[DecorationImage] adds scale property

packages/flutter/lib/src/painting/decoration_image.dart

@@ -47,10 +47,12 @@ class DecorationImage {
     this.centerSlice,
     this.repeat = ImageRepeat.noRepeat,
     this.matchTextDirection = false,
+    this.scale = 1.0
   }) : assert(image != null),
        assert(alignment != null),
        assert(repeat != null),
-       assert(matchTextDirection != null);
+       assert(matchTextDirection != null),
+       assert(scale != null);
 
   /// The image to be painted into the decoration.
   ///
@@ -129,6 +131,12 @@ class DecorationImage {
   /// in the top right.
   final bool matchTextDirection;
 
+  /// Defines image pixels to be shown per logical pixels.
+  ///
+  /// By default the 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
@@ -152,11 +160,12 @@ class DecorationImage {
         && other.alignment == alignment
         && other.centerSlice == centerSlice
         && other.repeat == repeat
-        && other.matchTextDirection == matchTextDirection;
+        && other.matchTextDirection == matchTextDirection
+        && other.scale == scale;
   }
 
   @override
-  int get hashCode => hashValues(image, colorFilter, fit, alignment, centerSlice, repeat, matchTextDirection);
+  int get hashCode => hashValues(image, colorFilter, fit, alignment, centerSlice, repeat, matchTextDirection, scale);
 
   @override
   String toString() {
@@ -175,6 +184,7 @@ class DecorationImage {
         '$repeat',
       if (matchTextDirection)
         'match text direction',
+      'scale: $scale'
     ];
     return '${objectRuntimeType(this, 'DecorationImage')}(${properties.join(", ")})';
   }
@@ -263,7 +273,7 @@ class DecorationImagePainter {
       canvas: canvas,
       rect: rect,
       image: _image.image,
-      scale: _image.scale,
+      scale: _details.scale * _image.scale,
       colorFilter: _details.colorFilter,
       fit: _details.fit,
       alignment: _details.alignment.resolve(configuration.textDirection),

packages/flutter/test/painting/decoration_test.dart

@@ -279,7 +279,7 @@ void main() {
       '   direction provided in the ImageConfiguration object to match.\n'
       '   The DecorationImage was:\n'
       '     DecorationImage(SynchronousTestImageProvider(), center, match\n'
-      '     text direction)\n'
+      '     text direction, scale: 1.0)\n'
       '   The ImageConfiguration was:\n'
       '     ImageConfiguration(size: Size(100.0, 100.0))\n'
     );
@@ -583,4 +583,34 @@ void main() {
       expect(call.positionalArguments[2].center, outputRect.center);
     }
   });
+
+  test('scale cannot be null in DecorationImage', () {
+    try {
+      DecorationImage(scale: null, image: SynchronousTestImageProvider());
+    } on AssertionError catch (error) {
+      expect(error.toString(), contains('scale != null'));
+      expect(error.toString(), contains('is not true'));
+      return;
+    }
+    fail('DecorationImage did not throw AssertionError when scale was null');
+  });
+
+  test('DecorationImage scale test', () {
+    final DecorationImage backgroundImage = DecorationImage(
+      image: SynchronousTestImageProvider(),
+      scale: 4,
+      alignment: Alignment.topLeft
+    );
+
+    final BoxDecoration boxDecoration = BoxDecoration(image: backgroundImage);
+    final BoxPainter boxPainter = boxDecoration.createBoxPainter(() { assert(false); });
+    final TestCanvas canvas = TestCanvas(<Invocation>[]);
+    boxPainter.paint(canvas, Offset.zero, const ImageConfiguration(size: Size(100.0, 100.0)));
+
+    final Invocation call = canvas.invocations.firstWhere((Invocation call) => call.memberName == #drawImageRect);
+    // The image should scale down to Size(25.0, 25.0) from Size(100.0, 100.0)
+    // considering DecorationImage scale to be 4.0 and Image scale to be 1.0.
+    expect(call.positionalArguments[2].size, const Size(25.0, 25.0));
+    expect(call.positionalArguments[2], const Rect.fromLTRB(0.0, 0.0, 25.0, 25.0));
+  });
 }