imageStream.removeListener(_handleImageChanged);
	_isListeningToStream = false;
	}

	@override
	void dispose() {
	assert(_imageStream != null);
	_stopListeningToStream();
	super.dispose();
	}

	@override
	Widget build(BuildContext context) {
	return new RawImage(
	image: _imageInfo?.image,
	width: widget.width,
	height: widget.height,
	scale: _imageInfo?.scale ?? 1.0,
	color: widget.color,
	colorBlendMode: widget.colorBlendMode,
	fit: widget.fit,
	alignment: widget.alignment,
	repeat: widget.repeat,
	centerSlice: widget.centerSlice,
	matchTextDirection: widget.matchTextDirection,
	);
	}

	@override
	void debugFillProperties(DiagnosticPropertiesBuilder description) {
	super.debugFillProperties(description);
	description.add(new DiagnosticsProperty<ImageStream>('stream', _imageStream));
	description.add(new DiagnosticsProperty<ImageInfo>('pixels', _imageInfo));
	}
	}